Note: Descriptions are shown in the official language in which they were submitted.
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ARCING FAULT PROTECTION SYSTEM
FOR A SWITCHGEAR ENCLOSURE
FIELD OF THE INVENTION
The present invention relates generally to protective devices for electrical
switchgear and, more particularly, to the protection of electrical switchgear
from arcing
fault currents.
BACKGROUND OF THE INVENTION
Switchgear enclosures are commonly employed in electrical power distribution
systems for enclosing circuit breakers and switching equipment associated with
the
distribution system. Typically, switchgear enclosures are comprised of a
number of
individual stacked or adjacent compartments. each of the switchgear
compartments
receiving electrical power from a power source and distributing the electrical
power
through a feeder circuit to one or more loads. Generally, each of the
switchgear
compartments includes circuit breakers for interrupting electric power in a
particular
feeder circuit in response to hazardous current overloads in the circuit.
Switchgear is a general term covering switching and interrupting devices and
their combination with associated control, instruments, metering, protective
and
regulating devices, also assemblies of these devices with associated
interconnections,
accessories, and supporting structures used primarily in connection with the
generation,
transmission, distribution, and conversion of electric power. The following
paragraphs
describe switchgear characteristics in accordance with ANSI/IEEE Standards No.
C37.20.2-1987.
A switchgear assembly generally refers to assembled equipment (indoor or
outdoor) including, but not limited to, one or more of the following:
switching,
interrupting, control, instrumentation, metering, protective and regulating
devices,
together with their supporting structures, enclosures, conductors, electrical
2 5 interconnections, and accessories. A switchgear assembly may be completely
enclosed
on all sides and top with sheet metal (except for ventilating openings and
inspection
windows) containing primary power circuit switching or interrupting devices.
or both.
with buses and connections referred to as metal-enclosed (ME) power
switchgear. The
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assembly may include control and auxiliary devices. Access to the interior of
the
enclosure is usually provided by doors or removable covers, or both.
Metal-enclosed power switchgear may include one or more of the following
features:
( 1 ) The main switching and interrupting device is of the removable (drawout)
type arranged with a mechanism for moving it physically between connected and
disconnected positions and equipped with self aligning and self coupling
primary
disconnecting devices and disconnectable control wiring connections.
(2) Major parts of the primary circuit, that is, the circuit switching or
interrupting devices, buses, voltage transformers, and control power
transformers, are
completely enclosed by grounded metal barriers, that have no intentional
openings
between compartments. Specifically included is a metal barrier in front of or
a part of
the circuit interrupting device to ensure that, when in the connected
position. no primary
circuit components are exposed by the opening of a door.
(3) All live parts are enclosed within grounded metal compartments.
(4) Automatic shutters that cover primary circuit elements when the
removable element is in the disconnected, test, or removed position.
(5) Primary bus conductors and connections are covered with insulating
material throughout.
2 0 (6) Mechanical interlocks are provided for proper operating sequence under
normal operating conditions.
(7) Instruments, meters, relays, secondary control devices and their wiring
are
isolated by grounded metal barners from all primary circuit elements with the
exception
of short lengths of wire such as at instrument transformers terminals.
(8) The door through which the circuit interrupting device is inserted into
the
housing may serve as an instrument or relay panel and may also provide access
to a
secondary or control compartment within the housing.
Switchgear leaving all of the above eight features is referred to as metal-
clad (MC) Switchgear. Metal-clad switchgear is metal-enclosed, but not all
metal-
3 0 enclosed switchgear is metal-clad.
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The ratings of a switchgear assembly are designations of operating limits
under
specif ed conditions of ambient temperature, temperature rise, etc. Where the
switchgear
assembly comprises a combination of primary and secondary circuits, each may
be given
ratings.
ME switchgear usually has the following ratings:
( 1 ) Rated maximum voltage
(2) Rated frequency
(3) Rated insulation levels
(4) Rated continuous current
(5) Rated short-time current
(6) Rated momentary current
The designated ratings in this standard are preferred but are not considered
to be
restnctive.
In addition to these ratings, a switchgear assembly may have interrupting or
switching capabilities, which are determined by the rating of the particular
interrupting
and switching devices that are integral parts of the switchgear assembly.
The rated maximum voltage of ME switchgear is the highest nns voltage for
which the equipment is designed, and is the upper limit for operation.
The rated insulation levels of ME switchgear includes two items.
2 0 ( 1 ) Low frequency 1 min withstand voltage
(2) Impulse withstand voltage
The rated maximum voltages, and corresponding insulation levels for ME
switchgear are listed in tabular form in ANSI/IEEE C37.30.2-1987.
The rated frequency of a device, or an assembly, is the frequency of the
circuit for-
t 5 which it is designed. (Ratings are usually based on a frequency of 60 Hz).
The rated continuous current of ME switchgear is the maximum current in nns
amperes at rated frequency, which can be carried continuously by the primary
circuit
components, including buses and connections, without causing temperatures in
excess of
specified limits for
3 0 ( 1 ) Any primary or secondary circuit component
(2) Any insulating medium, or structural or enclosing member
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The specified temperature limits applicable to switchgear assemblies are given
in
ANSI/IEEE C37.20.2-1987, ~~ 4.5.1 through 4.5.6.
The continuous current ratings of the main bus in ME switchgear are listed in
ANSI/IEEG C37.20.2-1987.
The continuous current rating of the individual circuit-breaker compartment
shall
be equal to the ratings of the switching and interrupting devices used, except
as may be
modified by lower continuous current ratings for current transformers, power
fuses etc.
The rated momentary current of ME switchgear is the maximum rms total current
that it shall be required to withstand. The current shall be the rms value,
including the
direct-current component, at the major peak of the maximum cycle as determined
from
the envelope of the current wave during a test period of at lease 10 cycles
unless limited
to a shorter time by the protective device.
The momentary current ratings of the individual circuit-breaker compartments
of
ME switchgear shall be equal to: The circuit breaker close and latch, switch
fault close,
or asymmetrical momentary current ratings of the switching devices used.
The rated short-time current of the ME switchgear is the average rms current
that
it can carry for a period of 2 sec. unless limited to a shorter time by the
protective device
or current transformer ratings.
The short-time current ratings of the individual circuit-breaker compartments
of
2 0 the ME switchgear shall be equal to the short-time ratings of the
switching and protective
devices used or the short time rating of the current transformers (see
ANSI/IEEE C57.13-
1978 (R 1986) [10]).
The limiting temperature for ME switchgear is the maximum temperature
permitted.
2 5 In addition to current overloads, the switchgear enclosure may encounter
other
hazardous conditions known as arcing faults. Arcing faults occur when electric
current
"arcs" or flows through ionized gas between conductors, between two ends of
broken or
damaged conductors, or between a conductor and ground in the switchgear
enclosure.
Arcing faults typically result from corroded, worn or aged wiring or
insulation, loose
30 connections and electrical stress caused by repeated overloading, lightning
strikes, etc.
Particularly in medium- to high-voltage power distribution systems, the
ionized gas
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associated with arcing faults may be released at pressures and temperatures
sufficient to
damage the switchgear equipment.
Presently, the most commonly employed method for enhancing the durability of
switchgear enclosures in the event of arcing faults is to provide arc-
resistant metal
switchgear compartments to the above-described MC (metal clad) standards, with
a
means for venting the gases from the compartments in the event of an arcing
fault. These
compartments are designed to withstand the pressures and temperatures of the
gases
associated with an arcing fault and reduce the likelihood or extent of damage
to
switchgear equipment by preventing the gases from entering adjacent switchgear
compartments. However, because these systems do not eliminate the generation
and
release of hot gases associated with arcing faults, they do not eliminate the
risk of
damage to the switchgear equipment.
Cronin, U.S. Patent No. 4,130,850 is directed to a high speed fault diverter
switch
for a gas-insulated substation. However, the switch referred by Cronin is a
High Voltage
switch which would be used in a GIS (Gas Insulated Substation). The lowest
typical
voltage application for this type of system would be at 60-145 kV which
requires an
impulse voltage level of up to 650 kV. For this application and in order to
withstand the
high voltage, the open gap of the contacts would be typically around 4 inches.
However,
Cronin discloses only a conventional switch, which would require 3 to 4 cycles
(i.e., 48-
2 0 64 in sec.) to operate. Cronin speaks of the rise of the high pressure
being extremely
rapid; indeed, experience has shown that the arc should be controlled within 4
milliseconds.
Therefore, the conventional switch will not work, because the contacts must
travel 4 inches in 4 milliseconds which gives an approximate average velocity
of 25
2 5 meters per second. Since, the acceleration time is 4 ms. and the initial
velocity is 0 then
the required constant acceleration is about 12,500 meter second squared.
And when the mass of the contact and the drive system is taken into
consideration
(assume a typical mass of 6 kg) the required force is 75 kilo-newtons which
for the
required 4 inch or about 100 mm stroke gives an energy requirement of 7.5 kilo-
newton-
3 0 meter. Presently available switch mechanisms are incapable of fulfilling
this
requirement, in fact, they are at least one order of magnitude short of this
level of energy.
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Therefore, Cronin presents a problem without a practical solution. That is,
the switch
required by Cronin does not exist. -
The use of the electrodynamic drive would not be feasible either because the
discharge pulse is generally in the order of 1 to 2 milliseconds. For example,
the
discharge pulse described in Diebold U.S. Patent No. 2,971,130. is a 1
millisecond pulse.
With a 1 ms pulse discharge and assuming that the transfer efficiency of the
system is
20% at best, the components required can be estimated. Energy from the
capacitor is E2
C =30.000 newton-meters. A reasonable voltage to charge the capacitor would be
3,000
volts in which case a capacitor of 0.0033 farads is needed (this is a rather
large capacitor)
or if a more common capacitor of 100 micro-farads is used then the charging
voltage
would be 300 kV.
Accordingly, there is a need for a system of protecting switchgear enclosures
and
from arcing faults in a manner which is very rapid and reduces or eliminates
the
generation of ionized gases at high temperatures and pressures. The present
invention is
1 S directed to addressing this need.
SUMMARY OF THE INVENTION
In one embodiment of the present invention, there is provided an arcing fault
protection system for a switchgear enclosure accommodating a plurality of
feeder
2 0 circuits. Each of the feeder circuits is electrically connected to a
source bus and carries
an electric current through the switchgear enclosure toward one or more loads
downstream of the switchgear enclosure. The arcing fault protection system
comprises
or_e or more arcing fault detectors for monitoring the feeder circuits for the
presence of
arcing fault currents and for producing an arcing fault detection signal upon
detecting
2 5 arcing fault currents in any of the feeder circuits. and an arc diverting
device for rapidly
diverting the current from the source bus in response to the production of an
arcing fault
detection signal. The arc diverting device diverts current carried on the
source bus to
ground, or to another phase in an ungrounded three phase system, and rapidly
eliminates
arcing fault currents occurring on any of the feeder circuits. The rapid
elimination of
3 0 arcing fault currents substantially reduces or eliminates the generation
of hot gases
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associated.with arcing faults and obviates the need to provide an arc-
resistant switchgear
enclosure or to vent gases from the enclosure.
In another embodiment of the present invention, the arc diverting device
comprises a mechanical switch rapidly movable from an open position to a
closed
position. The mechanical switch includes a movable contact and a stationary
contact.
One of the contacts is electrically connected to the source bus and the other
of the
contacts is electrically connected to ground or to another phase in a three
phase
ungrounded system. In the open position of the mechanical switch, the movable
contact
is in a first longitudinal position, apart from the stationary contact. In the
closed position
of the mechanical switch, the movable contact is in a second longitudinal
position.
electrically connected to the stationary contact. A latching mechanism
comprises a
portion of the mechanical switch. The latching mechanism holds the movable
contact in
the first longitudinal position, defining a latched position, or releases the
movable contact
from the first longitudinal position, defining an unlatched position,
depending on the
status of the mechanical switch. A driving mechanism rapidly drives the
latching
mechanism from the latched position to the unlatched position and accelerates
the
movable contact toward the second longitudinal position in response to
activation of a
triggering mechanism.
In one embodiment of the present invention, the latching mechanism includes a
2 0 latch core oriented adjacent to the driving mechanism, the latch core
being movable
coincident to the driving mechanism and communicating movement to the movable
contact in response to activation of the triggering mechanism. An outer
surface of the
latch core defines a holding surface and a recessed releasing surface. A
stationary latch
support is oriented transverse to the latch core and has an inner surface
defining a
2 5 retaining member. A plurality of ball bearings are disposed between the
latch core and
the latch support. The ball bearings are held into engagement with the
retaining member
by the holding surface of the latch core when the latching mechanism is in the
latched
position, and collapse inwardly toward the releasing surface and become
released from
the retaining member when the latching mechanism is in the unlatched position.
3 0 In still another embodiment of the present invention, the arc diverting
device
comprises a first and second thyristor connected from the source bus to
ground. The first
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and second thyristors include respective first and second gate terminals
responsive to the
arcing fault detection signal. The first and second-thyristors block current
flow when the
arcing fault detection signal is not applied to their respective gate
terminals and permit
current flow when the arcing fault detection signal is applied to their
respective gate
terminals.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings, which comprise a portion of this disclosure:
FIG. 1 is a block diagram of an arcing fault protection system for a
switchgear
enclosure according to one embodiment of the present invention;
FIG. 2 is a block diagram illustrating one embodiment of a portion of the
circuit
of FIG. 1 which generates an arcing fault detection signal;
FIG. 3 is a side sectional view of a prior art vacuum interrupter which forms
a
portion of a mechanical switch which may form part of the arc diverter in the
system of
FIG. 1;
FIG. 4 is a side sectional view of another portion of a mechanical switch
which
may be used in conjunction with the vacuum interrupter of FIG. 3, illustrating
both an
open and closed position of the mechanical switch;
FIGS. Sa through Sd are schematic diagrams of solid-state switches ur~hich may
2 0 form part of the arc diverter in the system of FIG. 1;
FIG. 6 is a schematic diagram of an additional circuit which may be added to
the
mechanical switch of FIGS. 3 and 4 or the solid state switches of FIGS. Sa-Sd;
and
FIG. 7 illustrates a hybrid mechanical/solid state switch.
2 5 DETAILED DESCRIPTION
Turning now to the drawings and referring first to FIG. I , there is shown a
switchgear enclosure, generally designated by reference numeral 10, including
individual
compartments 10a, l Ob, I Oc and l Od for housing various components of an
electrical
distribution system 12. A power source 14, which may comprise, for example, a
utility
30 company power transformer, supplies power for the distribution system 12
through a
main circuit 16. The main circuit 16 is typically routed through a main
breaker,
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designated here by reference numeral 18. An instantaneous current (or voltage)
sensor
20 such as a current transformer (C.T.) may also be provided for monitoring
the main
circuit 16 for characteristics of arcing faults. A source bus 22 connected to
main circuit
16 distributes electrical power from the power source 14 to a plurality of
feeder circuits
24a, 24b, 24c, each of which is routed through one of the switchgear
compartments 10.
Each of the feeder circuits 24 typically supplies power to one or more loads
(not shown)
downstream of the switchgear enclosure 10. It will be appreciated that the
number of
feeder circuits 24 shown here, as well as the number of switchgear
compartments 10, is
exemplary only, and may be varied according to the particular type and/or
application of
the switchgear enclosure 10.
The switchgear enclosure 10 typically includes switching and monitoring
equipment associated with the respective feeder circuits 24. For example, in
the
embodiment shown in FIG. 1, the switchgear enclosure 10 includes a plurality
of circuit
breakers 26a,b,c and a plurality of optical sensors 28a,b,c. In one
embodiment, the
circuit breakers 26 and optical sensors 28 comprise devices known in the art
which are
mounted within the respective switchgear compartments l0a,b,c and are
associated with
one of tile feeder circuits 24a,b,c. The circuit breakers 26 are provided for
interrupting
electric power in the respective feeder circuits 24 in response to current
overloads and the
optical sensors are provided for monitoring the respective feeder circuits 24
for the
2 0 presence of arcing faults. Again, however, it will be appreciated that the
electrical
components shown here are exemplary only, they may be replaced, eliminated or
supplemented with other components, according to the particular type and/or
application
of the switchgear enclosure.
In accordance with one aspect of the present invention, an arc diverter 32 is
2 5 connected between the source bus 22 and ground. In the case of an
ungrounded (i. e.
"delta") system (as shown in FIG. 6), the arc diverter 32 is connected between
the phase
lines of the system. The arc diverter 32, upon receipt of an arcing fault
detection signal
34, quickly connects or "crow-bars" the source bus 22 to ground (or to another
phase line
in an ungrounded system), thereby extinguishing arcing fault currents which
may have
3 0 occurred on any of the feeder circuits 24 before they are permitted to
generate gases at
dangerous pressures and/or temperatures. In one embodiment, for example, the
arcing
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fault currents are extinguished in less than about 4 milliseconds, effectively
eliminating
the generation of dangerous gases associated with the arcing fault. The
present invention
therefore can eliminate the need to manufacture the switchgear enclosure 10
according to
conventional metal-clad (MC) arc-resistant designs or to vent gases from the
enclosure
5 10.
As will be described in greater detail hereinafter, the arc diverter 32 may
comprise a mechanical switch, a solid-state switch or a hybrid mechanical and
solid-state
switch. The arc diverter 32 may be mounted in one of the switchgear
compartments, as
shown here, or may be mounted in a separate compartment external to the
switchgear
10 enclosure 10.
FIG. 2 illustrates one embodiment in which the arcing fault detection signal
34 is
generated by a combination of a current sensor 20 monitoring the main circuit
16, and
optical sensors 28 monitoring the feeder circuits 24. It will be appreciated,
however, that
the arcing fault detection signal 34 may be generated by any of several other
configurations of sensors including, for example, a system where optical
sensors 28 and
current sensors 20 are employed in each feeder circuit 24, or a system
including only
optical sensors or only current sensors.
The sensor 20 comprises an instantaneous current (or voltage) sensor, such as
a
current transformer (C.T.), for sensing the instantaneous magnitude of the
current (or
2 0 voltage) in the monitored line. The sensor 20 produces an arcing fault
detection signal,
designated in FIG. 2 by reference numeral 36, if it determines that an arcing
fault is
present on the main circuit 16 or a feeder circuit 24.
The optical sensors 28 may comprise any type of optical sensor known in the
art
such as, for example, the optical sensor described in U.S. Patent No.
4,369,364 and
2 5 commercially available from BBC Brown, Boveri & Company Limited, Baden,
Switzerland. The optical sensors 28 are sensitive to light impulses
representing the
occurrence of arcing faults within the switchgear enclosure 10 and produce an
arcing
fault detection signal, designated in FIG. 2 by reference numeral 38, if they
determine
that an arcing fault is present on any of the feeder circuas 24.
30 In one embodiment, as shown in FIG. 2, the respective arcing fault
detection
signals 36,38 are fed to an AND gate 40. which produces a consolidated arcing
fault
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detection signal 34 to trigger arc diverter 32 only when arcing fault
detection signals are
provided by both the current sensor 20 and cptical sensor 28. This arrangement
minimizes the chance that switching will occur due to "false" signals because
it is
unlikely that false signals will be detected by both the current sensor 20 and
the optical
sensor 28.
In one embodiment of the present invention, the arc diverter 32 in the system
of
FIG. 1 comprises a mechanical switch for rapidly shorting or "crow-barring"
the source
bus 22 to ground in response to the receipt of an arcing fault detection
signal 34. One
portion of the mechanical switch may consist of a standard, commercially
available
vacuum interrupter 52, also known as : "vacuum bottle," such as the one shown
in FIG.
3. The vacuum interrupter 52 is oriented generally about a longitudinal axis
6~ and
comprises a cylindrical chamber 54 for housing a movable contact 56 and a
stationary
contact 58. Alternatively or additionally, the vacuum interrupter 52 may
include a set of
contacts immersed in an insulating medium such as, but not limited to,
sulfurhexaflouride gas (SF6) or oil.
The stationary contact 58 is electrically connected to the source bus 22 {FIG.
1 )
by a connecting rod 60. The movable contact 56 is connected via a connecting
rod 62 to
a driving mechanism, one example of which will be described in detail later,
and is
shown in FIG. 4. Normally, in the absence of an arcing fault detection signal,
the
2 0 movable and stationary contacts 56, 58 are separated, defining an open
position of the
vacuum interrupter 52. In one embodiment, the separation or gap between the
artifacts
56, 58 when in the open position is from about 6 mm to about 10 mm, for
example. about
8 mm. As shown in FIG. 3, the movable and stationary contacts 56, 58 are
engaged,
defining a closed position of the mechanical switch.
2 5 According to oae aspect of the present invention, the closing of the
vacuum
interrupter 52, e.g., the movement from the open position to the closed
position, is
accomplished very rapidly so as to substantially eliminate the generation of
gases
associated with arcing faults. More specifically, the movable contact 56 is
rapidly
moved toward the stationary contact 58, from a first longitudinal position in
which the
30 movable contact 56 is separated from the stationary contact 58 (i.e., in
the open position
of the vacuum interrupter 52), to a second longitudinal position in which the
movable
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contact ~6 is engaged with and electrically connected to the stationary
contact ~8 (i.e., in
the closed position of the vacuum interrupter 52, W hick is shown in FIG. 3).
Preferably,
the closing of the switch is accomplished in less than about 4 milliseconds.
FIG. 4 illustrates a structure, designated by reference numeral 64, that may
be
used to drive the movable contact 56 into engagement with stationary contact
58
according to one embodiment of the present invention. The structure 64 is
oriented
generally about longitudinal axis 6~ and aligned with the vacuum interrupter
52. The
left-hand half of FIG. 4 shows the structure 64 as it would appear when the
vacuum
interrupter 52 is in an open position, in the absence of an arcing fault
detection signal 34.
The right-hand half of FIG. 4 shows the structure 64 as it would appear when
the vacuum
interrupter 52 is in a closed position, after having shorted or ''crow-barred"
the source
bus 22 to ground in response to receipt of an arcing fault detection signal
34.
The structure 64 consists generally of a latching mechanism 66, a driving
mechanism 68 and a triggering mechanism 70. The latching mechanism 66 includes
a
latch core 72, a latch support 74 and a plurality of ball bearings 76 disposed
between the
latch core 72 and the latch support 74. A piston 78 is positioned between the
latching
mechanism 66 and the connecting rod 62. Generally, the driving mechanism 68,
latch
core 72, ball bearings 76 and piston 78 are adapted for rapid upward movement
along
longitudinal axis 65 when the vacuum interrupter ~2 is actuated by the
triggering
mechanism 70, as will be described in detail hereinafter, to drive the movable
contact 56
into engagement with the stationary contact 58.
The outward-facing surface of the latch core 72 includes a holding surface 80,
an
inclined surface 82 and a recessed surface 84. The inward-facing surface of
the latch
support 74 includes a retaining groove 86. The ball bearings 76 are adapted to
move in
2 5 both longitudinal and transverse directions relative to the latch core 72
and latch support
74, depending on the operational status of the latching mechanism 66. In the
left-hand
side of FIG. 4, the latching mechanism 66 is shown in a latched position, in
which the
ball bearings 76 are held between the retaining groove 86 of the latch support
74 and the
holding surface 80 of the latch core 72. In this position, the piston 78 (and
hence
3 0 connecting rod 62 and movable contact 56) is restrained from longitudinal
movement,
maintaining the vacuum interrupter 52 in the open position.
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In the right-hand side of FIG. 4, the latching mechanism 66 is shown in an
unlatched position, in which the piston 78 (and hence movable contact 56) is
released
and permitted to move along longitudinal axis 65 toward the stationary contact
58. The
release of the latching mechanism 66, e.g., from the latched position to the
unlatched
position, occurs in response to upward movement of the latch core 72. More
specifically,
when the latch core 72 is moved upwardly along the longitudinal axis 65, the
holding
surface 80 of the latch core 72 is advanced beyond the longitudinal position
of the ball
bearings 76, causing the ball bearings 76 to collapse inwardly toward the
recessed
surface 84 of the latch core 72. The inward movement of the ball bearings 76
causes
them to become released from the retaining groove 86 of latch support 74,
thereby
"unlatching" the latching mechanism and permitting the vacuum interrupter 52
to move
toward its closed position.
The driving mechanism 68 is a cylindrical disk oriented about the longitudinal
axis 65 and adjacent to a bottom surface of the latch core 72. In one
embodiment, the
driving mechanism 68 comprises a metal disk, but it will be appreciated that
other
materials may be employed. The driving mechanism 68 is adapted for rapid
upward
movement along the longitudinal axis 65, in response to production of an
accelerating
force by the triggering mechanism 70. The driving mechanism 68, in conjunction
with
triggering mechanism 70, must accordingly be capable of developing significant
2 0 velocities over a short distance. For example, in one embodiment, it is
expected that the
closing of vacuum interrupter 52 will be accomplished in two to four
milliseconds.
Assuming a travel distance of about 6 to about 10 mm, (corresponding to a
typical
distance between movable and stationary contacts 56, 58, when in the open
position, in a
vacuum interrupter 52 of the type shown in FIG. 3), the driving mechanism 68
must be
2 5 capable of producing an average velocity of about 4 to 5 meters per
second. However, it
will be appreciated that this operating speed is exemplary only. The operating
speed
required for any particular application is dependent on the distance between
contacts as
well as the pressure and type of insulating medium (if any) between contacts.
In the left-hand side of FIG. 4, the driving mechanism 68 is shown adjacent to
the
30 triggering mechanism 70, as it would appear when the vacuum interrupter 52
is open. In
the right-hand side of FIG. 4, the driving mechanism is shown as it would
appear when
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the vacuum interrupter 52 is closed, having traveled a distance corresponding
to the
separation distance of the movable and stationary contacts 56, ~8. The upward
movement of the driving mechanism 68 causes corresponding movement of latch
core
72, causing a release of latching mechanism 66 and ultimately closing the
vacuum
interrupter 52. As stated previously, closing of the vacuum interrupter 52
quickly
extinguishes any arcing fault currents present in the distribution system, by
rapidly
shorting or "crow-barring" the source bus 22 to ground (or, to another phase,
in an
ungrounded system).
In one embodiment, the triggering mechanism 70 comprises a flat (pancake-type}
radially wound coil having a face located adjacent to the driving mechanism
68.
Generally, the coil 70 is connected to an energy source (not shown) which is
activated in
response to production of an arcing fault detection signal by the protection
system. In
one embodiment, the energy source comprises one or more capacitors (not shown)
charged to a voltage in the range of hundreds or thousands of volts, depending
on the
particular application or rating of the vacuum interrupter 52. In one example,
a 700
microfarad capacitor may be charged to 800 volts; where the gap between the
open
contacts 56, 58 is from about 6 to about 8 millimeters. Upon generation of an
arcing
fault detection signal, energy from the energy source is released into the
coil 70, causing
an electrical current to be conducted in the coil 70. The presence of
electrical current in
2 0 the coil 70 causes a repulsion force to be produced which is proportional
to the number
of turns of the coil 70 and the current carried by the coil 70. The repulsion
force is
directed along the longitudinal axis 65 toward the driving mechanism 68,
imparting a
high instantaneous acceleration to the driving mechanism 68 and causing a
quick release
of the latching mechanism 66, in the manner heretofore described. The
accelerating
2 5 force may be supplemented by an additional force associated with the
mechanical switch.
For example, the supplemental force may be provided by a compressed spring
(not
shown), or by pneumatic or hydraulic operation.
In one embodiment, the movable components of the mechanical switch of FIGS.
3 and 4 are relatively low mass components so as to minimize the amount of
energy
30 required to close the contacts 56, 58.
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is
Now turning to FIGS. Sa through ~d, there is shown a series of solid-state
switches which may be used in the system of FIG: 1. Components common to the
various switch embodiments will be designated by common reference numerals
throughout, although the different switch embodiments shown in FIGS. Sa
through Sd
will be designated by different reference numerals. Generally, each of the
respective
solid state switch embodiments of FIGS. Sa-d includes a pair of thyristors 90,
92
connected in parallel but in opposite polarity between the source bus 22 and
ground of an
electrical distribution system, so as to handle both positive - and negative-
going portions
of the AC power on the bus 22. In operation, each of the thyristors 90, 92
illustrated
diagramatically in FIGS. Sa-d. may comprise a number of thyristors coupled in
series and
parallel arrays to meet the voltage and current handling requirements of the
system. Gate
terminals of thyristors 90, 92, designated respectively by reference numerals
94 and 96,
are connected to the AND gate 40 (or, alternatively, directly to the arcing
fault sensors 20
and/or 28) to receive an arcing fault detection signal 34 upon the detection
of an arcing
fault by the protection system.
Generally, with no arcing fault signal 34 provided to the gate terminals 94,96
(e.g., with no arcing fault having been detected), the thyristors 90, 92 are
"off ' and do
not permit current to flow through the thyristor. With an arcing fault signal
34 applied to
the gate terminals 94,96 , the thyristor is turned "on" and current is
permitted to flow
2 0 through the thyristor, thereby effectively short-circuiting the source bus
22 and
extinguishing any arcing fault currents present in the system. Once the
thyristors 90, 92
begin to conduct electrical current in response to application of the gate
signal, the
current flow through the thyristors 90, 92 does not generally shut off, even
if the gate
signal is removed, until the current flow is reduced below a threshold level,
most likely
2 5 by the main breaker 18 (FIG. 1 ) in the distribution system. It will be
appreciated,
however, that the present invention is not limited to the use of thyristors,
but may utilize
other forms of solid-state devices such as. for example, insulated-gate
bipolar transistors
(IGBTs). A system employing IGBTs will operate generally the same as a system
employing thyristors, the difference being that current flow through an IGBT
is blocked
3 0 when the gate signal is removed.
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FIG. Sa illustrates a solid-state switch 98 in a basic embodiment including a
pair
of thyristors 90, 92 connected in parallel between the source bus 22 and
ground, as
heretofore described. With no arcing fault signal 34 provided to the gate
terminals 94,96
(e.g., with no arcing fault having been detected). the thyristors 90, 92 do
not permit
current to flow through the thyristor. With an arcing fault signal 34 applied
to the gate
terminals 94,96 , the thyristors conduct electric current from the source bus
22 to ground,
thereby short-circuiting the source bus 22 and extinguishing any arcing fault
currents
present in the system. The thyristors 90. 92 are biased in a manner such that
the first
thyristor 90 begins conducting current from the source bus to ground
coincident to a
positive half cyle of alternating electric current on the source bus 22, and
the second
thyristor 92 begins conducting current from the source bus to ground
coincident to a
negative half cyle of alternating electric current on the source bus 22.
FIG. Sb illustrates a solid-state switch 100 according to an alternative
embodiment of the present invention. The solid-state switch 100 includes
respective
thyristors 90, 92 having respective gate terminals 94,96 responsive to an
arcing fault
detection signal, as heretofore described. The solid-state switch 100 further
includes a
shunt shorting contact 102 connected between the source bus 22 and ground. The
shunt
shorting contact 102 generally comprises a relatively slow operating switch
(e.g., about
one-half cycle--16m sec.) which is also triggered in response to the detection
of an arcing
fault (e.g., upon receipt of the arcing fault detection signal 34) to provide
an alternate
current path 104 from the source bus 22 to ground. In one embodiment, the
shunt
shorting contact 102 is triggered to provide the alternate conducting path 104
before the
thyristors 90, 92 have conducted for much more than about one half cyle of
alternating
electric current. This is an advantageous feature because typical thyristors
are rated to
2 5 withstand high currents for no more than about one half cycle. The
provision of shunt
shorting contact 102 prolongs the operable life of the thyristors 90, 92
because it
decreases the likelihood that the high current rating of the thyristors 90, 92
will be
exceeded.
FIG. Sc illustrates a solid-state switch 110 according to another alternative
embodiment of the present invention. The solid-state switch 110 includes
respective
thyristors 90, 92 having respective gate terminals 94,96 responsive to an
arcing fault
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detection signal, as heretofore described. To protect the thyristors 90, 92
fiom excess
voltages and/or currents, the solid-state switch I lD further includes a
current-limiting
reactance I 12 and a voltage arrestor 1 14 such as a varistor. The current
limiting
reactance I 12 is connected between the source bus 22 and the thyristors 90,
92 so that,
' 5 when the thyristors 90, 92 are conducting, the current flowing through the
respective
thyristors 90, 92 is limited, preferably to a level that does not exceed the
current rating of
the thyristors 90, 92. Similarly, the voltage arrestor 114, such as a metal
oxide varistor
(MOV), is connected in parallel to the thyristors 90, 92 and is selected to
clamp the
voltage across thyristors 90, 92 to a level that does not exceed the rated
breakdown
voltage of the thyristors 90, 92.
FIG. Sd illustrates a solid-state switch 120 according to still another
alternative
embodiment of the present invention. The solid-state switch 120 includes all
of the
components of the solid-state switch I 10 shown in FIG. ~c, with the addition
of a shunt
shorting contact 122. The shunt shorting contact 122 serves substantially the
same
function as the shunt shorting contact 102 described in relation to FIG. Sb,
providing an
alternate conducting path 124 for the current flowing between the source bus
22 and
ground. Preferably, the shunt shorting contact 122 is triggered to provide the
alternate
conducting path 124 before the thyristors 90, 92 have conducted for more than
about one
half cyle of alternating electric current, for the reasons heretofore
described in relation to
2 0 FIG. Sb.
It will be appreciated that any of the solid-state switches described in
relation to
FIGS. Sa .through Sd may be used in combination with a mechanical switch. such
as that
described in relation to FIGS. 3 and 4, to define a hybrid solid-state and
mechanical arc
diverter circuit 32 in the system of FIG. I . One example of this is shown in
FIG. 7 and
2 5 described below.
Referring now to FIG. 6, there is shown an additional circuit which may be
added
in series between either the mechanical switch shown on FIGS. 3 and 4 or any
of the
solid state switches shown on FIGS. ~a-d and ground, or another phase line, in
an
ungrounded three phase system. This additional circuit component comprises the
3 0 combination of a resistor 125 and a fuse 126 connected in parallel. In
FIG. 6. two such
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lg
circuits are illustrated for a three-phase system, that is between each of the
A and C
Phase Lines and the B Phase Line.
This circuit will prevent a sudden short circuit condition from occurring in
the
protected line when the switch initially closes. The fuse 126 will be a
relatively fast
acting fuse, for example, on the order of '/4 to '/2 milliseconds, and may be
rated from I 0
to 250 amps, for example 30 amps. Taking into account stray inductance in the
circuitry
and wiring, it might be expected that about one millisecond will elapse before
the current
is totally transferred from the fuse 126 to resistor 125, following the
opening of the fuse.
However, since a typical arcing event lasts from 10 to 100 microseconds before
the
arcing space de-ionizes the '/4 to '/2 millisecond provided by the fuse is
long enough to
accommodate the arcing condition, and yet short enough to prevent significant
overcurrent from flowing through the protected circuit.
When the fuse opens, the resistor will provide an additional resistance in
series to
limit the current in the circuit, while still clamping the circuit to ground.
The resistor
value may be selected to limit the current to the circuit on the order of no
more then
about 2 to 2 '/z times the continuous current rating of the circuit. For
example, in a 1200
amp continuous rated circuit, the resistor might be selected to permit a
current flow of
about 3 K amps. The use of a fuse will avoid the delay which might be
experienced with
the addition of the resistance alone, that is the time in which it might
otherwise take the
arc voltage to rise sufficiently to drive significant current through the
resistor.
Referring briefly to FIG. 7, a simplified diagrammatic showing of a hybrid
mechanical and electronic switch is illustrated. The mechanical switch 130 may
be of
similar design to the switch shown on FIGS. 3 and 4, while the electronic
switch 140
may be of similar design to the electronic switches shown on FIGS. Sa through
Sd. In
this embodiment, the mechanical switch 130 and electronic switch 140 are wired
in
series between the buss 22 or other line to be protected and ground, or, in an
ungrounded
three-phase system between the protected phase and another phase. The same
triggering
line 34 carries the arcing fault detection signal. However, a delay (not
shown) is
employed to delay the triggering of the solid state switch 140 until the
mechanical switch
130 closes. Therefore, the mechanical switch closes before the flow of current
is
initiated to eliminate the phenomenon known as non-disruptive discharge which
may
AMENDED SHEET
CA 02310619 2000-OS-18
19
occur in vacuum interrupters, and to otherwise eliminate the problems such as
contact
welding or the like which may occur when the contacts of a mechanical switch
are closed
while a relatively high current is flowing. At the same time, the solid state
device 140
will not be required to withstand the voltage on the line 22. This
arrangement, therefore,
permits the use of fewer solid state devices to form the solid state switch
140 since the
voltage withstand capacity is greatly reduced, and also permits the use of a
smaller and
more economical mechanical switch 130. The gap between the contacts and the
mechanical switch 130 in the embodiment of FIG. 7 may be smaller than that
described
above, for example, about 4 millimeters rather than about 8 millimeters, such
that a
smaller, less expensive component may be utilized. The circuit of FIG. 6 may
also be
used in series with the hybrid switch of FIG. 7.
While particular embodiments and applications of the present invention have
been illustrated and described, it is to be understood that the invention is
not limited to
the precise construction and compositions disclosed herein and that various
modifications, changes, and variations will be apparent from the foregoing
descriptions
without departing from the spirit and scope of the invention as defined in the
appended
claims.
AMENDED SHEET
