Note: Descriptions are shown in the official language in which they were submitted.
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Commutating Circuit Breaker
Field
This disclosure relates to a circuit breaker.
Backeround
In order to open any DC circuit, the inductive energy stored in the magnetic
fields due to the
flowing current must be absorbed; it can either be stored in capacitors or
dissipated in resistors
(arcs that form during opening the circuit are in this sense a special case of
a resistor). Because
of the rapid inrush of current in a short circuit, the inductive energy can
easily be much greater
than just the inductive energy stored in the system at normal full load; if
the current goes to five
times the normal full load amps before being controlled, the inductive energy
would be up to
twenty-five times as large as in the circuit at normal full load (depending on
the location of the
short). The inductive energy that must be dissipated to open a high voltage DC
(HVDC)
transmission line circuit can be in the hundreds of megajoules (MJ). The other
major problem
with opening a DC circuit is that (unlike AC power), the current and voltage
do not go through
zero periodically, so it is very difficult to extinguish a DC arc.
Several prior art strategies are known for breaking a high power DC current.
Arc chute breakers
(US patents 2,270,723; 3,735,074; 7,521,625; 7,541,902 for example) are
effective to break DC
currents up to 8000 amps (8.0 kA, kiloamps) at 800 volts (0.8 kV, kilovolts)
DC, or 4000 amps
at 1600 volts (1.6 kV). One can go to higher voltage with arc chute breakers,
but the needed
physical separation of the electrodes and the number of plates in the arc
chute increases linearly
with voltage in such devices, and so they become very large at voltage higher
than 3.5 kV.
The concept behind arc chute breakers is to spread the arc current out into
many small arcs over
a large surface area between parallel metal plates. Since the arc is quite
hot, the higher surface
area of the many small arcs implies far greater radiative cooling. As the arcs
cool, the resistance
goes up so high that the arc current is ultimately quenched; this process
takes a while: 50-300
milliseconds (ms) is a typical time between striking the arc and arc
extinction in a megawatt
(MW) scale arc chute breaker. This long time to open the circuit has little to
do with the speed of
motion of the electrodes; in a GerapidTM circuit breaker from GE, for example,
the electrodes are
separated within 3 ms (milliseconds), but cooling the arc takes up to 100
times as long as that,
a
and the current can continue to increase in case of a short for up to ten ms
in an arc chute circuit
breaker before it begins to decrease.
Another means known in the prior art to create a high power DC circuit breaker
is to use the
charging or discharging of a capacitor to momentarily reduce the voltage and
current to a level
that a fast acting AC-type switch can open the circuit. US patent 3,809,959
describes an
arrangement in which two AC-type switches, a resistor, a spark gap, and a
capacitor are
combined to give an effective DC circuit breaker that can work up to HVDC
voltage. This is
faster than an arc chute breaker, and is applicable up to HVDC voltage levels.
Later refinements
of this idea include pre-charging the capacitor to an opposite polarity
compared to the flowing
current to be interrupted.
US patent 3,534,226 describes a particular way to insert resistance and
capacitance into a DC
circuit, to open the circuit. The basic
concept of switching in resistors to reduce the current in a stepwise manner
so as to control the
magnitude of voltage transients during opening of a DC circuit is well
described in US patent
3,534,226, which envisions using many individual switches and resistors. The
method of patent
3,534,226 involves two different kinds of switches that must be opened in a
precise sequence:
first a low resistance mechanical switch (through which most of the power
flows when the circuit
breaker is closed) is opened. This is a conventional switch in which the
electrical contacts are
separated. Although a plasma arc may briefly form between the separating
electrodes of the low
resistance switch, this arc is quickly extinguished as the current is
commutated onto a parallel
path through the resistors, which are switched via fast acting switches. The
initial resistance in
the resistive network must be quite low for the initial arc to extinguish and
commutate to the
parallel resistive path. By the time the last fast acting switch is opened the
current has been
reduced to less than 10% of its maximum value (which implies that >99% of the
magnetic
energy has been dissipated), which allows the final capacitor snubber to be
relatively small and
economical compared to the size it would have to be if it had to absorb most
of the magnetic
energy stored in the circuit at the time of initial opening. US patent
3,534,226 forms the basis for
several subsequent patents, including US patents 3,611,031 and 3,660,723 (both
of which also
use a low-loss mechanical switch to commutate the current to a resistive
network based on fast
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electronic switches), and US patent 6,075,684 which uses a fast electronic
switch in place of the
commutating mechanical switch.
Summary
Commutating circuit breakers work by switching increasing resistance into a
circuit in a pre-
determined sequence until the current is sufficiently reduced so that a final
circuit opening can be
performed using a relatively small snubbing circuit such as a varistor or a
capacitor to absorb the
last bit of stored magnetic energy. The resistance needs to increase slowly
enough that the
inductive energy can be quenched without creating voltage spikes that are
above the maximum
voltage that the system can tolerate. In the commutating circuit breaker the
sequential switching
of resistance into the circuit is accomplished by the motion of a shuttle. As
the shuttle moves, the
resistance increases because of one of these three "Cases":
1. The resistance across a variable resistance shuttle increases as the
shuttle moves;
2. The resistance across the circuit breaker increases as a commutating
shuttle commutates
the current over a sequence of stationary resistors; or,
3. A commutating variable resistance shuttle is used to commutate over a
sequence of
stationary resistors, but part of the inserted resistance is on board the
shuttle.
In the commutating circuit breaker, the current flows between a first Pole A
through a first stator
electrode (stator electrode #1) to a first shuttle electrode on the shuttle;
this part of the current
path from Pole A of the circuit breaker on to the shuttle can be accomplished
by any workable
means, either via a commutating connection or a stable continuous connection;
the stable
continuous connection can be accomplished by a flexible wire, a telescoping
tube, or a slip ring,
for example. Once the current is on the shuttle, it flows to a second shuttle
electrode which
connects to one or a series of second stator electrodes to complete the
circuit to Pole B in such a
manner that electrical resistance increases as the shuttle moves.
In Case #1 above of a variable resistance shuttle, a variable resistance
portion of the shuttle
connects Pole A of the commutating circuit breaker to Pole B through
stationary stator
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electrodes. Motion of the shuttle could be linear or it could be rotary. The
points of electrical
connection between the stationary stator electrodes and the moving shuttle
electrodes include at
least one discrete stator electrode along which the shuttle slides during
operation of the circuit
breaker, through which the current is transferred. The other connection of the
shuttle to the
circuit can also be a sliding contact, but may also be a flexible wire
connection or a telescoping
tube that remains attached to the shuttle as it moves (on only one side of the
shuttle circuit).
In Case #2 above of a commutating shuttle, the resistors remain stationary,
and the commutating
shuttle delivers the power to different stator electrodes as it moves, which
connect the power
flow through a sequence of stationary resistors in such a way that resistance
increases repeatedly
during opening of the commutating circuit breaker. In this case, at least one
of the shuttle
electrodes on the commutating shuttle must be a discrete electrode which is
bounded by
insulation on at least one side, though in the simplest case the surrounding
insulation can be a
fluid or vacuum. Insofar as the mass of resistors required to open a circuit
depends on the total
energy that must be absorbed, and can be in the hundreds of kilograms for a
commutating circuit
breaker designed for a high power, high voltage line, it is preferable in high
power applications
not to accelerate the resistors as in Case #1, but to rely instead on a
commutating shuttle as in
Case #2 to commutate the power over a series of stationary resistors. The
commutating shuttle
can both weigh less and be conveniently composed of stronger materials than
the variable
resistance shuttle of Case #1. The lower mass of a commutating shuttle
compared to a variable
resistance shuttle implies less momentum needs to be transferred to accelerate
the shuttle, which
minimizes the jolt due to acceleration of the shuttle, and also reduces shock,
vibration, and
fatigue for the structure that holds the commutating circuit breaker.
A commutating variable resistance shuttle as in Case #3 above is useful for
snubbing arc currents
that might otherwise arise as the trailing edge of a commutating stator
electrode leaves its
electrical connection to a particular moving shuttle electrode. Making the
last part of a shuttle
electrode lower in conductivity compared to the first part can suppress arcing
while still
preserving a low resistance path through the first part of the shuttle
electrode to conduct
electricity efficiently when the circuit is closed, or to connect to the next
stator electrode in the
sequence of stator electrodes contacted by the moving shuttle electrode. This
same type of
resistivity gradient is also desirable on the trailing edges of the stator
electrodes where the stator
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electrodes link to external resistors. Making the trailing edge of an
electrode much more resistive
than a metal implies placing a portion of the resistance insertion of a
commutating circuit breaker
on board the shuttle in the trailing portion of the shuttle electrodes, or
within the trailing portion
of the stator electrodes, or both.
Insofar as the mass of the shuttle must be accelerated during operation of a
commutating circuit
breaker, it is desirable to minimize the mass of the shuttle, and therefore to
prefer that the trailing
edge resistive gradient is primarily limited to the stator electrodes because
adding said gradient
on the trailing edge of the shuttle electrodes increases shuttle mass, which
makes the launching
mechanism heavier, and the momentum transferred to accelerate the shuttle
greater. Grading the
resistivity on the trailing edges of both the shuttle electrodes and the
stator electrodes provides
the best possible arc suppression as a particular stator electrode loses
contact with a particular
shuttle electrode. The graded resistivity on the trailing edges of the
electrodes connecting
through Path A commutates the current to a different higher resistance
electrical Path B through
next neighbor electrodes that share a parallel connection with the separating
electrodes. Well
before the final separation of the electrodes that are in Path A, it is
desirable that the resistance
through Path A has increased to at least ten times the resistance through
parallel Path B, and this
may be accomplished by graded resistivity in the trailing edges of the
separating Path A
electrodes.
There must be at least one commutation zone in a commutating circuit breaker
wherein the
movement of the shuttle changes the electrical path through the circuit
breaker, so that the
current is shunted onto paths of increasing resistance during opening of the
circuit breaker. This
zone may commutate the power from a shuttle electrode through a series of
electrically separated
stationary stator electrodes onto paths having increasing resistance, or the
stator commutation
zone may comprise a stack of electrically series connected stationary stator
electrodes such that
the path length through the resistor stack increases, leading to increased
insetted resistance as the
commutating shuttle moves, or the movement of a variable resistance shuttle
may simply place
greater resistance between Pole A and the stator electrode that links to Pole
B.
Commutating circuit breakers enable high power DC power transmission and
distribution above
3,500 volts. Medium voltage DC (MVDC) power distribution at 2,000-36,000 volts
(2-36 kV)
would be both capital efficient and energy efficient compared to MVAC power
distribution, but
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has up until now been economically infeasible due in part to the high cost,
low efficiency, and/or
slow action of DC circuit breakers. MVDC enables microgrids with many
different generators,
power demands, and storage units tied into a single grid, whereas this is far
more difficult to do
with AC power.
MVDC allows efficient power distribution in industrial facilities (especially
factories and
processing plants that use a lot of variable speed motors); on board ships;
and at mine sites and
other isolated off-grid sites. The provision of DC power to many different
variable speed motor
drives saves both capital and energy costs compared to the normal mode of
operation in which
each motor controller for a variable speed drive must first produce DC power
from AC power
within the drive, then either drive a DC motor or convert to AC at a
controlled frequency to drive
the variable speed motor. Variable speed drives are less expensive and more
efficient if they are
powered by MVDC, which has previously been impractical due to the lack of
fast, efficient,
economical MVDC circuit breakers.
High voltage DC (HVDC) power transmission is the most efficient way to
transmit high power
levels, over one gigawatt (GW) for example, for distances greater than 1000
km. Unlike AC
power, DC power lines can readily go underground or undersea, and for these
reasons HVDC is
the most efficient and feasible way to transmit vast amounts of renewable
electricity from distant
wind farms and solar arrays to cities and economical remote energy storage
sites, as will be
needed to build an efficient energy economy based on renewable energy. Until
recently, HVDC
power transmission was strictly via "line commutated converters" (LCC) which
only work as
point-to-point power lines, connecting two or a few nodes of the AC grid, with
LCC converters
at each connection point to the AC grid. An LCC HVDC system does not need HVDC
circuit
breakers, because the current can be broken on the AC side. A newer type of
AC/DC converter,
"voltage source converters" (VSC) allows for the first time, true multi-
terminal HVDC; however
these multi-terminal HVDC systems require HVDC circuit breakers. Development
of multi-
terminal HVDC power lines and eventually, HVDC supergrids, has been inhibited
by the high
cost, low efficiency, and poor reliability of prior art HVDC circuit breakers.
The commutating circuit breaker is a breakthrough in terms of capital cost and
operating
characteristics (long life, low switching transients) that will enable DC
grids all the way from the
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modest voltage relevant for data centers (-400 volts) to MVDC for microgrids,
ships, factories
and processing plants, to HVDC for long distance power sharing.
Brief Description of the Drawings
Figure 1 shows a linear motion ballistic circuit breaker with variable
resistance shuttle having
step changes of resistivity in the shuttle; two stator electrodes are arranged
in a circularly
symmetrical manner to avoid a Lorentz force torque.
Figure 2 shows a container for a resistor that is sometimes called a "Can"
herein. This Can is
filled with a potted disc shaped resistor to form a resistor cell.
Figure 3 shows a stack of resistor cells as in Figure 2 that are series
connected in such a way as
to facilitate commutation by a moving shuttle that fits around the stack as in
Figure 4.
Figure 4: Linear motion commutating circuit breaker with a pipe-shaped
commutating shuttle
that fits around a stationary column of disc-shaped resistors.
Figure 5: Linear motion multistage commutating circuit breaker with four
commutation zones in
two stages.
Figure 6: Rotary Motion Multistage commutating circuit breaker with six
commutation zones.
Figure 7: Quenching of current and energy for an optimized 18-stage
commutating circuit
breaker of Figure 6 and Table 1.
Figure 8: Single stage commutating shuttle with electrical stress control
behind moving
electrode; circuit shown just prior to actuating motion of the commutating
shuttle.
Figure 9: Single stage commutating shuttle with electrical stress control
behind moving
electrode; circuit shown at the end of the motion of the corrunutating
shuttle.
Figure 10: Shuttle electrode/stator electrode interface with increased
resistivity trailing edges.
Figure 11: Commutating circuit breaker with flexible wire lead from Pole A to
the shuttle.
Figure 12: Commutating circuit breaker with shuttle having the shape of a rod,
tube, or wire.
Figure 13: Variable resistance shuttle with elastomer sleeve for voltage
stress control.
Figure 14: Elastomer sleeve for voltage stress control following stator
electrode.
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Figure 15: Hybrid commutating circuit breaker with parallel fast switch.
Figure 16: Pipe-shaped commutating shuttle.
Figure 17: Rotary commutating circuit breaker, with two commutation zones and
external
resistors.
Figure 18: Simplified rotary fast-acting commutating circuit breaker in which
the stator
electrodes and resistors make up wedge-shaped keystone sections of the stator
wall.
Figure 19 shows the drive and control mechanism for a large diameter rotary
commutating
circuit breaker designed for high voltage.
Figure 20: Rotary commutating circuit breaker mounted on base plate, with
torque driver,
bearings, fast actuated release, and arresting brake.
Figure 21: Semi logarithmic plot comparing current versus time in a worst case
dead short (no
voltage sag, no resistance) versus a circuit with internal resistance.
Description of Embodiments
In a commutating circuit breaker it is necessary to accelerate a shuttle. The
shuttle can be either
a variable resistance shuttle as in Case #1, or a commutating shuttle as in
Case #2, or a blending
of these cases in which part of the insertion of variable resistance occurs on
the shuttle, and part
via stationary resistors, as in Case #3.
Commutating circuit breakers for relatively low power circuits of less than
about one hundred
kilowatts (kW) can be made with a variable resistance shuttle (Case #1) that
connects between
two sets of contacts, as in Figure 1. This simplifies the design of the
circuit breaker mechanism
and wiring, but requires fabrication of a fairly complicated shuttle with
higher strength than is
normally required for resistors. Stronger springs or launching mechanisms are
required than for
commutating shuttle (Case #2) designs for the same power level because the
entire mass of
resistors must be accelerated. The variable resistance shuttle must withstand
high acceleration
loads, and must have a surface that slides on the stator electrodes without
excessive wear.
Figure 1 is a partially exploded view of a commutating circuit breaker 100 in
which the inserted
resistance is on board the shuttle. In Figure 1 a spring 101 is under tension,
pulling on the shuttle
through a non-conductive rod 103; this rod extends to the back end of the
shuttle and is
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connected to permanent magnet 119, the "shuttle magnet." Shuttle magnet 119 is
in contact with
stator magnet 121 when the circuit breaker is closed, prior to triggering the
breaker.
Electromagnet coil 123 is oriented to repel the shuttle magnet and to trigger
opening of the
circuit breaker by the spring when a DC current passes through the coil.
Figure 1 shows a
variable resistance portion 110 of the shuttle having step changes of
resistivity in the shuttle core
segment layers 111, 112, and 113. Stator 107 has electrodes 105 and 115 that
are arranged in a
circularly symmetrical manner to avoid torque on the shuttle by Lorentz
forces. The two circular
stator electrodes 105 and 115 are at a set distance apart, far enough to
prevent arcing during
opening of the circuit breaker.
During the time that a single resistivity layer is exiting stator electrode
115, the resistance
increases smoothly due to insertion of a greater length of resistive segments
between Pole A and
Pole B as the shuttle moves left. As each resistive material boundary passes
out of contact with
stator electrode 115, there is a discontinuity in the resistance versus time
curve, which in turn
generates change of slope in the resistance vs. time curve, but no step
changes in resistance.
The shuttle in Figure 1 is shown in its closed circuit position, but an
exploded view is applied to
the stator magnet 121 and the electromagnet trigger 123 to make it easier to
depict. In the closed
circuit, power flows from Pole A to the stator electrode 115, then through the
portion of the
shuttle 109 to stator electrode 105; 109 is composed of a good electrical
conductor with low
resistivity ¨104 ohm-meter. After the shuttle begins to move, the resistance
increases as the
boundary between material 109 and material 111 exits the left side of stator
electrode 115; this is
the first commutation. After this, resistance rises smoothly while the 111
material exits the left
side of the stator electrode 115, then with increasing slope at the time of
the second commutation
when the boundary between material 111 and 112 exits the left side of stator
contact 115, then
again resistance rises smoothly for a while until the boundary between 112 and
113 exits stator
electrode 115. The circuit is finally opened when insulating material 117
extends from the left
side of electrode 115. When the circuit is finally opened a snubber of some
kind, as is familiar to
one skilled in the prior art, such as a varistor or a capacitor absorbs the
last bit of inductively
stored energy. Total travel during opening of the circuit is distance 125. Not
shown is the means
to arrest the forward motion of the shuttle.
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Two commutating circuit breakers of the type shown in Figure 1 can be arranged
on a common
support so that the momentum effect of accelerating one shuttle to the left is
balanced by the
momentum effect of accelerating the second shuttle to the right.
Figure 2 shows a single resistor cell of a stacked resistor column (shown in
Figure 3) in which a
disc-shaped resistor 127 is potted into a Can that facilitates stacking and
commutation. Resistor
127 is desirably an alumina/carbon resistor, such as those available from HVR
Advanced Power
Components of Cheektowaga, NY, USA. These resistors can handle pulsed power
very well, as
is needed during operation of a Commutating Circuit Breaker, and are available
over three orders
of magnitude in resistivity. The physical properties of this class of resistor
(especially density
and strength) would not be desirable for a design such as Figure 1 in which
the resistor per se is
accelerated to accomplish the circuit opening, and the stator electrodes ride
on the surface of the
resistor. The Can of Figure 2 is comprised of a conductive lower portion 129,
an insulating upper
portion 133, and an insulating sleeve portion 135. Said Can provides a nesting
site for a disc-
shaped resistor 127 (or 137, 138, 139, 140, or 141, as shown in Figure 3)
which is attached by
conductive adhesive 131 to the bottom of the Can 129. The conductive adhesive
131 is desirably
a metal brazing compound, a solder, or a conductive adhesive that is lower in
volume resistivity
than the resistive material that comprises disc resistor 127. Said bottom of
the Can 129 is
metallic and has a metal lip that extends part way up along, but some distance
away from the
sides of the disc resistor 127, 137, 138, 139, 140, or 141. Above and adjacent
to the metal part of
the can 129, and extending to nearly the same outer radius as the metal part
of the can 129 is an
electrically insulating section 133. Between the common inner radius of the
upper lip of the
metallic portion of the Can 129 and the insulating upper portion of the Can
133 and the outer
radius of the disc resistor 127, 137, 138, 139, 140, or 141, an insulating
sleeve 135 is inserted;
this sleeve guarantees that current flows vertically from top to bottom of
each resistor, so that I2R
resistive heat generation is distributed over the entire volume of the disc
resistor such as 127.
The resistors (127, 137, 138, 139, 140, or 141 ) are potted into six Cans that
each contain
components 129, 131, 133, and 135 with a void-free insulating polymeric system
(as is
commonly practiced in potted transformers, for example) to form the final
potted resistor cell, as
in Figure 2.
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Six resistor cells similar to the one shown in Figure 2 are then stacked as in
Figure 3 to form the
base of a stator; the entire outside radial wall of each Can and the entire
stator formed by
stacking the Cans plus a special top cell is a concentric sliding surface that
is smooth. The
bottom resistor cell contains disc resistor 127; the next cell up contains
disc resistor 137; the next
cell contains disc resistor 138; the next cell contains disc resistor 139; the
next cell contains disc
resistor 140; the next cell contains disc resistor 141; the resistivity levels
of each disc resistor
increases in the order 127< 137< 138< 139 < 140 < 141. At the very top of the
stack of resistor
cells there is a special variable resistivity resistor cell that differs from
the other cells in that it is
comprised of a metal base plate 145, and on top of that is a graded
resistivity cermet element 143
that has resistivity at the bottom that is approximately equal to the
resistivity of disc resistor 141,
with resistivity that increases until it is an excellent insulator at the top,
with resistivity > 1012
ohm-meter (ohm-m). All these cells are mechanically and electrically bonded
together, so that
the metal base of each cell is attached to the entire upper surface of the
disc resistor below it in
the stack.
Figure 4 shows how the stack of resistor segments of Figure 3 is combined with
a commutating
shuttle 147, which in this case takes the form of a metallic sleeve that fits
over the column of
resistor segments, a conductive slip ring 149 that is connected to Pole A and
to commutating
shuttle 147, and a conductive base plate 151 that is connected to Pole B to
form a commutating
circuit breaker. Figure 4 shows an intermediate state that occurs during
opening of the
commutating circuit breaker of Figures 2, 3, 4; in this intermediate state
three resistor cells
containing disc resistors127, 137, and 138 are in a series-connected state
between the moving
commutating shuttle 147 and the base of the resistor stack 151. Note that the
metallic sleeve
commutating shuttle 147 is lower in mass than the column of resistor segments,
and therefore
takes less force 150 to accelerate than would be required to accelerate the
resistor stack at the
same rate. Current flows from Pole A to the movable commutating shuttle 147
through slip ring
149 (in this case the entire length of 147 is the shuttle electrode). The
connection of the
commutating shuttle to Pole A could also be via a wire in principle. When the
commutating
circuit breaker of Figure 4 is closed, current flows with low resistance from
Pole A through the
slip ring 149, then through the commutating shuttle 147to the metal portion
129 at the bottom of
the lowest resistor cell (which contains resistor disc 127), in the on state
case (not shown), the
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current mostly flows directly into the metallic base plate 151 and on to Pole
B, bypassing disc
resistor 127 (some small current does still flow through disc resistor 127).
When the circuit breaker of Figure 4 is triggered, the commutating shuttle 147
is rapidly
accelerated upwards, causing the current to pass first through resistor 127,
then 127 + 137, then
127 + 137 + 138 (this is the state illustrated in Figure 4), and so on. The
commutating shuttle
continues to move upwards until it has moved beyond the last metallic portion
of the resistor
stack column, 145 of Figure 3, after which the final small remaining current
is quenched by the
graded resistivity cell 143. At the bottom of the commutating shuttle 147 is a
semiconductive or
insulating sleeve 153 that fits closely around the resistor column to suppress
arcing when the
conductive portion of the commutating shuttle 147 pulls apart from one of the
metallic parts 129
found at the bottom of each resistor shell. Said sleeve 153 is desirably
semiconductive where it
touches the commutating shuttle 147, but has a resistivity gradient such that
it becomes a high
dielectric strength, high resistivity material (greater than 1012 ohm-meter)
at the opposite end
(lower end in Figure 4). Said sleeve 153 can be made of a variety of
materials; a particularly
desirable composition is a high strength fabric-reinforced elastomer with a
slippery inner surface.
Not shown in Figure 4 are the means by which the commutating shuttle is pulled
upwards, the
sensors to detect a fault condition, and the means of triggering the circuit
opening; these
functions can all be accomplished by means known in the prior art. Not shown,
but optionally
present on the inner surface of the commutating shuttle 147, are flexible
electrodes that facilitate
better electrical contact between the commutating shuttle 147 and the outer
surface of the stack
of resistors shown in Figure 3.
Figure 5 is a two-stage commutating circuit breaker that has a commutating
shuttle 158 that
moves a distance 205 to open the circuit. The commutating shuttle contains two
shuttle electrode
pairs comprised of 210, 211, and 212 (shuttle electrode pair #1), and 215,
216, 217 (shuttle
electrode pair #2), both of which are embedded in a structural insulator 159.
There are four
commutation zones 161 to 164: 161 and 162 together form the first stage 157;
163 and 164
together form the second stage 219 of this two-stage commutating circuit
breaker. In each of
these zones there are four stator electrodes; for example commutation zone 161
contains stator
electrodes 166, 168, 170, and 172; stator electrode 166 connects through low
resistance
conductor 174 to Pole A. Stator electrode 168 connects to Pole A through
resistor 176; stator
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electrode 170 connects to Pole A through resistors 178 and 176 in series;
stator electrode 172
connects to Pole A through resistors 180, 178, and 176 in series; and
similarly for the other
commutation zones. Commutation zone 162 contains stator electrodes 181, 183,
185, and 187.
Stator electrode 181 connects to stator electrode 189 through low resistance
conductor 182.
Stator electrode 183 connects to low resistance conductor 182 through resistor
184; stator
electrode 185 connects to low resistance conductor 182 through resistors 186
and 184 in series;
stator electrode 187 connects to low resistance conductor 182 through
resistors 188, 186, and 184
in series. Commutation zone 163 contains stator electrodes 189, 190, 192, and
194. Stator
electrode189 connects to stator electrode 181 through low resistance conductor
182; stator
electrode 190 connects to low resistance conductor 182 through resistor 191;
stator electrode 192
connects to low resistance conductor 182 through resistors 191 and 193 in
series; stator electrode
194 connects to low resistance conductor 182 through resistors 195, 193, and
191 in series.
Commutation zone 164 contains stator electrodes 196, 198, 200, and 202. Stator
electrode 196
connects to Pole B through low resistance conductor 197. Stator electrode 198
connects to Pole
B through resistor 199; stator electrode 200 connects to Pole B through
resistors 201 and 199 in
series; stator electrode 202 connects to Pole B through resistors 203, 201,
and 199 in series.
When the circuit is closed there is a low resistance path from Pole A to Pole
B through the
commutating circuit breaker in this way: Pole A connects through conductor 174
to stator
electrode 166 to shuttle electrode 211, which then connects through insulated
conductor 210 to
shuttle electrode 212, which then connects to stator electrode 181 and from
there through
conductor 182 to stator electrode 189, then to shuttle electrode 216, then
through insulated
conductor 215 to shuttle electrode 217, then to stator electrode 196, then
through conductor 197
to Pole B. The commutating shuttle in this case is essentially a rigid body
that maintains a set
geometric relationship between the four shuttle electrodes 211, 212, 216, and
217 as it moves to
the right to open the circuit. It is desirable to have the times at which the
four shuttle electrodes
lose contact with the four on state stator electrodes that correspond to a
closed circuit (166, 181,
189, and 196) not to be simultaneous, since simultaneous commutation in all
four sets of
electrodes will increase the magnitude of the switching transient. The
trailing edges of the four
shuttle electrodes 211, 212, 216, 217 can have their axial positions adjusted
to time the four first
commutations off the on state electrodes, during which electrical connection
is lost with stator
electrodes 166, 181, 189, 196; in fact, all the subsequent commutations can be
timed by also
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adjusting the spacing between second, third, and forth electrodes within each
commutation zone.
Said timing may be accomplished by adjusting both the spacing between the
shuttle electrodes
and the stator electrodes; or, a standard spacing can be adopted between the
shuttle electrodes,
with all the timing control being done by adjusting the trailing edge
positions of the stator
electrodes only. It is optimal to insert the twelve resistors at controlled
time intervals. After the
twelve resistive insertions implied by Figure 5, the current is low enough so
that the shuttle
electrodes can move beyond their last connection through resistors without
damaging arcs as the
then greatly diminished current is cut off. It is desirable to grade the
resistivity of the trailing
edges of the stator electrodes, especially the particular stator electrode
that does the final power
shutoff. In Figure 5, the final shutoff occurs when shuttle electrode 211
loses its connection to
stator electrode 172, which is the last electrode in Zone 1. (It is best to
define which of the four
final commutations [one in each Zone] is the one that opens the circuit, so
that the extra high
voltage insulation that will be needed can be deployed only in this particular
zone; this saves
cost.) Since stator electrode 172 is the one to open the circuit, it is highly
desirable to grade the
resistivity of the trailing edge of this electrode all the way from
semiconducting to high
resistivity to provide a soft final shutoff of the residual current still
flowing after the twelfth
commutation of the commutating circuit breaker of Figure 5.
A long multistage chain of commutating circuit breakers as in Figure 5 can be
used to break an
arbitrarily high voltage. In order to efficiently move a long commutation
shuttle such as this
implies, it is desirable to use multiple drives along the length of the
commutating shuttle, such as
multiple springs positioned to accelerate the shuttle between the commutating
zones, or multiple
linear motors acting between the commutating zones. A long multistage breaker
with embedded
permanent magnets can be driven by known electromagnetic means, for example
(however,
greater force can be exerted with springs or electromagnets than by coupling
to permanent
magnets). A combination of drive mechanisms can also be used to achieve
greater acceleration
than can be produced by one means alone. A variety of triggers and releases
can be deployed in
such a multistage linear breaker, as is discussed in more detail later.
Figure 6 represents a notional rotary multi-stage commutating circuit breaker
designed for one
pole of a medium to high voltage DC or AC power circuit breaker. In this case,
six commutation
zones are shown, 221-229 (comprising shuttle electrode 221; stator electrodes
222, 223, 224, and
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225; conductive lead 226; and resistors 227, 228, and 229); 231-239
(comprising shuttle
electrode 231; stator electrodes 232, 233, 234, and 235; conductive lead 236;
and resistors 237,
238, and 239); 241-249 (comprising shuttle electrode 241; stator electrodes
242, 243, 244, and
245; conductive lead 246; and resistors 247, 248, and 249); 251-259
(comprising shuttle
electrode 251; stator electrodes 252, 253, 254, and 255; conductive lead 256;
and resistors 257,
258, and 259); 261-269 (comprising shuttle electrode 261; stator electrodes
262, 263, 264, and
265; conductive lead 266; and resistors 267, 268, and 269); and 271-279
(comprising shuttle
electrode 271; stator electrodes 272, 273, 274, and 275; conductive lead 276;
and resistors 277,
278, and 279). These zones are arranged in pairs that comprise commutation
stages: the first
commutating zone (defined by 221-229 in Figure 6) is closest to Pole A, and is
linked via
insulated conductor 220 to the second commutating zone (defined by 231-239 in
Figure 6); the
first commutating zone and the second commutating zone together with insulated
conductor 220
form the first of three commutation stages in the commutating circuit breaker
of Figure 6. The
other two stages include components 240-259 and 260-279. A stage is defined as
a complete
circuit that moves power on to the commutating shuttle and then off of the
shuttle. In Figure 5
there are two stages, and in Figure 6 there are three stages.
The multistage rotary commutating circuit breaker of Figure 6 works in much
the same way as
the linear multistage commutating circuit breaker of Figure 5, except that
actuation is via rotation
of a cylindrical commutating rotor 280 rather than linear motion of a
commutating shuttle as in
Figure 5, and there are three stages rather than two as in Figure 5. (As used
herein, "commutating
rotor" is a special case of a "commutating shuttle;" a "shuttle electrode"
refers to any moving
electrode, whether it moves linearly as in Figure 5, or via rotation, as in
Figure 6.) The circuit
breaker of Figure 6 has six commutation zones, each of which works in the same
way as does
each of the four linear motion commutation zones of Figure 5. In this case,
the commutating
shuttle rotates about 18.2 degrees counterclockwise to open the circuit, then
a further 7.9 degrees
to a final open circuit position, so that the total rotation during actuation
of the rotary
commutating circuit breaker is 29.1 degrees (281). The rotor is composed of
strong, electrically
insulating materials such as a fiberglass reinforced polymer composite, an
engineering grade
thermoplastic compound, or a polymer-matrix syntactic foam, except for the
shuttle electrodes
221, 231, 241, 251, 261, and 271 and the insulated conductive paths shown with
heavy black
lines (220, 240, and 260) within the shuttle that connect pairs of shuttle
electrodes (such as 221
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and 231). The shaft is desirably metallic, but electrically insulated from the
conductors 220, 240,
and 260. The entire rotating part is surrounded by a stator 290 in which the
stator electrodes are
mounted. The resistors are preferably outside the stator to facilitate heat
removal after the circuit
breaker trips.
The view in Figure 6 is an end-on view of a commutating shuttle which has the
shape of a
cylinder. The length of the cylinder (perpendicular to the cross-section shown
in Figure 6) can be
adjusted to keep the normal full load amps per cm2 of electrode contact area
within design limits;
thus, depending on the current, the cylinder 280 can look like a disc or a
barrel. The
circumferential insulated distance between stator electrodes (for example 222,
223, 224, 225) can
be adjusted to deal with the voltage gradient at each commutation; in
principle, both the width of
each stator electrode and the distance between each next neighbor pair of
stator electrodes would
be adjusted to reach an optimum design. Neither the distances between stator
electrodes, nor the
width of the stator electrodes, nor the composition of different stator
electrodes needs to be the
same for any two stator electrodes. Also, multiple series-connected
commutating circuit breakers
such as that of Figure 6 can be mounted on a single shaft, to create more
commutation stages (6,
9, etc.). In this case, each of the switch contacts 221, 231, 241, 251, 261,
and 271; and their
mating contacts 222, etc. only span a fraction of the length of the drive
shaft separated by
intervening insulating sections.
In the particular design of Figure 6, the on-state stator electrodes 222, 232,
242, 252, 262, and
272 are desirably liquid metal electrodes; these are the only stator
electrodes which carry high
current in the on state. Liquid metal electrodes are about 104 times as
conductive as sliding solid
metal electrodes in terms of contact resistance. Liquid metal electrodes can
therefore also be
narrower than sliding solid contact electrodes, which is a major advantage for
the first few
commutation steps of a commutating circuit breaker. Let's consider a specific
case: in Figure 6
the liquid metal stator electrodes 222, 232, 242, 252, 262, and 272 can be one
tenth as wide as
the solid stator electrodes 223, 224, and 225 for example, and still have one
thousandth of the
contact resistance of the solid stator electrodes. As a particular example,
consider the case where
the commutating rotor of Figure 6 is a 31.5 cm diameter barrel-shaped
commutating shuttle
designed for 30 kV DC or AC power. Making one of the liquid metal stator
electrodes 222, 232,
242, 252, 262, and 272 one millimeter (mm) wide in the circumferential
direction means that it
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would be possible to achieve the first commutation by only rotating the
shuttle 280 by 0.36
degrees if the first stator electrode is aligned with the rotor electrode so
that there is only one mm
to move to cause the first commutation (for example). This first commutation
is very important
in any circuit breaker in which it is critical to control the maximum fault
current, since as soon as
the first resistance is inserted the fault current is controlled. Using narrow
liquid metal electrodes
is one way to speed up the first commutation by reducing the distance that
must be moved by the
commutating shuttle to get to the first commutation
A consideration when using liquid metal electrodes is to avoid oxidized solid
metal contacts to
connect with the liquid metal electrode. One way to avoid oxidation at the
shuttle electrode
surface that mates with the liquid metal electrode is to enclose the circuit
breaker in a sealed
oxygen free environment; in this case, conventional copper- or silver-based
shuttle electrodes
can be used with a liquid electrode, as long as the liquid metal electrode
does not react with
copper or silver. Another known method is to use a "noble metal" such as gold,
platinum, or
palladium in air. A particularly desirable solution is to use a molybdenum-
surfaced electrode,
since molybdenum does not oxidize in air below 600 Celsius; even though
molybdenum has low
conductivity for a metal (resistivity 85 times higher than copper), a thin
coating of molybdenum
on a substrate metallic electrode results in an oxide-free surface that
couples very well with
liquid metal electrodes, without the added resistance due to an oxide layer;
the resistance through
the molybdenum per se is negligible if it is only a mm or less thick on the
electrode, as may be
easily obtained by plasma spray or various PVD (physical vapor deposition)
processes.
Liquid metal electrodes typically comprise a sintered porous metal structural
component formed
by a powdered metallurgy processes that is wetted and flooded by a liquid
metal such as gallium
or a low melting gallium alloy. Sodium, sodium/potassium eutectic, and mercury
have also been
used in liquid metal electrodes, but are less desirable than gallium-based
based liquid metal
electrodes. Gallium, gallium alloys, sodium, or sodium/potassium eutectic will
oxidize, so such
electrodes must be protected within an oxygen-free container which may contain
gas, liquid, or
vacuum in addition to the solid movable parts of the rotary motion multi-stage
commutating
circuit breaker of Figure 6. The added cost of the gas-tight containment
structure in order to be
able to use gallium or sodium based liquid electrodes is well justified in the
case of high power
circuit breakers, such as that of Figure 6. If an oxygen-free environment must
be maintained for
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the liquid metal, then there is also no need for the sliding surfaces of the
non-liquid-metal
electrodes to be oxidation resistant materials in principle (the non-liquid-
metal electrodes include
all the shuttle electrodes and all but one of each commutation zone's stator
electrodes); in such a
design the sliding electrode surface could be based on an copper, nickel,
chromium or silver pure
metal or alloy, or a cermet composite containing one of these metals or an
alloy thereof, rather
than molybdenum. Even if an oxygen-free environment is provided in the final
commutating
circuit breaker however, an oxidation-resistant surface on the electrodes that
contact the liquid
metal electrodes in the on state may be important to make it convenient to
fabricate the device
without having to maintain an oxygen-free environment between the time that
the electrodes are
manufactured and the circuit breaker is fabricated.
The six commutation zones of Figure 6, each of which can shut off the power,
give this design a
high shut-off redundancy and reliability. As a particular example, consider
again the case where
the commutating rotor of Figure 6 is a 31.5 cm diameter barrel-shaped
commutating shuttle
designed for 30 kV DC or AC power. The barrel-shaped rotary commutator 280 in
this particular
example is 99 cm in circumference and contains 6 conductive shuttle electrodes
that are 1.25 cm
wide in the circumferential direction (occupying 4.55 degrees at the outer
radius of the
commutating rotor). The shuttle electrodes are wide enough to be touching two
stator electrodes
at all times except for the final commutation; all the shuttle electrodes are
embedded in an
insulating polymeric material. The commutating rotor as a whole has a smooth
outer surface to
slide against the stator and its electrodes. The greater the number of amps,
the longer the barrel
has to be to pass the current in this design. In the specific case of the zone
1 commutation in
Figure 6, the stator electrodes 223, 224, and 225 are metallic electrodes that
can be, for sake of
demonstration 1.0 cm wide, with 0.25 cm of an insulator between each, so that
the 1.25 cm wide
shuttle electrodes are in full contact with the next stator electrode at the
moment that contact is
lost with a given stator electrode. The first stator electrode 222 is only .25
cm wide, and is a
liquid metal electrode, followed by an insulating gap that is 0.25 cm wide
between stator
electrodes 222 and 223; this means that the commutating rotor only needs to
rotate 0.91 degrees
to the first commutation in zone 1. At the moment that electrode 222 loses
contact with shuttle
electrode 221, shuttle electrode 221 is in full contact with electrode 223;
and at the moment that
electrode 223 loses contact with shuttle electrode 221, said shuttle electrode
is in full contact
with electrode 224; and so on.
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The trailing edges of the conductive electrodes of Figure 6 may be graded in
terms of
composition and electrical resistivity to reduce the chance that an arc will
initiate at the time the
electrodes separate. This is true of all the designs of commutating circuit
breaker discussed in
this document, and the trailing edge resistivity gradient can be in only the
shuttle electrodes, only
in the stator electrodes, or in both the shuttle and stator electrodes. This
is discussed more
generally elsewhere; in the specific case of the Figure 6 commutating circuit
breaker a single
graded resistivity zone at the trailing edge of one of the stator electrodes
could easily absorb the
last bit of magnetic energy in the flowing current after the last commutation
of Table 1, or a
capacitor may be more economical to absorb this last bit of inductive energy.
The outermost surface of the shuttle electrodes is best made from a highly
conductive metal or
composite which is also wear resistant, and which does not oxidize,
recrystallize, or interdiffuse
with the facing on state stator electrodes during use. Oxidation can either be
prevented by
excluding oxygen, or by using an oxidation resistant metal such as gold,
platinum, or
molybdenum. Where oxygen is excluded, a particulate hard particle/soft metal
matrix composite
with good electrical conductivity, such as silver- or copper-impregnated
porous structures based
on sintered metals; for example chromium powder as in US patent 7,662,208, or
tungsten
powder, as in commercial electrodes from Mitsubishi Materials C.M.I Co. Ltd.
are suitable.
Aluminum/silicon carbide electrodes are also suitable in an oxygen-free
environment. Where
oxygen is not excluded, molybdenum is a favored contact surface for all the
non-liquid-metal
electrodes; molybdenum that is plasma sprayed onto aluminum/silicon carbide
electrodes is
especially favorable. Although a version of the commutating circuit breaker of
Figure 6 could be
made to operate in an air environment, it would not be possible in that case
to use any other
liquid metal electrodes other than mercury.
To achieve a target of losing 1.0 kW to on state losses at 2000 amps in the
closed circuit
condition, the total resistance of the path from Pole A to Pole B in Figure 6
would be at most
2.5E-4 ohms. This low a resistance is only feasible with liquid metal on state
electrode junctions.
Achieving lower resistance entails using a more massive rotor, which requires
more torque to
accelerate; there exists an optimum design basis on-state resistance target
that will be somewhat
different for each particular case; in some cases, higher heat production than
one kW may be
well justified in combination with fan or liquid cooling, which makes it
easier to make a working
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commutating circuit breaker without resorting to liquid metal electrodes for
the on state electrode
connections.
The spring or other driver used to cause the counterclockwise radial
acceleration of Figure 6 may
accelerate the rotor throughout the time of the commutations, or
alternatively, a very stiff spring
could impart an initial acceleration using up only a small part of the 18.2
degrees of radial
motion that the commutating rotor moves during commutation. In this scenario,
the commutating
rotor is in free flight during most of the time that the commutating circuit
breaker rotor is moving
and causing commutations.
By making a few simplifying assumptions, an optimized sequence for the
eighteen resistor cut-
ins that the 18 commutations of the commutating circuit breaker of Figure 6
enables, for a 300
kV commutating circuit breaker, can be modeled. Table 1 gives the calculated
target
commutation times and inserted resistances, based on these assumptions:
1. Assumed circuit inductance of 100 millihenries (realistic high side
estimate);
2. Maximum allowed current is 1 OkA at the first commutation;
3. Upper voltage limit of 500kV (1.67X normal voltage); which then decays
exponentially
to a lower voltage limit of 360kV (1.2X normal voltage) before the next
commutation.
Since one cannot pick where a circuit fault occurs it is not logical to take
the normal system
inductance as being a realistic estimate of system inductance in a fault; this
means that the
system inductance may not be available to slow the inrush of current in a
fault. This case allows
us to consider a realistic high inductance fault (100 millihenries); in this
case the inductively
stored magnetic energy that must be dissipated to open a faulted HVDC circuit
at 10kA is 5
million joules (5MJ). The previously mentioned carbon/alumina sintered
resistors from HVR
International can absorb 111 J/gram in routine service, which means that 45 kg
of HVR disc
resistors would be needed to absorb 5MJ of inductive energy as modeled in
Table 1. In order to
be able to absorb the energy of three repeated circuit openings based on the
above assumptions,
135 kg of HVR International pulse-rated resistors would be needed.
The first commutation of Table 1 inserts 50 ohms, which is based on limiting
the voltage and
current at the design basis maximum (500 kV and 10 kA); this first commutation
needs to occur
within 2.667 milliseconds (ms) in order to hold the fault current to no more
than 10 kA (starting
from normal full load of 2 kA at time zero). After the first insertion of 50
ohms, it takes 0.657 ms
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for the voltage to decay from 500kV to 360kV; this is the time of the second
commutation, after
which the resistance is 69.4 ohms, and it takes only .473 ms for the voltage
to decay from 500kV
to 360kV, and each subsequent resistance level applies for less elapsed time,
because at higher
resistance, the exponential decay of current is faster. Each step of this
repeated exponential decay
of current (i) occurs according to this equation:
(1) i(t) = Ie-(R"
Where I is the current when the resistance R (in ohms) is first inserted, and
L is the inductance
(0.10 Henry in this example), and t refers to time (in seconds) since
resistance R is first inserted.
Resistance R is repeatedly reset during the operation of the commutating
circuit breaker (as in
Table 1); this is a highly efficient way to absorb inductively stored magnetic
energy during
opening of a DC circuit with a lot of stored magnetic energy. By holding the
voltage 20% above
normal operating voltage during opening of the circuit breaker, we can
guarantee that any
batteries and/or high energy capacitors that may be on the circuit will not
discharge through the
fault during the time the circuit is being opened.
Table 1: Optimized Commutation Times & Resistance Steps for Figure 6 Breaker
commutation time, ms R (ohms) A time at R, ms amps
(inductive energy, joules)
#1 2.667 50.0 0.657 10000.0 5000000
#2 3.324 69.4 0.473 7200.0 2592000
#3 3.797 96.5 0.341 5184.0 1343693
#4 4.138 134.0 0.245 3732.5 696570
#5 4.383 186.1 0.177 2687.4 361102
#6 4.560 258.4 0.127 1934.9 187195
#7 4.687 358.9 0.092 1393.1 97042
#8 4.778 498.5 0.066 1003.1 50307
#9 4.844 692.3 0.029 722.2 26079
#10 4.873 961.6 0.034 520.0 13519
#11 4.907 1335.5 0.011 374.4 7008
#12 4.918 1854.9 0.018 269.6 3633
#13 4.936 2576.2 0.013 194.1 1883
#14 4.948 3578.1 0.009 139.7 976
#15 4.958 4969.5 0.009 100.6 506
#16 4.967 6902.1 0.005 72.4 262
#17 4.972 9586.3 0.003 52.2 136
#18 4.975 13314.3 0.002 37.6 71
final circuit open 4.978 > 1E8 27.0
37
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Eighteen resistance insertions occur during the opening of the circuit in
Table 1; the resultant
voltage, current, and inductive energy changes are as shown in Figure 7; the
first six
commutations can be timed precisely by adjusting the exact angles of rotation
at which each of
the first six separations of stator electrode and shuttle electrode occur, as
the trailing edge of a
shuttle electrode moves away from the trailing edge of a particular stator
electrode. This fine
timing adjustment capability for the first switching event in each of the six
commutation zones
can be determined down to the microsecond time scale by careful design of the
structure of the
rotating commutating shuttle 280 and the mating commutating stator 290;
however, after that the
needed minimum spacing between stator electrodes to maintain electrical
isolation creates
limitations on timing subsequent commutations in each commutating zone.
In a realistic opening of a rotary circuit breaker as per Figure 6, switching
will not be fast enough
to keep up with the pace of the last several commutations of Table 1, since
for the last few
resistance levels, the indicated time delay between switching is only a few
microseconds. If
switching rate is uniform and slower than indicated in Table 1 for the last
twelve commutations,
130 microseconds between each commutation after Commutation #6, then the final
shutoff
would occur at 3.453 ms after the first commutation rather than at 2.311 ms
after the first
commutation as indicated in Table 1. The range of voltage from 500kV to 360kV
is an unusually
narrow control range for voltage excursions during opening of a circuit
breaker (voltage
switching transients), which is enabled in this case by the eighteen small
commutation steps of
Table 1 that the design of Figure 6 allows. The final open circuit condition
occurs when one of
the shuttle electrodes slides past the last of that zone's sequence of stator
electrodes into its
highly insulating final resting zone. Although in the design of Figure 6 all
six shuttle electrodes
slide past the last of each zone's sequence of stator electrodes into a highly
insulating final
resting zone, only the first shuttle electrode to do so is part of the circuit-
opening sequence of
switched-in resistances; after the circuit is opened, the remaining final five
commutations that
occur in the other five zones are redundant final circuit openings. Note from
Table 1 and Figure
7 that the commutating circuit breaker with 18 commutations through resistors
reduces the stored
inductive energy from 5 million joules to just 37 joules at the time when the
circuit is opened; the
current is squeezed down from 10 kA to 27 amps via the 18 commutations. One
still needs to
deal with the last bit of inductive energy; this can be accomplished with a
small capacitor, or by
using a graded resistivity in the trailing edge of the stator electrode that
does the final circuit
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opening. In order to accomplish the commutations implied by Table 1, it will
be necessary to use
resistivity gradients in the trailing edges of all the electrodes, and to
surround the electrodes by
high dielectric strength fluids, as discussed in more detail subsequently.
Although Figure 6 shows the shuttle electrodes on the outer radius of the
commutating shuttle, it
is equally possible to put the shuttle electrodes on the flat ends of the
shuttle. Both designs have
advantages and disadvantages. The design of Figure 6 is analogous to a drum
brake, where the
brake pads have an analogous role to that of the stator electrodes, and the
drum is analogous to
the rotary commutating shuttle. The alternative design with the shuttle
electrodes on the ends of
the commutating shuttle is analogous to a disc brake.
It is easier to submerge the cylindrical commutating rotor of Figure 6 in an
arc suppressing fluid
compared to a linear movement commutating circuit breaker such as that of
Figure 5 because
rotation of a circularly symmetrical cylinder does not produce form drag,
whereas linear motion
in a fluid necessarily involves form drag, which can significantly inhibit
rapid motion of the
commutating shuttle in a liquid. The cylindrical design also enables a liquid
submerged system
with a very low volume of liquid compared to a linear actuated design.
Sparking can be highly
inhibited by fluid surrounding the separating electrodes, especially if the
fluid is held at high
pressure. Limiting the dielectric fluid to only a few cubic cm is feasible in
a cylindrical
commutating circuit breaker such as that of Figure 6. This means that high
dielectric strength
fluids such as perfluorocarbon fluids could be economically used. The major
advantage of using
high pressure lubricants in a commutating circuit breaker is that the standoff
distance between
neighboring stator electrodes can be reduced if the gap between the solid
dielectrics is flooded
with a very high dielectric strength high pressure fluid. This will allow more
compact
commutating circuit breakers. It has not been practiced commercially in the
prior art to operate
switchgear at high liquid pressure, but the unique shape of the rotary
commutating circuit
breaker of Figure 6 allows for a very small volume of high pressure liquid,
which is not
dangerous in terms of stored energy.
The needed separation distance between next neighbor commutating electrodes
depends mainly
on the voltage change that occurs during the commutation step as current
flowing through one
resistive path is shunted to the next path when the separation of the shuttle
electrode and stator
electrode occurs. The voltage difference between these two alternate paths
carrying the same
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current is a reasonable estimate of the actual voltage difference driving arc
formation as two
electrodes separate; this driving force to form an arc has little to do with
the medium surrounding
the electrodes (vacuum, gas, or liquid) but whether an arc actually does form
also depends on the
dielectric strength of the fluid surrounding the separating conductors. This
in turn depends on
such factors as the pressure and chemical composition of the fluid and the
dissolved gases
present in the fluid if it is a liquid. Particularly desirable fluids to
surround the separating shuttle
electrode and stator electrode include paraffinic hydrocarbons, including
mineral oil and
kerosene; vegetable oils; methyl esters of fatty acids; perfluorocarbon
fluids; and liquid or
gaseous sulfur hexafluoride (including gas mixtures), and a high vacuum.
Sulfur hexafluoride-
containing gas mixtures are well known in the prior art for their high
dielectric strength (for a
gas) and excellent arc quenching properties, but liquid phase sulfur
hexafluoride is not used
commercially at present as far as I know as an intentional liquid dielectric.
The low liquid
volume required in rotary design commutating circuit breakers such as that of
Figure 6 make it
feasible to use SF6 in the liquid state as a dielectric fluid.
The properties that influence whether an arc, a small spark, or no spark at
all will be struck at the
moment of separation of shuttle electrode and stator electrode include:
1. the current that is flowing at the moment of separation;
2. the resistivity profiles of the parting electrodes;
3. the dielectric strength of fluids surrounding the parting electrodes;
4. the availability of a parallel path for the current to take.
Each time a commutation occurs the total voltage across the circuit breaker is
redistributed over
the six commutation zones proportional to the fraction of the total resistance
from Pole A to Pole
B that applies to the given commutation zone. When a new, higher resistance is
switched into the
circuit, the largest proportion of the total voltage gradient will be across
the most recently
switched commutating zone with the highest resistance. In the design of Figure
6, configured as
a stand-alone circuit breaker for 300 kV as implied by Table 1, the first
commutation represents
such a large increase in resistance that effectively the entire 500kV could be
across the first
switched-in resistor, and voltage withstand must be suitably high in that
commutation zone.
It is desirable to create multistage commutating circuit breakers as in Figure
5 (linear motion)
and Figure 6 (rotary motion), especially for high voltage DC applications; the
multiple stages
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divide the voltage, thus allowing for lower voltage per stage. In order to
accomplish this,
commutating shuttles containing pairs of shuttle electrodes which are
connected to each other
electrically but are insulated from each other at the surface of the
commutating shuttle are
required. Said insulating material can comprise a polymer, an inorganic glass,
a ceramic, a
cementitious material, or a composite of two or more of these components.
Specific examples of
insulators that may be used to insulate around the shuttle electrodes include:
1. fiber-reinforced composites based on a matrix phase curing polymer (such as
fiberglass-
epoxy, polyaramid-epoxy, boron fiber-epoxy, fiberglass-polyester, etcetera);
2. engineering-grade moldable plastics (defined as polymers with tensile
modulus > 2.5 GPa
and tensile strength > 40MPa, which may be unreinforced polymers; or polymers
reinforced by non-conductive reinforcing fillers;
3. cement composites, including fiber-reinforced and polymer latex toughened
cement
composites;
4. plasma sprayed or flame-sprayed coatings on metals;
5. polymeric syntactic foam (low density and high compressive and shear
strength);
6. nanocomposites.
Each shuttle electrode aligns with several different stator electrodes as the
shuttle moves, and in
most cases each shuttle electrode is also connected to a second shuttle
electrode at a different
location on the commutating shuttle, such that the two shuttle electrodes are
insulated from each
other on the surface plane.
The shuttle electrodes of a multi-zone commutating shuttle occupy less than
half of the total
surface area of the commutating shuttle, and in most cases occupy less than
10% of the surface
area of the commutating shuttle. The commutating shuttle can be fabricated
from previously
formed metallic and insulative components; or, the commutating shuttle can be
obtained by
overmolding an insulator onto a metallic core. Overmolding can be accomplished
via reaction
injection molding (RIM) of fast-polymerizing systems, by casting of slow-
polymerizing systems,
or by thermoplastic injection molding, for example.
Figures 8 and 9 depict a single stage commutating circuit breaker with
commutating shuttle 310
(which includes a highly conductive shuttle electrode 335, a semiconductive
transition plug 312,
an insulating plug 311, and an insulating sleeve 347 that surrounds part of a
highly conductive
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connecting rod 337). Connecting rod 337 attaches the shuttle electrode 335 to
Pole B through a
conductive slip ring 345 and a wire lead 346. Shuttle electrode 335 connects
the various stator
electrodes 321, 322, 323, 324 to Pole B as the shuttle electrode 335 moves to
the right. The stator
electrodes are connected through paths of varying resistance to Pole A of the
commutating
circuit breaker; in the on state (Figure 8), stator electrode 321 connects
through low resistance
lead wire 331 to Pole A; as the commutating shuttle moves to the right, stator
electrode 322
connects shuttle electrode 335 through resistor 332; next, the connection is
through stator
electrode 323 through resistors 333 and 332 to Pole A; then the connection is
through stator
electrode 324 through resistors 334, 333, and 332 in series. The commutating
shuttle 310 is
actuated by pressure P (301) behind the commutating shuttle insulating plug
311, which causes
the commutating shuttle to move from the closed (on) state shown in Figure 8
to the open (off)
state shown in Figure 9. Insulating plug 311 must be long enough to lie over
all the stator
electrodes (321, 322, 323, 324) at the end of travel of the commutating
shuttle, and to overlap
with insulating layer 340, as in Figure 9, in the fully open state to create a
total resistance
between Pole A to Pole B greater than 108 ohms in the fully open state.
Figures 8 and 9 depict a simplified commutating circuit breaker with just one
commutation zone;
these simplified depictions of a single commutation zone with only three
resistance insertions
prior to opening the circuit make it easier to describe and discuss certain
aspects of commutating
circuit breakers. The commutating circuit breaker of Figures 8 and 9 has only
5 primary
resistance levels. Power is linked from Pole B through slip ring 345 to the
shuttle electrode 335,
and from there through a series of different stator electrodes connected to
increasing resistances
given approximately by:
1. Resistance Level One is shown in Figure 8: current flows with minimal
resistance
through the circuit breaker.
2. Resistance Level Two: current flows primarily through stator electrode 322
and then
through resistance 332 to the opposite Pole A of the circuit breaker.
3. Resistance Level Three: current flows primarily through stator electrode
323 and then
through resistances 332 + 333 to the opposite Pole A of the circuit breaker.
4. Resistance Level Four: current flows primarily through stator electrode 324
and then
through resistances 332 + 333 + 334 to the opposite Pole A of the circuit
breaker.
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5. Resistance Level Five is the open circuit condition shown in Figure 9 in
which total
resistance > 108 ohms (see Figure 9).
Actuation of the circuit breaker begins with the commutating shuttle 310
(composed of
components 311, 312, 335, 337, and 347) in the closed circuit state of Figure
8; the resistance
through the commutating circuit breaker in the closed circuit case is also
known as the "on-state
resistance" of the circuit breaker. The on-state resistance of the circuit
breaker of Figure 8 is
actually comprised of two component resistances R1 and R2 through parallel
circuits:
= RI is resistance of slip ring 345 + lead resistances 346 + 337 + contact
resistance
between shuttle electrode 335 and stator electrode 321 + lead wire resistance
331
= R2 is resistance of slip ring 345 + lead resistances 346 + 337 + contact
resistance
between shuttle electrode 335 and stator electrode 322 + resistance 332;
the total on state resistance is then given by:
R1xR2
(2) Rtotal = -
R1+R2
Thus, in general, when the shuttle electrode 335 is touching two stator
electrodes, the actual
resistance should be calculated as a parallel path resistance. In the on-state
closed circuit
condition, R2>> R1 (because R2 includes resistance 332, the first in a series
of inserted
resistances); so most of the current goes through the low resistance path R1,
and the total
resistance Rtotal is only a little less than the resistance through this path
alone. To make this
concrete, consider the case of a normal voltage of 1200 volts, with normal
full load of 1200
amps, and a design basis maximum heat loss in the on state due to ohmic losses
(I2R) of 100
watts; this requires that Rtotal in the closed circuit case (on state) can be
no more than 69 micro-
ohms. The first inserted resistance would be 0.40 ohms (based on maximum
design current in a
fault = 6000 amps, maximum voltage = 2400 volts), so equation 2 implies that
the resistance of
the parallel circuit of equation 2 would only be .017% lower than the simple
connection through
only one resistive path (R1). Note though that in subsequent commutations, for
example when
there are parallel paths available through stator electrodes 323 and 324, the
current is more
evenly split between the parallel paths, though even in this case the major
portion of the current
will flow through the less resistive path through electrode 323.
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During commutation, the contact area between shuttle electrode 335 and stator
electrode 331
goes to zero, and the resistance through R1 increases until is surpasses R2,
just before
commutation [because contact resistance scales with 1/(contact area)]. By
grading the resistivity
of the trailing edges of shuttle electrode 335 and stator electrode 331, the
desired commutation
can be forced to occur well before the two electrodes lose contact with each
other. In this case, a
semiconductive trailing portion of shuttle electrode 335 is provided by
transition plug 312.
As the commutating shuttle 310 moves to the right from the initial position of
Figure 8, there will
also be an electric current path through transition plug 312 to a sequence of
stator electrodes
(321, 322, 323, and 324). This means that at some points during the opening of
the circuit
breaker there will be electrical paths through three different stator
electrodes, with the leftmost
connections being through the semiconductive transition plug 312. When shuttle
electrode 335
leaves contact with stator electrode 321, there is a sudden increase in
resistance through 321 and
331 as current through this path must then pass through the transition plug
312 after the metal
electrodes 335 and 321 separate, which quickly commutates the current to the
path through R2,
but much more softly than if the trailing (left) edge of shuttle electrode 335
would abut an
insulator such as 311 rather than semiconducting transition plug 312.
A consideration during this commutation is that current through the
semiconducting transition
plug 312 must not cause melting or damage to the material used to create
semiconducting
transition plug 312. This can be avoided by making the resistivity of
transition plug 312 high
enough so that only a minor portion of the current flows through transition
plug 312 in every
commutation except the last one. At the end of the motion of commutating
shuttle 310,
semiconducting transition plug 312 performs the final quench of the last of
the inductive energy.
At the final commutation, as shuttle electrode 335 moves to the right of
stator electrode 324, the
only electrical connection remaining between Pole A and Pole B goes through
the
semiconducting transition plug 312. Because of the graded resistivity in
transition plug 312, a
soft shut off can be provided if current and voltage is low enough to not
damage the
semiconducting material that makes up transition plug 312 during the shut off.
At equilibrium in the commutating circuit breaker of Figures 8 and 9 (which
can only occur
when the shuttle electrode 310 is stationary), the current is partitioned
between all parallel-
connected resistive paths in inverse proportionality to the path resistance.
During a commutation
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a true equilibrium does not actually pertain, but it is nonetheless useful to
consider a pseudo
equilibrium condition which is evaluated moment by moment during opening of
the
commutating circuit breaker. In general, electrical equilibration is fast
compared to mechanical
motion of the commutating shuttle, or resistive heating of conductive shuttle
components, so this
pseudo- equilibrium condition is at least reasonable. It is desirable to
minimize the inductance of
the resistive paths shown in Figures 8 and 9, since each pathway will store an
amount of energy
Lpath.I2 when the current is flowing which must be dissipated in order to
commutate the current
to a different path. In this case, Lpath refers just to the inductance of the
current path from the
point where the current turns from another alternative path to go through the
given path, such as
L331, which is the inductance from stator electrode 321 through connector 331
to Pole A, or
L332, which is the inductance from stator electrode 322 through resistor 332
and its lead wires to
Pole A. It is thus desirable in particular that resistors 332, 333, and 334
have relatively low
inductance, as will be familiar to a person skilled in the art of electrical
engineering.
Stepping through the actuation process for the device of Figures 8 and 9:
pressure 301 creates
force 300 by acting on the surface area of insulator 311; the force 300 moves
the shuttle to the
right inside the barrel 302, for a total distance 305; the electrical
resistance increases in stages:
1. prior to the first commutation the resistance is the parallel path
resistance in which R1 =
331 and R2 = 332 as defined by equation 2 above including contact resistances
between
stator electrode 321 and shuttle electrode 335 and between stator electrode
322 and
shuttle electrode 335;
2. after the contact between shuttle electrode 321 to 335 is lost, the
resistance is nearly R2
for a time, but slightly reduced by the parallel path through the
semiconducting plug 312
to electrode 321;
3. next there is a period in which the resistance corresponds to a parallel
path between R2 =
332 and R3 = 333;
4. after the contact between shuttle electrode 322 to 335 is lost, the
resistance is nearly R3 =
333 for a time (and so on through the sequence of resistive connections).
As described previously, the application of equation 2 to calculating the
actual resistance through
parallel paths as described above only slightly modifies the resistance steps
defined at the
beginning of the discussion of Figures 8 and 9. The designation of the two
poles in Figures 8 and
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9 as Pole A and Pole B may equally well be reversed; the polarity through a
commutating circuit
breaker may be reversed due to the arbitrary nature of the poles. For any of
the figures shown,
Pole A can be exchanged with Pole B and the commutating circuit breaker will
still work.
Depending on which pole is live after the commutating circuit breaker has
opened the circuit,
there will be different portions of the commutating circuit breaker that are
de-energized in the
case of a one directional power flow when the circuit is opened. If the power
source is on the A
side of the breaker of Figures 8 and 9 then when the circuit breaker is open
as in Figure 9, shuttle
electrode 335 and the slip ring 345 are de-energized (which facilitates
maintenance of the slip
ring 345). If on the other hand the power source is on the B side of the
breaker of Figures 8 and
9, then when the circuit breaker is open as in Figure 9, the stator electrodes
321-324 will be de-
energized (which facilitates maintenance of the stator electrodes 321-324).
Three particularly desirable kinds of material for dielectric insulating plug
311 are:
1. Rigid syntactic foam is especially desirable for insulating plug 311, due
to its high
strength to density ratio, in terms of both compressive strength and shear
strength;
2. A hollow insulating tube that is quite strong and rigid, and capped with a
strong end at
the boundary with transition plug 312 could also work as insulating plug 311.
3. A highly insulating elastomeric plug which is compressed when pressure is
applied to
drive the commutating shuttle forward may also be used for insulating plug
311; in the
case where elastomeric plugs are employed for insulating plug 311 or
semiconductive
transition plug 312, the interface between these plugs and the wall 302 should
be well
lubricated, and the inner surface of the tube 302 should be quite smooth and
have low
friction with the elastomer plugs.
Elastomers are desirable for at least a portion of transition plug 312, both
because of the
convenience of preparing chemically similar elastomer layers with controlled
resistivity, and
because compression of an elastomer layer such as transition plug 312 results
in a pressure
against the wall which facilitates tight contact with the stator barrel 302,
which inhibits arcing
between the plug 312 and the tube wall 302. The relative convenience of
creating a stack of
layers of elastomer compounds which are mutually cure compatible, mechanically
similar and all
with good sliding properties makes it fairly inexpensive to process, mold and
fabricate cured
elastomer plugs such as may be used in transition plug 312 with graded
resistivity from 10-2 to
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1012 ohm-m; it is much easier than creating all those layers in a plastic, for
example. Two
compatible elastomer masterbatches can be used to create the graded resistance
portion of
transition plug 312. This elastomeric portion of the transition plug may be
bonded to a more
conductive material, such as amorphous carbon or a sintered alnico layer for
example, to cover
the range of resistivity from 104 to 10-2 ohm-m, which may be desirable at the
leading edge of
transition plug 312, where it abuts against shuttle electrode 335. It is a
conventional, known
method to blend two elastomer masterbatches in various ratios to get
elastomers ranging from
being good insulators to being semiconductive with resistivity as low as 10-2
ohm-meter. It is
difficult to create intimate electrical contact between two separately molded
semiconductive
thermoplastic polymer discs, or between a thermoplastic, semiconductive
polymer and a metal or
ceramic surface, but the high compliance of elastomers facilitates better
electrical connection to a
surface, as long as the elastomer/metal interface is under pressure.
It is helpful to have a lubricant available to fill the surface voids that
always are present in sliding
friction. This interfacial lubricating layer between the shuttle and the
stator barrel 302 can be
thinner if the mating surfaces of the shuttle and the stator are smooth, and
match each other's
shape. Insofar as the surfaces of the shuttle and the rotor are not perfectly
smooth, the boundary
layer can also be thinner if the stator is somewhat flexible and is pressed
against the rotor.
A useful design feature of a commutating shuttle or a variable resistance
shuttle is to use a
polytetrafluoroethylene (PTFE) coated elastomer on some of the sliding
surfaces between the
shuttle and the stator such as on the outside of an elastomer cylinder like
311. Pure or formulated
PTFE can be sintered and then skive cut to create a PTFE film which can then
be used to create a
sleeve. PTFE and/or PTFE compounds can also be ram-extruded to form a thin-
walled tube that
can then be cut in lengths to use as a sleeve. Such a sleeve may then be
adhered to an elastomer
by first chemically etching it, for example with FluoroEtch available from
Acton Technologies,
Inc., and then co-molding it with a curing elastomer. It is however not nearly
as easy to vary the
resistance level of a PTFE layer as is the case for ordinary elastomers, so
PTFE coating of
elastomer surfaces is more desirable in the arc suppressing insulative sleeve
of Figure 4 (153) or
in purely insulating segments, such as 311 of Figures 8 and 9, rather than for
the semiconductive
components such as transition plug 312 of Figures 8 and 9.
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Figure 10 shows diagrammatically a sliding connection between two stator
electrodes and one
moving shuttle electrode; 355, 370, and 371 are highly conductive metallic
electrodes, while
360, 375, and 376 are semiconductive electrodes that are functionally similar
to 312 of Figures 9
and 9. Components 375 and 370 together form the ith stator electrode, and 371,
376 together form
the jth stator electrode, with stator insulator 380 between and surrounding
them; the jth stator
electrode connects through resistance 372, while the jth stator electrode
connects through
resistance 373, which is higher resistance than 372. A sliding shuttle
electrode (composed of the
two layers 355 and 360) is electrically connected to both the and the jth
stator electrode at the
moment shown in Figure 10. The shuttle electrodes 355 and 360 are surrounded
by highly
insulating regions of the shuttle 365. The shuttle electrode slides to the
left (indicated by 350)
below the stator electrodes and the trailing edge of the highly conductive
portion of the shuttle
electrode 355 is about to lose electrical connection to the highly conductive
first portion of the ith
stator electrode 370. One can see that this event will not completely open the
circuit connection
through the ith stator electrode through resistor 372, since the circuit is
still open through the
semiconductive electrode portions 360 and 375. By the time the final opening
of the circuit
through resistor 372 occurs, when the two semiconductive electrodes 360 and
375 separate, the
current flowing through B1 will have been reduced to less than one ampere.
Figure 10 illustrates another case for how electrical smoothing layers may be
implemented on the
trailing edges of electrodes, showing the case where electrical smoothing
elements (360, 375,
and 376) are connected to the trailing edges of both a shuttle electrode 355
and two stator
electrodes (370, 371). Here is a partial list of materials that may be used to
modify the resistivity
of electrodes as is useful in this invention:
I. Cold sprayed silver (resistivity ¨ 1.5 x104 ohm-meter), or other low
resistivity metal or
composite;
2. Nichrome alloys (resistivity ¨ 1.5 x10-6 ohm-meter) or another high
resistivity metallic
alloy or composite;
3. Cermet resistors (resistivity ¨ 10-6 to 10-3 ohm-meter) or another high
resistivity metallic
alloy or composite;
4. Alnico alloy #8 (resistivity ¨ 4.7 x l0-3 ohm-meter);
5. Quasicrystalline alloys (resistivity ¨ 10-4 to 100 ohm-meter)
6. Amorphous carbon (resistivity ¨ le to 10-2 ohm-meter);
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7. Conductive filled elastomer layers(resistivity ¨ 10-2 to 1012 ohm-meter);
These materials or a subset thereof can be deployed in the trailing edges of
metallic electrodes,
or in semiconductive components such as 153, 312, 360, 375, and 376. It is
possible to use
however many resistivity steps are needed.
The variable resistivity layer 360 is part of the moving shuttle, and so needs
to be stronger than
the stationary graded resistivity layers 375 and 376 at the trailing edges of
the stator electrodes
370 and 371. Appropriate materials for the shuttle electrode graded
resistivity feature 360
include cermets, quasicrystalline metal alloys, or highly loaded, stiff,
slippery polymers, whereas
transition plugs 375 and 376 can be made of weaker materials. It is also
desirable to keep the
stiffness and wear rate of all the layers that are engaged in frictional
relative motion in a
commutating circuit breaker approximately equal (for long device life).
A particular stator electrode is relevant to minimizing on-state heat
generation due to ohmic
losses only if a major portion of the on-state current flows through that
particular stator electrode
when the circuit is fully closed and the shuttle is stationary in the on state
(such as electrode 321
in Figure 8). The stator electrodes that carry the main current in the closed
circuit on state such
as 321 should be highly conductive (like copper or silver, or a liquid metal
electrode as discussed
previously), but the other stator electrodes such as 322, 323, 324 can be made
of a variety of
metals and/or cermets, chosen more for friction, wear, cost, and corrosion
resistance properties
rather than especially low resistivity. Nickel and/or nickel alloys are
particularly useful electrode
materials, for stator electrodes that only carry current for a short time.
Figure 11 shows the case where electric power is delivered to the shuttle of a
commutating
circuit breaker by a flexible wire 417 from Pole A. In this case, a
commutating shuttle design
with sharp conductor/insulator boundaries is depicted, but variable resistance
electrodes as in
Figures 8, 9, and 10 can also be used with a tethered wire attachment
mechanism as in Figure 11.
The connecting wire 417 must have high strength and very good fatigue
resistance. Total
movement of shuttle electrode 425 to the right is such that at the end of its
travel 445 the
electrode is surrounded by a high dielectric strength, high resistivity tube
430. A shock absorbing
insulating element 427 is at the end of the travel of the front (right hand)
face of electrode 425. In
the closed state, which is depicted in Figure 11, nearly all the current from
shuttle electrode 425
flows through stator electrode 431 and then through low resistance current
path 440 to a second
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terminal B of the circuit breaker. As the shuttle electrode 425 moves to
right; the current is
sequentially diverted through stator electrodes 432, 433, and 434 and the
respective resistor
sequence; at the first commutation resistance increases from 440 to 441, then
to 441 + 442, then
to 441 + 442 + 443, before the current is quenched in a small spark or by
charging a small
capacitor (not shown) as shuttle electrode 425 passes beyond the edge of
stator electrode 434.
The actuator of motion 400 could be any suitable fast acting device; the
thrust delivered by the
actuator passes through a metal shaft 405 to an electrical isolation coupling
410, and from there
via a non-conductive shaft 413 to the coupling 415 which links the metal shaft
420 to Pole A of
the circuit breaker via the wire lead 417. Shaft 420 is surrounded by an
insulating sleeve 423 that
aligns and supports the shaft within the non-conductive stator barrel 430,
though which the stator
electrodes 431, 432, 433, and 434 are installed.
Figure 12 shows a variant on the simple commutating circuit breaker concept
shown in Figure 4.
A cylindrical shaped stack of hollow disc resistors 460 with metal washers 451
between each
pair of next neighbor disc resistors (such as 450) is bonded together by some
suitable means such
as conductive adhesive, soldering, or brazing. This is simpler and less
expensive to implement
than the disc resistor stack of Figure 3, based on a metal Can to hold each
disc resistor as shown
in Figure 2. The metal washers 451 are very simple examples of stator
electrodes, and preferably
have a slightly smaller hole through them than the hole 455 through the disc
resistors themselves
(such as 450), so that the washers protrude into the central cavity through
the resistors; this
protects the inner surfaces of the disc resistors from damage via direct
contact with the moving
shuttle electrode 465, which in this case is simply a metal rod or tube that
extends clear through
the stack of resistors 460. At the bottom end of the shuttle electrode is an
optional end 466 of the
commutating shuttle 465 which may function as an electrical stress control
device with a similar
function to 312 in Figures 8 and 9, but which may also have additional
functionality as described
below, by providing a gripping surface to hold back the rod 465 in the on
(closed circuit) state. In
the closed circuit state, electrical connection to Pole A is made by low
resistance stator electrode
490 which can be a high conductivity metal electrode or a liquid metal
electrode that mates with
the end of commutating shuttle 465. There is a parallel path from Pole A to
the bottom of the
stack of resistors 485. Connection from Pole B to the commutating shuttle 465
can be made
through electrical slip ring 470, or by other means as described below. The
upper end of the
commutating shuttle 475 is a feature for connecting to a force 480 that pulls
the commutating
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shuttle out of the disc resistor stack 460 to open the circuit. Although
Figure 12 shows all the
disc resistors as having the same outside diameter, that is not necessarily
the case; in particular,
because the first disc resistors inserted into the circuit absorb far more
inductive energy than
subsequent resistors. It is desirable that the lowest disc resistor in Figure
12 (this is the first one
inserted into the circuit) should have the greatest mass and therefore the
largest outside diameter.
It is important that the metal discs such as 451cover the entire face of the
resistors to which they
are attached, so that the current can flow evenly through the entire volume of
each disc resistor.
The circuit breaker of Figure 12 has several unique features. It uses the
simplest possible
commutating shuttle, a metal rod or tube. The maximum force 480 that can be
applied to the rod
or tube depends on the strength of the material, and the cross-sectional area
of the rod or tube
wall. If all the force on the commutating shuttle originates from
acceleration, then the maximum
acceleration that is possible for any given material is strictly a function of
the strength/density
ratio of the material forming the commutating shuttle, and the length of the
commutating shuttle.
If a is the tensile yield strength of a material in pascals, D is its density
in kg/m3, and L is the
commutating shuttle length in meters, then the maximum acceleration in
meters/second2 Amaõ
that can be applied to a commutating shuttle like 465 is given by:
(3) Amax = a/LD
Results from this equation appear in Table 2 for a 2 meter long column of
metal pulled from one
end as in Figure 12; maximum feasible acceleration varies from less than 1000
m/s2 for sodium
to 114,000 m/s2 for aluminum matrix alumina-fiber wire. Table 2 also shows the
mass of various
materials at 20 C that are needed to create a 2 meter long 25 micro-ohm
column of material; at
this loss level the 2 meter long notional commutating shuttle would transmit
2000 amps with 100
watts of I2R waste heat production. (Waste heat scales linearly with conductor
mass, one tenth as
much mass conductor means ten times as much heat generation, for example.) The
mass of metal
required to create a 25 micro-ohm column of material varies from 3.7 kg of
sodium up to 618 kg
for the strongest alloy shown, titanium beta-C alloy (which enables maximum
acceleration
among the metals of Table 2). Table 2 also contains data on additional metals
that are discussed
in different parts of this document in reference to electrode surfaces or
resistivity grading on the
trailing edges of electrodes, for example.
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The best overall solution for a commutating shuttle 465 as in Figure 12
depends on the relative
cost for conductive material versus mechanical structure (including springs
and triggers and the
structural supports that maintain 465 in a stressed state, or apply stress to
it), and critically, on
the needed acceleration. The structural cost scales with the mass of conductor
that must be
accelerated times the acceleration. Acceleration determines time to the
critical first commutation,
so there is a good reason to push towards high acceleration in order to
minimize the time to first
commutation, if and where that is important (it is more important to get to
the first commutation
very fast if the system inductance in a fault is low than if the system
inductance in a fault is
high). Simply pulling a conductive tube so fast that one comes to the
engineering limit for
maximum tensile strength of the material (see Table 2 "maximum acceleration"
column) is the
fastest theoretical way to accelerate a linear motion commutating shuttle.
Table 2: Data Related to Accelerating a Conductor as in Figure 4 and Figure 12
Dens(ty, tensile yield maximum resistivity kg to
movement Nure of max force,
Conductor kg/mA3 strength (Pa) acceleration ohm-m
pass 2kA 4 ms (cm) Merit M pascals
sodium 971 1.00E+06 5.15E+02 4.76E-08 3.7
0.41 0.047 1.905E+03
calcium 1550 /./1E+07 3.56E+03 3.36E-08 4.2
2.85 0.456 1.485E+04
magnesium 1738 2.00E+07 5.75E+03 4.39E-08 6.1
4.60 0.564 3.512E+04
Magnesium AM60A,I3 1800 1.30E+08 3.61E+04 1.20E-07 17
28.89 1.294 6.240E+05
Magnesium A291 C,E T6 temper 1800 1.45E+08 4.03E+04 1.51E-07 22
32.22 1.147 8.758E+05
aluminum 2700 5.01E+07 9.28E+03 2.82E-08 6.1
7.42 1.415 5.651E+04
6061 aluminium alloy, T6 temper 2700 2.21E+08 4.09E+04 3.99E-
08 8.6 32.74 4.411 3.527E+05
Aluminum matrix alumina-fiber wire (3M ACCR) 3294 7.50E+08
1.14E+05 7.62E-08 20.1 91.07 6.424 2.286E+06
AlSiC-9 (CPS Technologies) 3000 4.88E+08 8.13E+04 2.07E-07 49.7
65.07 1.690 4.041E+06
copper (annealed) 8960 7.00E+07 3.91E+03 1.68E-08 12.0
3.13 1.000 4.704E+04
copper (cold worked) 8960 2.20E+08 1.23E+04 4.20E-08 30.1
9.82 1.257 3.696E+05
titanium elemental 4506 3.20E+08 3.55E+04 4.20E-07 151
28.38 0.363 5.371E+06
titanium beta-C alloy 4830 1.03E+09 1.07E+05 1.60E-06 618
85.41 0.287 6.602E+07
Tantalum 16600 , 2,10E+08 6.32E+03 1.35E-
07 .. 179 .. 5.06 .. 0.201 .. 1.134E+06
Inver 36 8050 2.07E+08 1.29E+04 8.23E-07 530
10.28 0.067 6.810E+06
Nichome (20% chromium) 8400 3.54E+08 2.10E+04 1.30E-06 874
16.84 0.070 1.839E+07
molybdenum 10240 4.80E+08 2.34E+04 1.44E-06 1,180
18.75 0.070 2.765E+07
Nickel/Chromium (80/20 Nichrome) 8400 3.45E+08 2.05E+04 1.25E-
06 840 16.42 0.071 2.20E+11
Alnico Grade 8 (cast, fully dense) 7300 6.90E+07 4.73E+03 4.70E-
03 730 3.78 0.000 2.20E+11
The fastest actuation commutating circuit breaker of Figure 12 using a
material from Table 2
would be based on the highest strength/density ratio material, aluminum matrix
alumina-fiber
wire. This cermet wire is the mechanical strength element (replacing steel in
the more standard
ASCR aluminum steel core reinforced wire) in 3MTm Aluminum Conductor Composite
Reinforced (3M ACCR) wire, which is commercially available from 3M. Using only
the list of
materials shown in Table 2, a desirable combination of fast actuation combined
with a
reasonably low total mass to accelerate can also be obtained by making
commutating shuttle 465
from a high strength titanium alloy shell with sodium inside. Among the single
component
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potential material solutions for commutating shuttle 465, pure aluminum and
pure magnesium
have essentially equal mass to meet the 25 micro-ohm resistance target, but
pure aluminum is
stronger and so is a better solution for commutating shuttle 465. The
penultimate column in
Table 2 is a dimensionless figure of merit M
M = {(strength)/{density x resistivity]] / {(strength)/[density x resistivity]
for annealed copper}
This figure of merit M is indexed to a reference value for annealed copper of
1.00; of the single
component materials (not composites or fabricated structures) shown in Table
2, cold worked
copper has a modestly improved figure of merit M (1.257) compared to copper,
and all the forms
of magnesium and aluminum examined also have slightly higher M value than
annealed copper,
ranging from 1.147 to 4.411 for high strength aluminum alloy 6061-T6. The
highest figure of
merit M in Table 2 (43.4) is for a cermet wire, composed of alumina glass
fibers in a matrix of
pure aluminum. Similar wires that are comprised of carbon fiber reinforced
aluminum have also
been reported, but are much more difficult to prepare, and are not (as far as
I know)
commercially available at present. Such a cermet wire can serve as both
conductor and actuator
of the motion of the commutating shuttle 465 in Figure 12.
Because the modulus of the cermet wire (core wire of 3M ACCR) is so high (4550
MPa),
stretching it just a few percent can store a large amount of elastic energy
(comparable to a very
stiff spring) that could supply force 480 while obviating the need for slip
ring 470. This design
could be used for a very fast actuating design capable to very high voltage.
In the most extreme
version, it is possible to stress a cermet ACCR wire up to close to its
breaking strength (1400
MPa), with the wire strung through a resistive stack such as that shown in
Figure 12, then release
the wire below the stack of resistors to open the circuit. This design, in
which a high strength
fiber reinforced wire 465 extends through a stack of resistors 460, and is
restrained below the
stack, in a zone 466 that is strongly attached to the stressed wire 465
enables the fastest possible
actuation of a linear motion commutating circuit breaker. There are several
known options to
rapidly release such a highly stressed fiber-reinforced wire version of 465:
1. The feature 466 can be a stiff, strong rod that is held in place by
a ring of piezoelectric
thrusters that hold the wire end 466 in place via a normal force that can be
released
within 20 microseconds (the needed normal force can be reduced if part of the
restraint to
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motion of 466 can be due to correlated magnetic domains on the surface of 466
that
match up with similar domains that are imprinted on the surface of sleeve
490);
2. The wire 465 or a wire end 466 can be cut with high explosives;
3. Fracture of the wire per se or a wire end 466 can be initiated with pulsed
lasers.
This type of circuit breaker would be resettable without replacing components
only for option 1.
The last two methods would still be useful as a form of fast fuse for HVDC
circuits that only
blow rarely; they too can be reset, however one part (the fuse) needs to be
replaced each time. A
commutating circuit breaker of Figure 12 can be reset if piezoelectric grips
are used to hold the
bottom end of the commutating shuttle 465, through the abutting rod-shaped
gripping surface
provided by feature 466 in Figure 12.
The design of Figure 12 minimizes the mass of non-essential parts of a
commutating shuttle, by
eliminating most of the insulation attached to the commutating shuttle and
minimizing the mass
of the trailing edge electric field control technology described elsewhere in
this disclosure. Only
the conductor is absolutely required for the breaker of Figure 12; the
optional graded resistivity
trailing edge component 466 is not a requirement, though it is expected to
reduce arcing inside
the core of the resistor stack during operation, and so is a desirable
feature. This design can also
be deployed with a high vacuum, or with an arc-quenching gas mixture
containing sulfur
hexafluoride surrounding the commutating shuttle 465 and the resistor stack
460.
A major consideration in accelerating and decelerating the shuttle of a
commutating circuit
breaker is the mechanical integrity of the shuttle under a given acceleration.
The setups shown in
Figures 1, 4, and 13 accelerate the commutating shuttle linearly strictly with
a pulling force; in
such a method of acceleration of the shuttle, there is no tendency for the
shuttle to buckle,
regardless of the slenderness ratio of the shuttle (length/diameter for a
circular cylindrical
commutating shuttle). Note though, that during deceleration the long, slender
shuttles of Figures
1, 4, and 12 would have a high tendency to buckle if braking force is applied
at the front, which
would limit the maximum deceleration to a lower value than the maximum
acceleration.
Buckling of a long slender commutating shuttle such as 465 in Figure 12 can be
prevented by
surrounding the commutating shuttle with a strong stiff stator; however making
the stator
perform a mechanical function in addition to its primary electrical function
(greatly reducing the
volume where arcing can occur) will make the entire device more expensive.
This is one major
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advantage of a rotary motion commutating circuit breaker such as that of
Figure 6 versus a
design in which the shuttle moves linearly. Insofar as long slender
commutating shuttles have
distinct advantages in terms of cost at very high power levels (Figure 12), it
is useful to discuss
options for braking a linear motion shuttle from the rear.
The feature 466 at the end of the conductive rod 465 may comprise permanent
magnets, as
indicated for feature 119 in Figure 1, which may both restrain the rod 465
from moving in the on
state and which can also provide a braking force (generated by inducing a
current in metal, a well
known means of braking) after the commutating circuit breaker has completed
its motion
through the stack of resistors. Other types of mechanical constraints,
including a non-conductive
rope attached to the end of the commutating shuttle, for example at the
position 466 in Figure 12,
and attached at the other end to a mechanical brake that can arrest the
forward motion of the
commutating shuttle after the circuit has been opened, or friction brakes that
only engage with
feature 466 at the end of travel, are also viable options to brake from the
rear.
Figure 13 shows a variable resistance shuttle design of the commutating
circuit breaker in the on
state, in which a highly conductive material 540 bridges between the two
stator electrodes 505
and 510. There are two significant changes from the similar design of Figure
1: first, a
continuously variable resistance shuttle core 530 is used rather than the step-
graded core 110 of
Figure 1. Figure 1 illustrates the case of a moving resistive core 110 with
well-defined
boundaries between materials with different resistivity (111, 112, 113, 117),
while Figure 13
shows the case of a variable resistance core 530 that is a continuously graded
cermet that has
resistivity increase smoothly from right to left, with no sudden changes in
resistivity. Cennet
resistors with stratified resistivity ranging from low to high resistivity can
be prepared by known
means (see for example, "Functionally Graded Cermets," by L. Jaworska et al,
Journal of
Achievements in Materials and Manufacturing Engineering; Volume 17, July-
August 2006).
Substituting a continuously graded resistor for step changes in resistance
eliminates switching
transients, so this is a desirable implementation of the invention that is
feasible either with
resistors on the shuttle (as in Figure 13), or stationary resistors. Second, a
new feature is shown
in Figures 13 and 14, the stator electrode trailing edge elastomeric sleeve
500, which is
functionally similar to the trailing edge feature 153 shown in Figure 4. Said
trailing edge
elastomeric sleeve 500 overlaps with electrode 505, and occupies region 535 to
the right of
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electrode 505. Figure 14 shows a close-up view of stator electrode trailing
edge elastomeric
sleeve 500, which is attached to stator electrode 505 as shown in Figure 14.
The sleeve 500
inhibits arcing and makes it possible to operate the commutating circuit
breaker of Figure 13 in
open air at a higher voltage differential between stator electrode 505 and
downstream stator
electrode 510 than would be possible in the absence of sleeve 500. By the time
the variable
resistance material 530 is exposed to the air upon exiting elastomeric sleeve
500, the voltage
gradient at that point is greatly reduced compared to what the voltage
gradient is upon exiting
electrode 505. The maximum voltage gradient can be higher under the elastomer
sleeve 500
without causing electrical breakdown compared to the voltage gradient that
could be sustained
without breakdown at an air interface at the trailing edge of 505 if the
variable resistance portion
of the commutating shuttle 530 exits the end of the metallic stator electrode
505 into air. The
downstream stator electrode 510 does not need a sleeve like 500, because the
current only flows
between Pole A and Pole B. The total movement of the shuttle core 550 is far
enough so that the
highly insulative portion of the commutating shuttle 533 fills a zone that
extends from left to the
right of stator electrode 505, to somewhere under elastomer sleeve 500. Figure
13 also provides
an example of actuation of motion of the shuttle with gas pressure 525.
The sleeve 500 fits around the circular cross-section of the tube-shaped
stator electrode 505, and
has a lip feature 555 to attach the elastomer sleeve 500 to the trailing edge
of said stator
electrode. The shape of 500 as molded will be substantially different than how
it looks in the
deformed state shown in Figure 14. As will be familiar to one skilled in the
art of design of
rubber boots for mechanical devices (steering boots and the like), it is
possible to work
backwards from the final deformed shape of the elastomer sleeve (Figure 14) to
calculate the
dimensions of the mold to make the rubber sleeve. An example of an appropriate
design criterion
would be to set the extension ratio A. (which is the ratio of diameter in the
deformed state to
diameter as molded) at the interface between the elastomer sleeve and the
shuttle at location 556
to about 1.1 to 1.25. It is desirable that the inner surface of elastomeric
sleeve 500 be coated by
PTFE, and that the sleeve is made of a strong elastomer with a low rate of
stress relaxation. In
the case of sleeve 500, stress must be maintained for the life of the
elastomer part, so slow
relaxing elastomer types, such as peroxide cured elastomers with carbon-carbon
crosslinks are
preferred. It is also desirable that sleeve 500 has electrostatic dissipative
resistivity between
about 105 to 109 ohm-meter. In addition, the sleeve of Figure 14 will have to
last many years in a
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potentially high ozone environment around electrical equipment, in an extended
state. Therefore
this sleeve also must be highly ozone resistant; for these reasons, peroxide
crosslinked HNBR
(hydrogenated nitrile-butadiene elastomer), EPR (ethylene-propylene rubber),
and EPDM
(ethylene-propylene-diene monomer) are particularly appropriate as base
elastomers for sleeve
500.
The method of using a flexible insulating material pressed into close contact
with a moving
electrode just behind a brush electrode to suppress sparks in a commutator was
first described by
Nikola Tesla in US patent 334,823, using a mica board just behind the brushes
of a DC motor. I
have invented an improved version of this concept having a tight-fitting
elastomeric insulating
layer just behind the electrical stator electrode 505 to inhibit arcing as the
most conductive part
of the variable resistance shuttle moves away from the stator electrode. By
creating contact
pressure, elastomeric sleeve 500 increases the intimacy of contact between the
sleeve and the
outer surface of the variable resistance shuttle. This mechanism can be
applied to commutating
shuttles as well, as in the trailing edge feature 153 shown in Figure 4 and a
semiconductive
elastomer plug such as one version of 312 in Figures 8 and 9.
Commutating circuit breakers can also be deployed in a hybrid circuit breaker
design such as
Figure 15, in which the critical first commutation is done by a very fast
switch 605; this fast
commutation switch is connected to a common buss bar 601 that connects both
fast switch 605
and commutating circuit breaker 610 to Pole A. Similarly, Buss Bar 615
connects both 605 and
610 to Pole B through a no-load disconnection switch 602, which is normally
closed (but which
is shown as open in Figure 15). In the on state, switches 602, 605 and
commutating circuit
breaker 610 are all closed, and current flows through both connections. When
fast switch 605
opens, the full current is rapidly commutated to the commutating circuit
breaker, which then
finishes opening the circuit over a period of -40 ms. After the current is
quenched, no-load
switch 602 is also opened, which facilitates re-setting of both fast switch
605 and the
commutating circuit breaker. The hybrid switch of Figure 15 still has the soft
circuit opening
capability of a stand-alone commutating circuit breaker, but can get to the
first resistance
insertion much faster than a purely electromechanical commutating circuit
breaker. The hybrid
circuit breaker design of Figure 15 can relax the requirement of very low on
state resistance
through the commutating circuit breaker 610, since in the on state, most of
the current flows
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through the parallel path through the fast switch 605. For example, when a
rotary multistage
commutating circuit breaker of Figure 6 and Table 1 is used in a parallel
circuit with a fast
commutation switch as in Figure 15, the resistor insertion sequence of Table 1
is modified so that
the on state resistance of the commutating circuit breaker (prior to
actuation) is equal to the first
inserted resistance of Table 1 (50 ohms in this example). In this case there
is no need to use
liquid metal or other very low resistance electrodes in the commutating
circuit breaker, which
significantly simplifies the design, because the fast switch carries most of
the on state current.
The fast commutating switch shown in Figure 15 can be:
= a type II (ceramic) superconducting shunt that is designed so that
resistance goes very
high when current exceeds a pre-determined limit. [Such ceramic
superconductors are
used in superconductive fault current limiters (SFCLs)]; this is the fastest
and preferred
option where control of short circuit over-current is the primary risk, and is
intrinsically
failsafe even for low inductance short circuits);
= an electron tube including the type of cold cathode vacuum tube mentioned
in US patent
7,916,507 (as in Example 1);
= a mercury arc valve;
= a semiconductor switch such as a GTO, IGBT, or IGCT (although this
implies high on
state losses compared to a mechanical switch);
= a fast mechanical switch of a different type than the commutating circuit
breakers of this
invention, such as that of US patent 6,501,635;
= a MEMS (Micro-Electro-Mechanical Systems) switch array;
= a vacuum circuit breaker (see for example US patent 7,239,490);
In the case of a hybrid circuit breaker as in Figure 15, based on commutating
circuit breaker 610
having the design of Figure 6 and the set of resistance insertions of Table 1,
the initial resistance
of the commutating circuit breaker (prior to any movement of the rotor) would
be 50 ohms,
which could be spread out among the six commutation zones equally by making
the resistance
of each of the six lowest resistance electrical links (226, 236, 246, 256,
266, and 276 in Figure 6)
8.33 ohms each, for example. The 50 ohms initial resistance could also be
divided between five
of the six commutation zones; the remaining commutation zone with low
resistance will then be
the zone where the second commutation occurs (this second commutation is the
first
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commutation caused by movement of rotary commutating shuttle 280 of Figure 6);
according to
Table 2, this second inserted resistance would be 19.4 ohms (inserted in
series with the previous
50 ohms, so that total resistance goes to 69.4 ohms). From this point forward,
all subsequent
commutations and resistance insertions would be handled by the commutating
circuit breaker
610.
The fast switch 605 can in some cases commutate power to the commutating
circuit breaker in
less than one microsecond, and then the commutating circuit breaker shuttle
begins to move and
may take 5-50 ms to fully open the circuit, but is instantaneously able to
clamp the current inrush
due to a dead short to protect the connected components, such as a VSC
(voltage source
converter), or a transformer for example. This fast commutation feature is
particularly important
in a multi-terminal HVDC grid. In this application, superconducting fault
current limiters and
cold cathode vacuum tubes are especially desirable for fast switch 605.
Figure 16 illustrates a simple method to create a linear motion commutating
shuttle that is
functionally similar to a single stage 157 of the two stages of the linear
actuated commutating
circuit breaker shown in Figure 5. The design of Figure 16 is based on a piece
of metallic or
metal-matrix cermet pipe 620, onto which conductive sleeves 625, 626, and
insulating sleeves
630, 631, and 632 are fitted and/or attached. Said conductive sleeves 625 and
626 correspond to
shuttle electrodes 211 and 212 in Figure 5, and are metallic sliding
electrodes. Sleeves 630, 631,
and 632 are electrically insulating sleeves that correspond to the insulating
material 159
surrounding conductor 210 in Figure 5. Said sliding metallic electrodes can be
mechanically and
electrically bonded to the pipe-shaped core 620 by a friction fit based on
assembling accurately
machined parts at different temperatures (shrink fit); by using solder or
brazing; or by plasma or
flame sprayed metal applied directly to the pipe-shaped core 620. The
electrically insulating
sleeves can be glazed onto the metallic substrate 620 as a glass; a preformed
insulating sleeve
that is accurately machined can be placed over the pipe-shaped core 620 by a
friction fit based on
assembling accurately sized parts at different temperatures (shrink fit); by
plasma or flame
sprayed ceramic insulation applied directly to the pipe-shaped core 620; or,
an insulating,
adherent polymer coating can be applied to the metallic substrate 620 to
insulate it everywhere
except at the sliding electrodes 625 and 626. Alternatively, the commutating
shuttle of Figure 16
can be prepared by lathe cutting a conductive pipe so as to leave raised
ridges behind to form the
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two shuttle electrodes 625 and 626, followed by coating the remaining portion
of the pipe with
an insulator, such as epoxy or polyurethane resin, or by insert molding using
a thermoplastic.
After forming the conductive and insulating sleeves, smoothing the surface of
the coated pipe so
that the outer radius of the insulating sections 630, 631, and 632 is equal to
the radius of the two
electrodes 625 and 626, and there are no sharp edges at the boundaries between
conductive
sleeves and insulating sleeves is important.
Figure 17 depicts a single stage, two zone rotary commutating circuit breaker
with external
resistors that is well suited to high current, medium voltage DC (MVDC)
applications. Figure 17
is similar to Figure 6 in that it depicts an end-on view of a circular rotary
commutating shuttle
and the mating parts of the stator, but it is designed to have a smaller and
simpler rotating
commutating shuttle, to push up the speed of actuation. The compact circular
cross-section of the
outermost surface 670 of the commutating rotor (comprising major components
650, 671, 672,
673) of Figure 17 is smooth on its outer surface, which enables it to fit
snugly inside a stator
assembly 652, not shown in detail, which holds all the stator electrodes (675,
680, 690, 700, 710,
676, 720, 730, 740, 750). The stator electrodes 680, 690, 700, and 710 connect
to external
resistors 681, 691, 701, and 711; similarly stator electrodes 720, 730, 740,
and 750 connect to
external resistors 721, 731, 741, and 751 as shown. The two on state stator
electrodes 675 and
676 are liquid metal electrodes that connect via low resistance lead wires to
Pole A and Pole B of
the commutating circuit breaker. The entire stator assembly 652, including the
inner surfaces of
the stator electrodes has a smooth inner surface in contact with the rotary
commutating shuttle
(650, 671, 672, 673). The entire stator surface other than the stator
electrodes is composed of a
highly insulating material, such as a polymer or polymer composite. A
lubricating interfacial
film (not shown in Figure 17) desirably resides between the rotor outer
surface 670 and the stator
652. The stator electrodes are desirably held against the shuttle with a
uniform pressure, which
can originate from an elastic force, a pressure on the outside of a flexible
stator, or both.
The commutating rotor core 650 is desirably composed of an aluminum-matrix SiC
composite
shaft or some similar low density, low thermal expansivity, high electrical
conductivity material
which is coated on its outer perimeter with an adherent electrically
insulating shell 671, for
example a ceramic such as plasma-sprayed alumina, aluminum nitride, quartz
glass, or a
polymer, except that the insulating shell is interrupted in the two shuttle
electrode regions 672,
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673 where the metallic tube is coated with a thin layer of conductive metal
that is the same
thickness as the insulating layer, but which is conductive and has good
properties as a sliding
electrode; two particularly desirable metals for the major part of shuttle
electrodes 672, 673 are
silver, nickel, and/or molybdenum. The shuttle electrodes 672 and 673 are wide
enough to make
full connection to the first two stator electrodes in the on state. The timing
of the commutations
can be set by varying the width of the two on state electrodes 675, 676 and
adjusting the gaps
682 and 692 between said on state stator electrodes and the next two stator
electrodes 680 and
720.
Figure 18 depicts an end-on view of a single stage, two zone rotary
commutating circuit breaker
800 with resistors that are incorporated into the stator, but which is
otherwise similar to the
rotary commutating circuit breaker of Figure 17. In Figure 18 hollow keystone-
shaped stator
electrode resistors (811, 821, 831, 841, 861, 871, 881, 891) act as both
stator electrodes and
resistors; these keystone-shaped stator electrode resistors actually form part
of the inner walls of
the stator and contact the commutating rotor (which is in this case a strong
metallic hollow or
solid shaft 855, selected to allow very high torque for maximum radial
acceleration and very fast
actuation). This design allows for continuously graded resistivity in the
stator electrode resistors,
which eliminates sudden voltage increases due to discreet commutations through
a series of
different resistors, similar to the linear motion graded resistors of Figure
13. Resistance insertion
occurs on both sides of the rotary commutating shuttle as the shuttle
electrodes 802 and 852 turn
clockwise out of contact with the liquid metal electrodes 801 and 851 (this is
the first
commutation, synchronized on the A and B sides of the circuit breaker in this
case, though the
first commutation off the liquid metal electrodes need not be simultaneous).
The liquid metal
electrodes 801 and 851 are connected to Pole A and Pole B of the circuit
breaker, and also
electrically connected to the neighboring stator electrodes 811 and 861, which
may be made of
Nichrome alloy, cermet, quasicrystalline alloys, or amorphous carbon, for
example. In a similar
manner, stator electrode resistors 811 and 861 are also electrically connected
to stator electrode
resistors 821 and 871 and so on, up to the final stator electrode resistors
841 and 891. In each of
these two series (Pole A side: 801 to 811 to 821 to 831 to 841; Pole B side:
851 to 861 to 871 to
881 to 891) the resistivity of the material forming each sequential stator
electrode resistor
increases compared to the prior stator electrode resistor in the series, and
also has graded
resistivity internally. After commutating through all the stator electrode
resistors, there are two
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highly insulating portions of the stator (825, 826); the shuttle electrodes
rotate under these highly
insulating portion of the stator when the circuit is opened. In both Figure 17
and Figure 18, the
total rotation of the commutating shuttle is 135 degrees during actuation of
the circuit breaker
from the on state (closed) to the off state (open).
Although Figure 18 shows all the stator electrode resistors as having the same
outer diameter, the
outer diameter of the various stator electrode resistors can vary according to
the amount of
energy each stator electrode resistor is expected to absorb during normal
operation of the
commutating circuit breaker; the first resistors to be switched into the
circuit (811, 861) absorb
far more energy than the last resistors (841, 891), and so should have higher
mass. This can be
accomplished by increasing the outer radius of 811 and 861. The outer radius
of the intermediate
stator electrode resistors (821, 831, 871, 881) would then be intermediate in
terms of outer
diameter between the diameters of the first resistors (811, 861) and the last
resistors (841, 891).
As in Figure 17, the outer surface of the rotor shaft 855 is coated with an
insulating ceramic,
glass, or polymer layer 803, 853 over most of its surface, but also is coated
in two shuttle
electrode regions 802 and 852 with suitable metals, as previously described.
The outer wall of
the commutating rotor extends out to radius 804, and is polished smooth so
that there is at most
only a very small unevenness in going from an insulating part of the wall
(803, 853) to the
neighboring conductive parts of the wall (802, 852). A tight clearance is
maintained between the
outer edges of the rotor and the keystone-shaped pieces forming the inner part
of the stator (801,
811, 821, 831, 841, 826, 851, 861, 871, 881, 891, and 825), which occurs at
radius 804; there
may be a liquid or dry non-conductive lubricant at this interface.
It is also possible to hybridize the designs of Figures 18 and 19, by using a
combination of stator
electrodes linked to external resistors, as in Figure 17, for the first few
resistance insertions
(which absorb most of the energy that is dissipated during operation of the
commutating circuit
breaker) and stator electrode resistors like in Figure 18 to accomplish the
last several
commutations through increasing resistance of the stator wall.
To get to a high voltage, multistage commutating circuit breakers, which can
be either large
diameter rotors or long axial motion devices, are desirable. It is highly
desirable to drive such
large commutating shuttles from multiple areas on the surface of the
commutating shuttle rather
than by applying force at one or both ends of a long axial motion multi-stage
breaker, or to the
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shaft of a large diameter rotary breaker. For example, in a three stage rotary
commutating
breaker with six commutation zones along its outer surface (as in Figure 6),
designed for 800 kV
the rotor will likely have to be more than a meter in diameter to allow
adequate insulation
between alternative electrical paths through the rotor. At that diameter,
driving rapid rotation
from a center shaft would require a great deal of torque, and structure to
support that torque.
Large diameter rotors are most effectively driven by many small springs or
actuators all along
the outer radius of the commutating shuttle that can distribute the needed
force to accelerate the
commutating shuttle over the surface of the commutating shuttle in such a way
that the force
needed to accelerate the commutating shuttle is delivered to the shuttle near
to where it is needed
to accelerate portions of the shuttle, as in Figure 19.
Figure 19 illustrates an actuation mechanism that is particularly well suited
to drive a large
diameter multistage rotary commutating circuit breaker similar to Figure 6.
Multiple flat or
gently curved springs 905 are disposed around the outer radius of the
commutating rotor 900.
Each spring engages with the rotor via a matching feature 910 attached to the
rotary
commutating shuttle. The commutating rotor is held in place via quick release
brakes 915 that
restrain the rotor from moving until a signal from controller 925 traveling
through control signal
wires 920 releases the brakes. As discussed previously, the brakes are
desirably based on
piezoelectric actuators that apply a normal force against polished surfaces to
resist movement by
friction. When the controller 925 causes the piezoelectric actuators 915 to
quickly change shape
so as to relieve the normal force, the commutator rotates to open the circuit
breaker.
Figure 20 shows a general setup of a shaft-driven rotary commutating circuit
breaker assembly.
At the left, 930 is a torque drive that applies torque to the shaft 945, which
drives the rotation of
the rotary commutating circuit breaker 940 when the fast brake 950 is
released. Rotary
commutating circuit breaker 940 can be of a variety of designs, such as
Figures 7, 18, or 19 for
=
example. All components are mounted on a strong base plate 960 (which could
also take the
shape of a pipe or a truss that surrounds the commutating circuit breaker
assembly). Torque
source 930 can be a torsion spring, a ring of flat springs acting on a drive
wheel, as in Figure 19,
an electromechanical or fluidic drive, or even a length of twisted shaft. The
rotary commutating
circuit breaker 940 is between two bearings 935. The fast release brake 950 is
on the opposite
side of rotary commutating circuit breaker 940 from the torque drive, which
holds the torque
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from the torque drive 930 in the on state of the circuit breaker, so that the
torque that is applied
to the shaft 945 is held back by the fast release brake 950; as soon as the
fast brake is released
the shaft and the rotary circuit breaker rotate to an open position. In the on
state there is an equal
and opposite torque on the base plate 960 between the torque drive 930 and the
fast release brake
950. The shaft 945 extends beyond the fast release brake 950 to an arresting
brake 955 that is
mounted to the shaft by a spline so that it does not encumber motion of the
shaft until after the
opening of the circuit by the commutating circuit breaker is complete, after
which the arresting
brake quickly stops the rotation of the shaft while also preventing rebound
and reversal of the
shaft rotation. At this point a no-load electrical switch 965 is opened, which
de-energizes the
rotary commutating circuit breaker so that it can safely be reset.
The arresting brake 955 also incorporates a feature for re-setting the rotary
commutating circuit
breaker, by twisting the shaft back to its initial position after the rotary
commutating circuit
breaker has opened. After the shaft is reset to its initial on state position,
the fast acting brake is
reset, then the arresting brake is returned to its normal on state position
and locked so that it
cannot rotate with respect to the base plate. Finally, the no-load switch 965
is reclosed to return
the rotary commutating circuit breaker assembly to its original on state,
ready to again carry
current from pole A to pole B, while also being able to rapidly open again as
needed.
The fast brake can be a variety of different prior art mechanical releases, or
a piezoelectric brake
as described elsewhere in this disclosure, or a combination of correlated
magnetic domains to
hold back part of the applied torque, combined with a piezoelectric brake to
enable very fast
actuation. It is possible to apply the principle of matching printed magnetic
domains to hold a
commutating shuttle stationary while stress is applied, either in a rotary
mode of actuation or a
linear mode of actuation. This is based on a method of accurate positioning
that is being
developed by Correlated Magnetics of New Hope, AL (see for example, US patent
8,098,122).
Using this concept, a "fingerprint" pattern of matching magnetic domains can
be created on the
commutating shuttle and the mating stator of commutating circuit breaker 940,
or on a shaft and
sleeve that form a part of the fast brake 950 that is capable of restraining
the rotation of the
commutating shuttle in respect with the stator because of a large aggregate
attractive force
between the correlated magnetic domains; let us assume that the matching
magnetic domain
patterns can prevent rotation of the shuttle out of the "magnetic energy well"
up to an applied
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torque of Tc. It is then possible to combine the braking effect of
piezoelectric actuators with the
correlated magnetic domains; in this case, a torque would be applied by drive
930 that is slightly
greater than the maximum that can be restrained by the correlated magnetic
domains alone, for
example 1.1(Tc) would be applied by drive 930, which is partially restrained
by correlated
magnetic domains, and partially by piezoelectric actuators that apply force
perpendicular to
polished metal or ceramic tabs, as in feature 915 of Figure 19. As soon as the
piezoelectric
actuators are released, the shuttle begins to move, because the applied torque
exceeds the
maximum that can be resisted by the correlated magnetic domains. This reduces
the normal force
that needs to be applied by the piezoelectric actuators, which is economical.
This retains the
same desirable failure mode as is the case for piezoelectric brakes alone, as
spring force alone
will knock the shuttle out of the magnetic energy well and open the circuit if
the control circuit
power to the piezoelectric actuators is lost.
Correlated magnetic domains have the additional important feature that they
can accurately
position the commutating shuttle rotor in a precise relationship to the
commutating stator (within
microns). This is especially important in versions of commutating circuit
breakers that use
thin liquid metal electrodes, which must be accurately aligned in the on
state. It is easy to arrange
things so that once the commutating shuttle begins to move, the magnetic
domains do not
restrain the motion significantly, and yet a second set of correlated magnetic
domains can arrest
the commutating shuttle in a desired off state at the end of its rotation.
The principle of matching printed magnetic domains to hold a commutating
shuttle stationary
while stress is applied, via matching "magnetic fingerprints" is also capable
of restraining linear
motion of a commutating shuttle of a variable resistance shuttle. The matching
magnetic domain
patterns can prevent motion of the shuttle out of the "magnetic energy well"
up to an applied
force of Fc. There are two distinct possibilities as to how these correlated
magnetic domains can
be used in a fast-acting linear motion commutating circuit breaker. The first
option is that fast-
acting springs are deployed which apply a force below that which would release
the shuttle from
the magnetic energy well, for example 0.95(Fc); the magnetic domains are in
this case adequate
to restrain motion of the shuttle out of the magnetic energy well. A
relatively small additional
force of only 5% or more of the spring force can be applied to knock the
commutating shuttle out
of its "magnetic energy well" after which it will be rapidly accelerated by
the springs. This
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additional force could be applied electromagnetically, by piezoelectric
actuators, or by gas
pressure for example.
The second way to use correlated magnetic domains in a fast commutating
circuit breaker is to
combine the braking effect of piezoelectric actuators with correlated magnetic
domains that are
not quite able to restrain motion of the shuttle by themselves (as was
discussed in relation to
rotary motion in the discussion of Figure 20 above). In this case, an applied
force that is greater
than the maximum that can be restrained by the correlated magnetic domains
alone, for example
1.1(Fc) is applied to the shuttle of a commutating circuit breaker that is
partially restrained by
correlated magnetic domains, and partially by piezoelectric actuators that
apply force
perpendicular to polished metal or ceramic tabs, as in feature 915 of Figure
19. This method of
partial restraint via magnetic domains could also be applied for example to
replace the magnetic
restraint features 119 and 121 in Figure 1, or to supplement the restraining
force applied by
piezoelectric actuators to hold feature 466 of Figure 12. As soon as the
piezoelectric actuators are
released, the shuttle begins to move, but the piezoelectric actuators only
need to provide about
10% of the total restraining force, which is economical. This method has the
advantage that if
control power is lost, the circuit breaker will open automatically, so its
failure mode is far less
dangerous than the other method previously described above to restrain motion
using correlated
magnetic domains, in which spring force per se is not adequate to knock the
shuttle out of the
magnetic energy well if the control circuit power is lost.
In any commutating circuit breaker, the motion of the variable resistance
shuttle or the
commutating shuttle implies rapid acceleration, which will cause a mechanical
jolt unless two
opposed motions with equal and opposite momentum changes are combined into a
single circuit
breaker. In order to minimize fatigue of the connections between the breaker
and its enclosure, or
the mounting fasteners holding the enclosure to the building or vehicle
structure, and to reduce
noise and vibration due to opening a commutating circuit breaker, it is
desirable to have two
opposed and balanced motions, so that the momentum that must be transferred to
the circuit
breaker enclosure and the structural supports of the enclosure are minimized.
Three mechanisms to contain the momentum effects of commutating circuit
breaker actuation
within the stator (housing of the commutating circuit breaker moving core,
whether the moving
core is a variable resistive element or a commutating shuttle) are possible:
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1. accelerating two linear variable resistance shuttles or commutating
shuttles in opposite
directions within a common stator housing (which is capable of absorbing the
shock
loading that will result when the shuttle cores reach the end of their travel
and must be
arrested) which will contain the momentum effects of two symmetrical and
balanced
cylinders which move axially in opposite directions;
2. in the case of rotating shuttles (which may comprise rotating variable
resistors or shuttle
commutators), balancing the momentum effects perfectly would require coaxial
counter-
rotating discs; it is much easier however, to use two opposed counter-rotating
shuttles on
a common support base; the modest twisting forces due to having the centers of
rotational
momentum of the two disks offset slightly can be tolerated; this precession
force is small
compared to the rotational momentum required to accelerate and decelerate the
rotating
commutating shuttles, which can be balanced;
3. for either linear motion or rotating commutating circuit breakers, the
balancing
momentum component can be a mass that is not a commutating circuit breaker per
se.
It is important in most circuit breakers to deal with the inrush of current in
a dead short. A
complete analysis requires an understanding of the entire electrical system in
which the circuit
breaker is imbedded, including especially system voltage response,
capacitance, resistance, and
inductance in a fault. The rate at which current can increase in a fault is
moderated primarily by
inductance, and it is always possible in principle to add inductance to slow
the inrush of current
in an anticipated fault. There is a trade-off between speed of operation that
is required for the
circuit breaker and system inductance. Adding inductance can allow the
insertion of resistance to
be slower while still clamping the current inrush at an acceptable level, but
at a cost: both for the
inductor per se, but also adding inductance can increase the mass of resistors
that are needed to
squelch the current. In general, the commutating circuit breakers of the
present invention work
best when the ratio of system voltage V (in volts) to inductance L (in
Henries) is less than 40
million at most; more preferably the ratio of V/L should be less than or equal
to 8 million.
Higher ratios than 40 million can be allowed in hybrid circuit breakers of
Figure 15.
Commutating circuit breakers for relatively low power circuits may desirably
incorporate the
resistors into the moving variable resistance shuttle, such as Figures 1 and
13; this principle may
also be used in rotary commutating circuit breakers, by using a variable
resistance rotor.
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Commutating circuit breakers for relatively high power circuits (more than
about 100 kW) are
preferably made with a commutating shuttle that connects the current through a
sequence of
increasing resistance paths by making sequential contacts through stator
electrodes connected
with multiple stationary resistors, as in Figures 4, 5, 6, 8, 9, 11,12, 17,
and 18. This is especially
true in the case of circuits with high system inductance (such as HVDC
transmission lines), since
the inductively stored energy must be dissipated as heat during opening of the
circuit, which can
imply a need for hundreds of kilograms of resistors.
It is desirable in some cases to have a snubber circuit integrated into the
commutating circuit
breaker that has the effect of minimizing the voltage spike that occurs when
the contacts slide off
the connection (whether direct or indirect) to one set of resistors onto the
next set of resistors of
higher resistivity. I have discussed using graded resistivity on the trailing
edge of the electrodes
to soften the voltage spikes due to commutation, but there are also numerous
known snubber
circuits that can reduce or "filter" voltage transients, such as varistors,
Zener diodes, capacitors,
capacitors connected to the circuit through diodes, and other known types of
snubber.
Consider several specific design approaches to solve the challenge of creating
illustrative designs
for a medium voltage DC (MVDC) commutating circuit breaker for 2kA and 6kV.
These basis
assumptions are used in developing Examples 1 to 4:
= Full load = 2000 amps;
= 6kV voltage source; two cases were modeled: Case #4 has no voltage sag
due to internal
resistance (a worst case assumption, similar to a large capacitor bank); Case
#5 has the
current come from a large battery bank with realistic internal resistance of
.36 ohms);
= normal full load resistance of (6 kV) /(2 kA) = 3 ohms
= Maximum design amps in dead short = 10kA (this determines how fast the
commutation
to switch in the first resistance level must be);
= First resistance switched in is (max voltage)/(max amps in a fault) = 1.2
ohms (just high
enough to clamp the current and reverse dI/dt)
= 1.0 microhenries is the assumed worst case system inductance Lo in a dead
short;
= Additional inductance is Lx is added as needed to slow the inrush of
current; different
values of Lx are considered in each of Examples 1 to 4;
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= Maximum voltage during commutation = 12kV (double the normal system
voltage;
occurs due to switching in resistance).
Table 3 shows calculated times to go from full load (2kA) to maximum overload
(10kA) in two
different overload cases:
Case #4: a worst case dead short, zero resistance, no voltage sag; the
increase of current
with time follows equation (3)
Case #5: power supplied by batteries, with internal battery resistance = 0.36
ohms; the
increase of current with time follows equation (4).
Table 3: Time to Max Amps (10kA) for Various System Inductances (6kV, 2kA
circuit)
System inductance, mH Time (2kA---10kA), ms Case #4 Time (2kAl0kA), ms Case #5
.001 .00133 .00163
.150 .200 .333
.750 1.00 1.63
3.750 5.0 8.17
At time zero, resistance goes to zero in Case #4 (a worst case dead short),
after which only the
system inductance constrains the current rise dI/dt. In Case #4, the fault
current I(t) is a linear
function of time after the fault, given by (4); on the other hand if the
circuit contains resistance R
(Case #5), the increase of current with time follows equation (5):
(4) I(t) = Vt/L dI/dt = V/L (Case #4)
(5) I(t) = (V/R){1-exp[-t/(L/R)]}
(Case #5)
Figure 21 shows a plot of these two equations for an intermediate inductance
case (150
microhenries); up to normal full load of 2kA, the two plots are nearly the
same, but they diverge
significantly at higher current, longer time. Given the very low assumed value
of minimum
system inductance L (1.0 microhenries; see Table 3), in the absence of added
inductance, dI/dt
(change of current with time in a dead short) is six billion amps/second. In
order to limit this
current rise to no more than 10kA (starting from 2kA, normal full load), it
would be necessary to
insert the first resistance at 1.33 microseconds. This is not possible for a
mechanical system; only
hybrid designs such as Figure 15 with the very fastest types of switches (IGBT
transistors,
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superconducting fault current limiters, or cold cathode vacuum tubes) can work
in less than two
microseconds as is needed if system inductance is only one microhenry.
Time to the first resistance insertion (commutation) is an important attribute
of a commutating
circuit breaker, because the first resistance reverses or greatly slows the
increase of current; this
is true whether it is a standalone commutating circuit breaker or a hybrid
design as in Figure 15;
or indeed for any DC circuit breaker based on sequential insertions of
resistance. (There are also
many types of faults in an AC system (lightening strikes for example) where
the inrush of current
is too fast to wait for an ordinary AC-type circuit breaker to work.) If the
first inserted resistance
is (max voltage) / (max amps in a fault) = 1.2 ohms in the case of the above
basis assumptions,
and if this resistance is inserted on or before the time when the design
maximum 10kA current in
the circuit is reached (Table 3), the first voltage spike will be less than or
equal to the maximum
design voltage, and current will decay back from that point onwards. If
current = 10kA, then
after switching in the 1.2 ohm resistor, the voltage across the resistor will
be 12kV. The selected
resistance for the first insertion is just high enough to clamp the current
and reverse dI/dt, but
without causing voltage to increase above 12kV. As discussed in detail above
around Figure 6
and Table 1 (which relates to a high inductance transmission system), one then
must allow
enough time for the current to decay down to some desired level before the
next commutation.
Adding in extra inductance Lx slows down not only the inrush of current in the
short (as in
Equations 3 and 4), but also extends the time until the circuit is opened
(since current decays as
exp[-t(R/L)], as the following examples will show.
Examples of the Disclosure
Example 1
Consider a circuit breaker of the style of Figure 15, in which the fast switch
is a cold cathode
vacuum tube of the type disclosed in US patent 7,916,507. Such a tube has an
on-state voltage
drop of about 10 volts, which implies energy loss of about 10/6000 or ¨.17% of
transmitted
power (better than an IGBT and not needing water cooling), for the basis
assumptions cited
above. This kind of tube can switch in less than 0.1 microsecond, easily
commutating power to
the commutating circuit breaker before the current inrush passes the 10kA
maximum level, even
at one microhenry inductance, provided of course that it can be triggered fast
enough.
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In this case, the vacuum tube is doing the first commutation, and if the
system inductance is only
one microhenry, then there is very little inductive energy to dissipate: only
100 joules if the
current is interrupted at 10kA, so that a small capacitor or varistor could be
used to absorb this
energy. The advantages offered by the commutating circuit breaker would be
negligible in this
case, except if (as is often the case) the inductance of the fault could be
highly variable
depending on its location. In the scenario of highly variable inductance in a
fault, one can rely on
the vacuum tube for fast switching to clamp down on the inrush in case of a
low inductance fault,
and the commutating circuit breaker can be optimized for the maximum expected
inductance, so
as to minimize voltage spikes during opening of the circuit breaker. In
particular, voltage spikes
can be kept below the voltage that would be experienced if a varistor were
used to absorb the
inductive energy.
Example 2
Consider the case of minimum inductance in a fault being 150 microhenries.
This implies very
fast actuation and movement of a commutating circuit breaker to get to a first
commutation in
200-333 microseconds (per the basis assumptions of Table 3). This is so fast
that (as is the case
for Example 1) only a hybrid commutating circuit breaker in a parallel circuit
with a fast
electronic switch (as in Example 1 and Figure 15) can feasibly reach the first
commutation
within 200 microseconds, but in the case that 333 microseconds are available
to reach the first
commutation (in Case #5 of a circuit with internal resistance) it is feasible
(but difficult) to use a
fast commutating circuit breaker to get to the first commutation within this
time. These
calculations are predicated on use of the fastest known method to actuate
release of a rotating
commutating circuit breaker, a piezoelectric actuator that moves 20 microns in
20 microseconds.
In the case of a rotary device, the torque required per unit angular
acceleration scales with radius
squared, whereas the circumferential distance (available for placing
electrodes) scales with
radius. Therefore, for a given available torque the fastest actuation will
occur for the smallest
workable radius of commutating rotor. To push the limits of a rotary
commutating circuit breaker
in which torque is applied through a shaft towards the fastest possible
actuation, it is thus
desirable to minimize the radius of the commutating shuttle. This in turn
means minimizing the
number of stator electrodes, the width of the stator electrodes, and the
standoff distance between
the stator electrodes, because each stator electrode and each separator
between neighboring stator
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electrodes must fit along the circumference of the rotating shuttle. The wider
is each stator
electrode, and the higher the number of stator electrodes, the longer must be
the circumference.
As this example is designed to probe the limits of speed of action of a
commutating circuit
breaker, it uses several simultaneous tricks, as detailed below and shown in
Figure 18.
The release of the rotor of Figure 18 which is under high torque is assumed to
occur within 50
microseconds of the fault, which includes 30 microseconds for the control
computer to detect the
fault and deactivate a pair of piezoelectric actuators to release the normal
force clamping against
a polished metal or ceramic brake that is also part of the rotary commutating
shuttle, but outside
the region where the shuttle electrodes are found, and on the opposite side of
the rotary
commutating shuttle from the device that applies the torque (as in Figure 20).
Ordinary springs
will not suffice to apply the torque for such fast motion; only elastic stress
in a very stiff material
can keep up with the needed motion; for example, a twisted titanium alloy tube
or a tube-shaped
carbon fiber reinforced composite that is the same diameter as the rotary
commutating shuttle
can supply the spring force and keep up with the motion of the rotary
commutating shuttle.
For purpose of calculation I took the axial length of the rotary commutating
shuttle of Figure 18
to be 10 cm, which implies a needed circumferential overlap of the rotor
electrodes 802 and 852
with the liquid metal stator electrodes 801 and 851 of less than one mm in the
closed circuit on
state; this may be too small a contact area for accurate routine alignment of
the electrodes in an
industrial circuit breaker; therefore, for purposes of this discussion I took
the circumferential
width of the liquid metal stator electrodes 801 and 851 to be 2.0 mm, which
allows for modest
misalignment between the rotor electrode trailing edge and the leading edge of
the liquid metal
electrode. At the selected outer radius of the rotating shuttle (2 cm), this
implies that the shuttle
must rotate by 5.73 degrees (0.100 radians) to the first commutation (where
the shuttle electrodes
802 and 852 slide off the liquid metal electrodes 801 and 851); in order to
achieve that
movement in 150 microseconds, the radial acceleration must be 8.89 million
radians/second.
This would require a torque of 2158 newton-meters which is higher than the
maximum torque
that can be applied to even a solid titanium beta-C shaft of 2 cm radius. (For
purposes of
calculation, the entire rotor which contains the 10 cm long rotary commutator
is assumed to be
equivalent to a 20 cm long titanium beta-C alloy shaft, 4 cm in outside
diameter and 20 cm long,
and weigh 1.214 kg.) In the case of a resistive circuit (Case #5), the
internal resistance delays the
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crossing of 10kA in a dead short, so that 283 microseconds is available to
reach the first
commutation (after the 50 microseconds allowed for fault detection and release
of the
piezoelectric brakes); this reduces the needed angular acceleration to 2.5
million radians per
second and the required torque to 606 newton-meters, which is just barely
within the strength
limitations of the assumed solid titanium alloy rotor. This is not a practical
design, but it does
show that it is technically feasible to reach the first commutation within 333
microseconds using
the rotary design of Figure 18.
Example 3
Consider the case of minimum inductance in a fault in the circuit of Table 3
being 750
microhenries. I will continue the discussion based on Figure 17, a good bit of
which has already
been discussed above in regard to Example 2, since the rotor diameter is the
same as before, for
the rotary commutating circuit breaker of Figure 18. Increasing minimum
inductance in a fault to
750 microhenries increases the time for current to rise to 10kA from the
presumed starting
current of 2kA by a factor of five: for the worst case, zero resistance fault
(Case #4) this gives
1.0 milliseconds to reach the first commutation, and for the Case #5 circuit,
1.63 milliseconds.
Using the same assumptions described above for Example 2 (50 microseconds for
releasing the
brake, rotary moment of inertia equivalent to a 20 cm long titanium beta-C
alloy shaft 4 cm in
outside diameter and 20 cm long), this drops the needed angular acceleration
to 222000
radians/second for Case #4 fault, and 80100 radians/second for the Case #5
fault. The
corresponding torque for these accelerations is 54 and 19 newton-meters;
within a range of
practical torques. In fact, these torques do not require such a strong solid
titanium shaft as would
be needed in Example 2, which means a hollow aluminum alloy shaft can be used,
which reduces
both weight and moment of inertia of the rotor, which reduces the needed
torque even more.
Note though that the speed of actuation required here will still rule out
conventional multi-turn
steel coil springs for actuation; a fast acting spring will still be needed
though not quite as fast as
in Example 2. This demonstrates that practical rotary commutating circuit
breakers with about a
one millisecond time to first commutation can be manufactured.
After the first commutation away from the liquid metal electrodes in Figure
17, the other eight
stator electrodes are not liquid metal electrodes, and as a consequence have
to be wider than the
liquid metal electrode in order to carry the fault current safely and without
damage to the
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electrodes. Further, as is illustrated by Table 1 and Figure 7 for a different
but similar case, the
optimum interval between commutations also changes as the current and stored
inductive energy
are quenched by repeated resistance insertions. I have not taken the step to
couple the equation of
motion of the rotor 650 with optimized times for resistance insertion (as in
Table 1 and Figure 7
for a different specific case), so as to calculate the optimal width of each
particular stator
electrode for the assumed worst case fault (10kA, zero system resistance). I
note though that this
is a straightforward calculation once the details of the torque source and the
rotor are known.
Figure 17 illustrates this principle by the fact that the first two metal
sliding stator electrodes 680
and 720 are wider (one cm wide in the circumferential direction) than either
the initial liquid
metal stator electrodes 675, 676 (which are 0.2 cm wide) or the three
subsequent stator
electrodes 690, 700, 710, 730, 740, 750 (which are 0.6 cm wide). In this case,
the two sets of
stator electrodes (those from 720-750 and those in from 680-710 are equal in
size to their
counterpart electrode in the opposite commutation zone. Syncopation of
switching between
commutating zone 760 and 770 is accomplished by making the width of the first
insulating gap
682 between liquid metal stator electrode 675 and stator electrode 680) 0.45
cm, whereas all the
other insulating gaps (including the insulation gap 692) are 0.30 cm; this
offsets the
commutations of rotor electrode 672 off of the stator electrodes (680, 690,
700, 710) in the upper
right commutation zone by 4.30 degrees behind the corresponding commutations
of rotor
electrode 673 off of the metal sliding electrodes (720, 730, 740, 750) in the
lower left
commutation zone. Using this method to create the syncopated commutations has
the advantage
of standardizing the stator electrode widths, and allowing the commutating
rotor to have a
symmetrical design. This is not an optimized configuration, but illustrates
the principle of using
asymmetric stator electrode circumferential spacing to make the commutations
in two different
commutation zones occur at different times during operation of a commutating
circuit breaker;
and shows that altering the gap spacing between only one set of stator
electrodes can achieve
syncopated commutations between one commutating zone (at the upper right in
Figure 17) and a
second series-connected commutating zone (at the lower left in Figure 17).
The best available conductors near room temperature are silver and copper;
silver-matrix
electrodes in which silver is infiltrated into a sintered porous metal
substrate of chromium or
tungsten are well known, for example. If silver or copper is used in contact
against liquid metal
electrodes, it can react; silver reacts with gallium and mercury, so even if
one made silver-
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mercury electrodes for example, the surface of the silver electrode will be a
silver-mercury
amalgam. Silver can be used with the sodium-potassium low melting eutectic,
but this introduces
safety concerns. A particularly desirable surface for the shuttle electrodes
672, 673 so that the
electrode surface is compatible with mercury or a gallium alloy is to cold
spray silver onto a non-
oxidized aluminum or aluminum composite substrate in a moderate thickness
layer 100-1000
microns thick, and then to polish the surface smooth before applying a
molybdenum layer, which
can desirably be accomplished by physical vapor deposition (PVD) methods to
lay down a fairly
thin film (1-5 microns) on the polished silver surface, which PVD-applied film
reflects the
surface finish of the silver substrate below, and does not require further
polishing. Plasma spray
techniques can also be used to apply a thicker molybdenum surface layer on a
copper, silver,
aluminum/SiC composite, or chromium substrate in principle. Plasma co-spraying
of a substrate
metal and molybdenum can be used to create a fuzzy boundary layer between
silver and
molybdenum (for example) to reduce the chance of delamination. However, a
thick layer of
molybdenum on a silver, copper, or aluminum substrate is intrinsically
unstable due to the
difference in thermal expansivity of the molybdenum compared to the substrate,
and is therefore
less favored than a thinner coating of molybdenum applied by PVD. In either
case, the reason to
apply a surface film of molybdenum is to coat the solid electrode with a non-
oxidizing metal
(below about 600 C) which does not react with gallium or mercury to form an
amalgam.
Because the electrode layers 672, 673 on the surface of the commutating rotor
650 of Figure 17
are relatively thin (less than one mm), and also for simplicity of
manufacturing, it is desirable for
the entire thickness of the electrodes to be composed of molybdenum that is
plasma sprayed onto
the substrate metal tube 651. In this scenario, the insulating layer 670 could
logically be a plasma
sprayed alumina layer (the surface of the commutating rotor would in this case
be ground smooth
after plasma spaying). Because molybdenum and alumina both have low thermal
expansivity
compared to conductive metals, it is desirable to minimize the thermal
expansivity of the
substrate conductive tube or shaft 650 in the commutating circuit breaker of
Figure 17. Two
potential materials for the core of a rotary commutating circuit breaker such
as that shown in
Figure 17 were considered:
= Solid shaft made of AlSiC-9 infiltrated composite;
= Hollow titanium shaft for high shock loading capabilities.
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These two shaft materials have very similar thermal expansivities. AlSiC-9 is
an aluminum-
infiltrated silicon carbide composite from CPS Technologies that has 8-9 ppm
(parts per million
per degree Celsius) thermal expansivity from 30 C to 200 C (less than half
the thermal
expansivity of aluminum), and titanium has 8.6 ppm (parts per million) thermal
expansivity from
30 C to 200 C. Both materials form bonds with plasma sprayed alumina and
molybdenum
which are more resistant to thermomechanical fatigue than similar thickness
plasma-sprayed
alumina or molybdenum layers on aluminum, copper, silver, or their alloys.
Using a solid shaft
made of AlSiC-9 for the core of the commutating rotor 651 in Figure 17 leads
to a resistance
between the two shuttle electrodes of about 0.0026 micro-ohms, with a
corresponding resistive
heat dissipation of only 0.01 watts at 2 kA. To compare a solid AlSiC-9 shaft
to a hollow
titanium tube, the tube wall thickness that gave the same moment of inertia
about the axis of
rotation as the solid AlSiC-9 shaft was calculated; in this case the mechanism
to accelerate both
tubes can be the same, as is desirable in comparing the two options
economically. The titanium
tube wall thickness (pure titanium) that matches the moment of inertia of a
solid A1S1C-9 solid
shaft (outside diameter of both is 4.00 cm), is only 0.149 cm thick. At a pure
titanium tube wall
thickness of 0.149 cm, the resistance between the two shuttle electrodes would
be about 88.5
micro-ohms, which implies on state losses at maximum full load (2000 amps)
around 350 watts
just from resistance heating of the 10 cm long titanium shaft section between
electrodes 672 and
673. The same type figures for a titanium beta-C alloy tube with the same
rotary moment of
inertia as a pure titanium tube were also calculated; because of the slight
density difference from
titanium (see Table 2), the wall thickness is a little less for a titanium
beta-C alloy tube (0.138
cm): the resistance between the two shuttle electrodes would in this case be
about 365 micro-
ohms, which implies on state losses at maximum full load (2000 amps) around
1,460 watts just
from resistance heating of the 10 cm long titanium shaft section between
electrodes 672 and 673.
(Though I consider this to be unacceptable, it only corresponds to 0.01% of
the transmitted
energy, far less than would be dissipated by an IGBT switch or even a cold
cathode tube switch.)
I note that the resistance for a titanium tube core rotating electrode can be
greatly reduced by
inserting an aluminum tube core inside the titanium tube shell in such a way
as to avoid any
oxides at the interface.
In the case where very fast actuation is required, which also implies shock
loading, it is
necessary to use a very strong, shock resistant material as the substrate for
the commutating rotor
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of Figure 17 or 19, such as titanium or a titanium alloy tube electrically
bonded to an aluminum
alloy core. In any scenario where the commutating shuttle can be protected
from shock loading,
AlSiC-9 will be a more appropriate material for the core of a rotating shuttle
such as 650 of
Figure 17, and aluminum alloy tubes may also be used on some cases.
Example 4
In this example, minimum system inductance is taken to be five times higher
than the minimum
inductance of Example 3 (3.75 mH). According to Table 3, this allows 5 ms to
the first
commutation in Case #4, or 8.13 ms to the first commutation in Case #5. Given
the same high-
side estimates for the total moment of inertia for the rotor of Figure 17 or
Figure 18 made for
Examples 2 and 3 above (corresponding to a 20 cm long, 4 cm diameter solid
shaft of titanium
beta-C alloy), the angular accelerations needed are 8160 radians/second for
Case #4 (required
torque = 2.0 newton-meter), or 3060 radians/second for Case #5 (required
torque = 0.7 newton-
meter). These accelerations and torques are within the range that can be
actuated by standard
steel coil springs.
Example 5
Many medium voltage DC circuits are arranged with a "floating neutral" which
means (unlike a
car battery and an automotive electrical system for example) that both poles
are considered "hot"
and any circuit breaker must simultaneously cut off power from both poles to
isolate a device or
circuit. One desirable way this can be done has already been mentioned: two
single pole
commutating circuit breakers can be simultaneously triggered, one for the
relatively positive side
of the circuit, one for the relatively negative side of the circuit. In this
case, it is especially
desirable if the necessary acceleration of the shuttles can be done in such a
way that a paired set
of commutating circuit breakers are simultaneously triggered so that the
momentum effect due to
accelerating and decelerating the shuttle mass of the first commutating
circuit breaker is
counteracted by the momentum effect of accelerating and decelerating the
shuttle mass of the
second commutating circuit breaker so that the momentum that must be
transferred to the
mounting system for the pair of commutating circuit breakers is greatly
reduced.
It is also sometimes desirable to place two separate commutating circuit
breakers on a single
common shuttle. For example, the two stage axial circuit breaker of Figure 5
can readily be
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modified to break two circuits simultaneously by eliminating the connection
between the two
stages 182 and wiring the two now electrically independent halves to break the
circuit on the
positive side and the negative side of the DC circuit simultaneously.
Similarly, a rotary
commutating circuit breaker can also be designed to open two circuits
simultaneously. Such a
rotary 2-pole circuit breaker cannot use a conductive shaft that is in the
circuit as in Figures 17
and 18, but would instead need to maintain electrical separation between the
stages, similar to
Figure 6. The three commutating stages in Figure 6 can also be adapted to
interrupt all three
phases of a three phase AC circuit simultaneously, by eliminating the series-
connecting wires
236 and 256 and instead connecting each stage to one phase of the three phase
circuit.
A number of embodiments have been described. However, there are many other
implementations which have not been described in detail that will be apparent
to a person skilled
in the art utilizing the design principles elucidated herein.
What is claimed is:
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