Note: Descriptions are shown in the official language in which they were submitted.
PYROTECHNICALLY-ASSISTED
C~RRENT INTERRUPTER
. .
Background of the Invention
This invention relates to apparatus for
automatically interrupting current in an electrical
circuit under overload conditions, and more particularly
to current interrupters ~or use in relatively high
current applications.
The current in electrical circuits must be
interrupted automatically under overload or fault
conditions, especially those caused by short circuits
and the like, to prevent possible damage to the circuit
components. In relatively high power equipment~ such as
power distribution and transmission apparatus used by
utilities, the current must be limited in magnitude and
interrupted very rapidly when an overload condition
occurs, preferably within one quarter of a cycle, before
the current reaches even one amplitude peak.
Current limiting devices have been developed
which are capable of limiting the current in about 200
microseconds, and interrupting it in less than one
quarter of a cycle. Such devices generally include a
fusible element which is placed in sand or the like.
The fusible element includes one or more portions of
reduced cross-section. An overload current melts the
element at the portions of reduced cross-section,
creating arcs. The sand absorbs enough energy from the
arcs to extinguish them, and the current is interrupted.
Such high voltage fuses ha~e a relatively low continuous
current carrying capability of about 200 ~mperes.
In one known type of current limiting
interrupter with higher continuous current carrying
capability, a large cross-section~conductor or bus bar
carries the current under normal operating conditions.
When a fault such as a short circuit occurs, a linear
pyrotechnic charge breaks the conductor into segments.
..... . .
, . .. . . .
~ ..... . .
.
--2--
In relatively low voltage applications, the sum of the
arc voltage drops across the gaps thus created is
sufficient to interrupt the current. In high voltage
applications, the sum of these arc voltages would be too
small to effect current limited interruption.
Therefore, the current is commutated to a parallel
current limiting fuse and the element of the current
limiting fuse melts and causes current limited
interruption in a conventional manner.
While current interrupters of the type
described above have proven effective, they have been
relatively expensive. A substantial portion of the cost
of commercially available current interrupters of the
above-described type is attributable to electronic
equipment which is used to sense excessive current and
ignite the explosive charge. This equipment may include
isolation transformers, a current sensing transformer,
and solid-state triggering logic. An external line
voltage power source is generally needed for this
equipment which further adds to the cost of the system
and its installation. There is a need for a less
expensive triggering system for sensing excessive
current and detonating the pyrotechnic charge in
pyrotechnic current interrupters. There is also a need
for a triggering system which does not require external
power sources or external connections. Past attempts to
address these needs are disclosed in U.S. Patent No.
4,538,133 and U.S. Patent No. 4,479,105.
A general aspect of this invention is to
provide new and improved apparatus for interrupting
current in an electrical circuit.
A more specific aspect is to provide new and
improved means for igniting a pyrotechnic charge in a
current interrupter in response to excessive current
flow.
--3--
Summary of the Invention
The invention is embodied in a current
interrupter in which a pyrotechnic charge segments a bus
bar. An overload detection deviee which provides a
voltage drop that increases with increased current flow
therethrough is connected in series with the bus bar,
and the pyrotechnic charge is detonated by a low energy
detonator which is connected in parallel with the
overload detection device. During normal eurrent
eonditions the voltage across the overload detection
deviee and detonator is insufficient to trigger the
detonator, but when excessively high current flows
through the interrupter, the voltage inereases
sufficiently to trigger the detonator.
15In aeeordance with one feature of the
invention, the overload deteetion device may eomprise a
resistive element such as a strip of metal which has
resistance characteristics related to the trigger level
of the detonator such that the detonator is triggered
while the strip of metal is operating at a temperature
below its melting point. In aecordance with another
feature of the invention, control means may be provided
to control current flow through the detonator and enable
various trigger levels to be available for an
interrupter having a particular overload detection
deviee and detonator.
Brief_Deseription of the Drawing
Figure 1 is a side elevational view, taken
partially in seetion with portions broken away, of
apparatus in aeeordanee with a preferred embodiment of
the invention.
Figure 2 is a bottom view, taken partially in
seetion with portions broken away, of the apparatus of
Figure 1.
35Figure 3 is a seetional view taken along line
3-3 in Fig, 1, shown with portions broken away.
--4--
Figure 4 is a scllematic diagram illustrating
apparatus of FIG. 1 in combination with control means
and a parallel current limiting fuse.
Figures 5-10 are schematic diagrams
illustrating six different control circuits in
accordance with six respective embodiments of the
invention.
Detailed Description of Preferred Embodi~ents
The invention is generally embodied in a
pyrotechnic current interrupter 10 which employs a bus
bar 12 to conduct current under normal conditions, and
employs a linear pyrotechnic cutting charge 14 to sever
the bus bar 12 at spaced locations when excessively high
current flows through the bus bar. To sense excessively
high current and detonate the charge in response
thereto, overload detection means such as the
illustrated resistive means 16 are connected in series
with the bus bar 12, and a detonator 18 is connected in
parallel with the resistive means 16. During normal
current conditions the voltage drop across the resistive
means 16 and detonator 18 is insufficient to trigger the
detonator, but when excessively high current flows
through the interrupter 10 the voltage drop increases
sufficiently to trigger the detonator 18.
The detonator 18 has a predetermined trigger
level which may be expressed as the quantity of energy
input required to effect detonation. For purposes of
the present analysis, this energy input is dependent
upon the integral of the square of the current over
time. Similarly, the interrupt~r 10 has a trigger level
which is determined by the trigger level of the
detonator 18 and the relationship between current flow
through the detonator 18 and current flow through the
bus bar 12.
In accordance with one feature of the
invention, the electrical resistance of the detonator 18
~ ' ~
2~
--5--
is -elat~d to that of the resistive means 16 such that
when fault current begins to flow through the
interrupter 10, the current level through the detonator
18 reaches a sufficient level for a sufficient time to
trigger the detonator while the temperature of the
resistive means is below its melting point. In
accordance with a second feature of the invention,
control means 20 (FIGS. 4-10) may be provided to vary
the trigger level of the interrupter 10.
The bus bar 12 has a series of portions of
reduced cross-section 22 formed therein to facilitate
formation of gaps by the pyrotechnic charge~ The
pyrotechnic charge 14 in the illustrated embodiment is a
cord such as Primacord, and is arranged so as to have a
respective portions 14a extending transversely across
the bus bar beneath each of the portions of reduced
cross-section in the bus bar. Upon detonation, the
portions of reduced cross-section 22 are cut and folded
upward.
The bus bar 12 is partially enclosed by a
generally cylindrical housing 24. The ends 26 of the
bus bar 12 protrude from the housing 24. The housing 24
includes a generally cylindrical side wall 28 and a pair
of divider walls 32a and 32b which separate the interior
of the housing into a central compartment 34 which
contains the pyrotechnic charge 14 and detonator 18, a
first end compartment 36 which contains the resistive
element 16, and a second end compartment 38. The
central compartment 34 may be filled with air or with a
dielectric gas.
The first end compartment 36 is filled by two
plugs 30a and 30b, and a layer of sand 31 which is
disposed therebetween. The plugs 30a, 30b are
preferably made of a resin reinforced by glass fibers,
and are preferably formed in the compartment 36 so as to
conform to the shapes of the bolts, bus bar, etc.
.~:
.
.
--6--
To fill the compartment 36, the interrupter is
placed on end so that the compartment 36 is uppermost,
and a sealant is applied to the periphery of the
transverse wall 32a. Glass fibers and resin are poured
into the compartment 36 up to a level just beneath the
resistive element 16 to form the first plug 30a, then
the layer of sand 31 is added, followed by a second
layer of glass fibers and resin to form the second
plug 30b. The compartment 38 at the opposite end is
similarly filled, but with a single plug 33.
The bus bar 12 has a transverse gap 42 formed
therein and the resistive means 16 preferably comprises
one or more thin strips of silver which extend
longitudinally of the bus bar 12 across the gap 42. The
number of silver strips employed is determined by the
current rating of the interrupter~ To enable the
interrupter to carry a continuous 200 ampere A.C.
current, one silver strip is used. To enable the
interrupter to carry higher continuous current, one
additional silver strip is employed for each additional
200 amperes.
The resistance of the resistive means 16 is a
function of (1) the total cross-sectional area of the
strip or strips, (2) the width of the gap 42 -- i.e.,
the effective length of the strip or strips -- and ~3)
the resistivity of the strip or strips, which is a
function of temperature. In the embodiment of FIGS.
1-3, each of the silver strips of the invention has a
width of about 0.275 in. and a thickness of about
0.004 in. which provides a cross-sectional area of about
.0011 in.2, or .0071 cm.2. The width of the gap 42
in the bus bar is 0.5 in., or 1.27 cm. When the
interrupter operates at its rated amperage, the strips
operate at or near 50 to 100 C. Each silver strip is
soldered at both ends to the bus bar 12 and is immersed
in sand 31.
,
--7--
The preferred detonator 18 is a low-energy,
fast-acting device comprising an electrically conductive
bridgewire which contacts a primary explosive.
Detonation is effected by transfer of heat produced by
the current through the bridgewire to the primary
explosive. The preferred detonator has a resistance of
about 2-5 ohms, several orders of magnitude above that
of the resistive means 16.
The bus bar 12 is fastened to a stack of
generally rectangular insulating plates 44 by a
plurality of ver~ically extending bolts 46. The
detonator 18 is connected in parallel with the resistive
means by fastening its leads 48 to a pair of the bolts
which contact the bus bar 12 on opposite sides of the
gap 42 in end compartment 36.
The detonator 18 is supported on a bracket 50
which is mounted on one of the bolts 46. The leads 48
extend through the divider wall 32a to the adjacent
compartment 36. To prevent flow of current between the
portions o~ the bolts 46 which extend beneath the
plates 44 after detonation of the charge 14,
transversely oriented channel members 52 and angle
members 54 are bolted to the bottom of the stack of
plates 44 so as to provide transversely-extending
vertical walls between the bolts 46 that will be on
opposite sides of a gap after segmenting of the bus
bar 12 occurs.
Although the low energy detonator 18 is of
known construction, both the response time and the
trigger level of the preferred detonator are
significantly lower than those of more commonly used
commercially available high energy detonators. The
preferred detonator has an "all fire" response time of 10
microseconds when subjected to current produced by
discharge of a 0.4 microfarad capacitor charged to 50
volts. The energy input required to ensure detonation
under standard conditions is about 0.0005 joules. For
'; ; ~
., '':
,
i2~
purposes of ccnparison, more commonly used high energy
detonators may require about .003 joules for detonation.
The above-described interrupter provides
reliable current-limited interruption of high voltage
alternating current in the 200A-1000 range when
connected in parallel with a current limiting fuse, and
has a relatively low let-through current. For example,
the let-through current for the 600 ampere interrupter
described above should be about 1~,000 amperes for a
prospective fault current of 25,000 amperes (rms, sym.).
To enable more precise control of the trigger
level of the interrupter 10, the interrupter may include
control means 20 as illustrated in FIG. 4. The control
means 20 preferably includes a device which provides a
voltage drop or voltage threshold in series with the
detonator.
Referring particularly to Fig. 5, the control
means 20 shown therein comprises two series of diodes
connected in series with the detonator, in parallel with
one another and in opposite directions. The diodes as
shown provide a voltage drop or threshold in series with
the detonator to raise the trigger level of the
interrupter. Two parallel series of opposite polarity
are provided so that the trigger level of the
interrupter will not vary according to the instantaneous
direction of the A.C. current. Switches are provided to
enable current to bypass one or more of the diodes in
each series so as to provide stepwise variability of the
sensitivity of the interrupter. Because the switches
enable external adjustment of the control unit identical
control units can be used for interrupters having
various trigger levels, and selection of the trigger
leyel of a particular interrupter can be made in the
~actory simply by setting of the switches to appropriate
positions.
- 9 -
To enable continuous rather than stepwise
adjustment of the trigger level of the interrupter,
variable resistors are provided in the embodiments of
FIGS. 7 and 10 in series with the detonator 18. The
variable resistors provide a continuously variable
voltage drop in series with the detonator. To prevent
unintended triggering of the detonator due to power
surges of very short duration, a capacitor may be
connected in parallel with the detonator as shown in
FIG. 10.
FIG~. 6, 8 and 9 illustrate control means 20
which are not externally adjustable. In these
embodiments of the invention, the trigger level of the
interrupter is selected simply by selection of
appropriate components for the control unit 20 rather
than by manipulation of a control. FIG. 6 illustrates a
control unit 20 wherein a pair of zener diodes are
arranged in parallel with one another and in series with
the detonator, and oriented in opposite directions. As
in the control means of FIG. 5, the orientation of the
diodes in parallel and in opposite directions enables
the control means to provide a threshold-type trigger
level for the interrupter which is not dependent on the
instantaneous direction of the AC current. The control
means of FIG. 9 employs a metal oxide varistor (MOV)
which has relatively high resistance when subjected to
relatively low voltage in either direction, and has
significantly lower resistance when sub~ected to higher
voltage in either direction. As in the previously
described control means of FIGS. 5~7 and 10, the control
means of FIG. 9 is independent of the instantaneous AC
current direction.
It may be desirable in some applications to
provide an interrupter having a trigger level which
differs depending upon the instantaneous current flow.
For such an application, the device of FIGo 8 employing
--10--
a single zener diode in series with the de~on~:or may be
suitable.
From the foregoing, it should be evident that
the invention provides a novel and improved current
interrupter. The invention is not limited to the
particular embodiments described above or to any
particular embodiments, but is defined by the following
claims.
. .
