Note: Descriptions are shown in the official language in which they were submitted.
llSW 04245
10813~5
This invention relates to means for detecting a
loss of vacuum in any one of the vacuum-type circuit
interrupters of a polyphase a.c. vacuum circuit breaker
and, more particularly, relates to detecting means of
this type which is very simple and inexpensive and relies
for its operation to a large extent upon components
already present in typical switchgear comprising a circuit
breaker of the type used for protecting medium voltage
three-phase a.c. power distribution circuits. Such
switchgear is usually referred to as medium voltage metal-
clad switchgear. Medium voltage denotes a voltage
rating of between 2.4 kV and 38kV.
Typical switchgear of the above type includes over-
current-sensing means comprising three current transformer
secondaries, one for each phase, connected in parallel
with each other across a shorting conductor. In each of
the parallel-connected branches of this overcurrent-sensing
circuit and in series with the associated current transformer
secondary, there is an input circuit of an overcurrent
relay. The current transformer secondary for each phase
delivers to the associated overcurrent relay a current
substantially proportional to the primary current through
the associated phase conductor. This secondary current
flows through the series combination of the current
transformer secondary, the input circuit of the associated
overcurrent relay, and the shorting conductor. When
the pahse current exceeds a predetermined value, the
associated overcurrent relay responds to the resulting
increased secondary current by operating to initiate
tripping of the circuit breaker.
An object of my invention is to utilize the
current transformer secondaries of the above-described
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` 10813ZS llSW 04245
overcurrent-sensing circuit for supplying input intelli-
gence to my loss-of-vacuum detecting means and to do this
in such a way that there is no interference with the
ability of the overcurrent-sensing circuit to respond
in the usual manner to over-currents.
Another object is to provide loss-of-vacuum
detecting means that performs as in the immediately-
preceding paragraph and yet is very simple and inexpensive.
Another object is to provide loss-of-vacuum
detecting means that~meets the above two objectives, is
sensitive to a wide range of leakage currents through a
faulty vacuum interrupter, and despite its sensitivity -
to low leakage currents, is capable of withstanding without
damage the effects of very high currents through the
interrupters.
In carrying out the invention in one form, I
provide an~"a" switch in the above-described shorting
conductor of the typical overcurrent-sensing means used
in medium voltage switchgear. This "a" switch is closed
when the circuit breaker is closed and is opened when the
~ circuit breaker is opened. Rectifying means is connected
I across said switch for developing a d.c. voltage when :-
said switch is opened and one or more of the secondary
windings of the overcurrent sensing means is energized.
Time-delay means is arranged to be energized by this d.c.
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voltage and is operable when the voltage thereacross `
' exceeds a predetermined level for a predetermined period
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-~i foLlowing opening of said switch. Operation of said
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relay means is indicative of a loss of vacuum in one
of the vacuum interrupters of the circuit breaker.
;~ For a better understanding of the invention,
reference may be had to the accompanying drawings, wherein:
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Fig. 1 is a schematic showing of switchgear
including a vacuum circuit breaker and loss-of-vacuum
detecting means for the circuit breaker embodying one
form of my invention.
Fig. 2 is a schematic showing of loss-of-
vacuum detecting means embodying another form of the
invention.
Referring now to Fig. 1, there is shown a
polyphase a-c power distribution circuit comprising
three phase conductors 11, 12 and 13. Current through
this circuit is controlled by a three-phase circuit
breaker 14 comprising three circuit interrupters 15, 16
and 17. Each interrupter is a vacuum-type circuit
interrupter of a conventional form, for example,
the form disclosed in U. S. Patent 3,441,698 - Sofianek-
dated April 29, 1969, assigned to the assingee of the present
invention. Since the details of the interrupter constitute
no part of the present invention, they have not been
shown in Figure 1.
The movable contacts of the three interrupters
are mechanically coupled together by means of an insulating
operating element 20. An opening spring 22 acts on this
operating element to bias the movable contacts of the three
interrupters from their closed position shown toward an open
position in which the contacts of each interrupter are
fully separated.
The movable contacts are latched in their
closed position illustrated by conventional latching
means 24 operable by means of a tripping solenoid 25.
When solenoid 25 is energized, it operates to release
the latch and allow the circuit breaker to open under
the influence of opening spring 22.
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10813Z5 llSW 04245
The switchgear in which circuit breaker 14 is
located includes overcurrent-sensing means 30 that will
cause the breaker to trip open in response to an over-
current in any of the three phases 11, 12 or 13. This
overcurrent-sensing means comprises three current-
transformer secondary windings 31, 32, and 33, one for
each phase. These three current-transformer secondary
windings are connected in parallel with each other across -
a shorting conductor 35. The shorting conductor 35
is in effect connected in series with the three
parallel-connected branches that contain the current-
transformer secondary windings. Three overcurrent relays
41, 42, and 43 are provided, and these relays have
their respective operating coils 45, 46 and 47 connected
in the three parallel branches in series with the associated
secondary windings.
Each of the overcurrent relays has a set of
normally-open contacts that is connected in a tripping
circuit 50 in series with the tripping solenoid 25.
These contacts, which are designated 51, 52 and 53,
respectively, are connected in parallel with each other in ;
the tripping circuit 50. When current through each phase
conductor is below a predetermined value, the secondary
current in each of the parallel-connected branches is
insufficient to operate the associated overcurrent relay.
But when the current in any phase exceeds a predetermined
minimum value, the associated current transformer develops
sufficient secondary current in its associated branch to
operate the associated overcurrent relay. This secondary
~- 30 current follows a path through the associated branch and
through the shorting conductor 35 connected thereacross.
When the associated overcurrent relay thus operates, it
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it completes the trip circuit 50, causing solenoid 25 to
trip the circuit breaker.
The overcurrent-sensing means 30, as described
up to this point is generally conventional. A basic object
of my invention is to utilize the current transformers of
such an overcurrent-sensing means to detect a loss of
vacuum in any one of the three vacuum circuit interrupters
15, 16 or 17 without interfering with the ability of
the overcurrent-sensing circuit to respond in the usual
manner to overcurrents.
I attained this objective, first, by providing
a set of "a" switch contacts 60 in the shorting conductor
35 and, second, by connecting across the "a" switch
contacts 60 a rectifier 62 and time-delay relay means 64,
sometimes referred to hereinafter as loss-of-vacuum
relay means.
The "a" switch (61) is a switch that is closed
when the circuit breaker is closed, and is open when the
circuit breaker is opened. In the schematic illustration
of Fig. 1, a cam 65 is provided on circuit breaker operating
element 20 for operating the "a" switch. When the circuit
breaker opens, cam 65 immediately depresses the plunger
of the"a" switch, forcing the contacts 60 to open. The
"a" switch includes another set of normally-closed contacts
66 which are also opened when the circuit breaker is
opened. These additional contacts, which are connected
in tripping circuit 50, serve to protect the tripping
solenoid 25 from continued energization after the circuit
breaker has opened.
Rectifier 62 is a conventional full-wave rectifying
bridge having its a-c terminals 68 and 69 connected
across the contacts 60 of the "a" switch and its d.c.
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~081325
terminals 70 and 71 connected across the time-delay relay
means 64. The time-delay relay means, in the illustrated
embodiment, comprises a sensitive telephone-type relay
73, a resistor 74, and a capacitor 75 in series with the
resistor 74. The operating coil 76 of the relay is connected
in series with resistor 74 and in parallel with capacitor
75. The capacitor is normally maintained discharged by
the low resistance relay operating coil 76. A shunting
path 77 is connected across the d.c. terminals of the
rectifier 62 and includes non-linear resistivity conducting
means in the form of a plurality of diodes 78 connected
in series in the shunting path.
When a voltage is applied across the d.c.
terminals 70 and 71 of the rectifier 62, the capacitor
75 charges at a rate determined by the RC time-constant
of circuit 74, 77, 75. When a predetermined voltage is
attained across the capacitor 75, the relay 73 picks up,
thereby closing its normally-open contacts 80 and 81. Such
operation of relay 73 is used to indicate a loss of vacuum
in an interrupter, as will soon be explained.
So long as the circuit breaker 14,is closed,
as shown, the "a" switch contacts 60 are closed and
shorting conductor 35 is complete. With the shorting
conductor 35 complete, no voltage is applied to the a.c.
terminals of the bridging rectifier 62, and the overcurrent-
; sensing means 30 is, in effect, electrically isolated
from the time-delay relay means 64. This electrical
isloation, while maintained, allows the overcurrent-
sensing means 30 to perform in its normal manner to
initiate circuit-breaker tripping in response to an ;~
overcurrent without affecting or being affected by the
presence of time-delay relay means 64. It is only
~0813Z5 llSW 04245
when the circuit breaker 14 is opened that the isolation
between these two circuits ~30 and 64) is removed (by
opening of "a" switch contacts 60).
Under normal interrupting conditions, when a
vacuum interrupter has reached its fully-open position,
the inter-contact arc has been extinguished and interruptiong
therein has been completed. Occasionally, in a normally-
functioning vacuum interrupter, arcing will persist
for a few half-cycles following full contact-separation
and will then be terminated. If arcing persists for
longer than a predetermined minimum period of, say, 3
cycles following complete contact separation, this indicates
a failure of the interrupter, such as might result from a
loss of vacuum. In the embodiment of Fig. 1, my loss-of-
vacuum relay 73 will respond to such a condition by operating
to close its contacts 80 and trip a back-up circuit
breaker that interrupts the current through the deficient
interrupter. The back-up breaker, which is not shown,
is connected in the power circuit 11, 12, 13 on the source
side of the circuit breaker 14 and has a trip circuit 79
which is completed by closing of the contacts 80 of
relay 73.
For preventing false operation of the relay 74 ~
in response to a short period of current flow through an ~ -
~ interrupter following complete opening of the circuit
; breaker, I rely upon the time-delay characteristics of
relay 64 and upon the shunting circuit 77. If current
flows through an interrupter 15, 16, or 17 after "a"
switch contacts 60 have opened (in response to circuit-
breaker opening), a voltage will ~uickly be developed
across the d.c. terminals 70, 71 of the rectifier bridge
62. This voltage will be limited by the forward drop of
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` . 108~3Z5 llSW 04245
the series-connected diodes 78 in the shunting circuit
77. This voltage will begin charging the capacitor 75/
but if current flow through the interrupters 15, 16, and -
17 is terminated prior to the capacitors' reaching the
predetermined pick-up voltage of relay 73, the relay
will remain inactive. Upon such termination of current
flow through the interrupters, the voltage across the
a.c. terminals 68, 69 of the rectifier bridge quickly
drops to zero, following which the capacitor 75 discharges
through the operating coil 76 of relay 73 without supplying
sufficient current to operate relay 73.
Consider next the situation in which the circuit
breaker is standing in its open position and leakage current
begins flowing through one of the interrupters in response
to a loss of vacuum. This leakage current will be
sensed by the associated current transformer. If the
leakage current exceeds a predetermined minimum value,
e.g., a value that results in current in the current
transformer secondary winding of 1% or more of rated
,
secondary current, then a sufficient voltage will be
developed across the d.c. terminals of the rectifier
bridge 62 to operate the loss-of-vacuum relay 73.
My time-delay relay means 64 can be set to
I
respond to very low values of a leakage current (e.g.,
~ 1% of rated current transformer primary current) without
¦~ being damaged by very high values of fault current (e.g.,
20 or more times rated current transformer primary current)
,~
because the high secondary currents developed during
such fault current conditions are readily shunted without
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damage through the diodes 78 of the shunting circuit 77.
The voltage developed across the d.c. terminals 70, 71 of
the rectifier bridge 62 under these fault conditions is
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~(~8~325 llSW 04245
only slightly greater than the voltage developed when
low values of leakage current flow through the interrupter.
It is to be noted that the "a" switch 61 is
operated immediately upon opening of the circuit breaker
- 14. Operating this switch 61 immediately with no intentional
time delay is desirable because such operation (i.e.,
opening of contacts 66) quickly interrupts the tripping
circuit 50 and protects the tripping solenoid 25 against
the effects of prolonged energization. This immediate
operation of the "a" switch, however, by opening the
contacts 60, immediately terminates isolation between
loss-of-vacuum relaying means 64 and the current-sensing
circuit 30. It is at this instant that the time-delay
characteristics of the relay enter the picture and enable
the relay to remain inactive should for some reason
current continue to flow through one of the interrupters
for a normal short period after complete separation of
the interrupter's contacts.
To provide a clear indication that the loss-of-
vacuum relay 73 has operated, a target relay 83 is provided.
When the loss-of-vacuum relay 73 picks up, it closes its
contacts 81 to complete an energizing circuit 82 for
target relay 83. The target relay immediately picks up
and actuates a clearly visible target 86 that indicates
operation of the loss-of-vacuum relay 73. The target
relay seals itself in through a set of seal-in contacts
85, thus remaining picked up even if the loss-of-vacuum
relay 73 should drop out, e.g., when the back-up circuit
breaker operates. To restore the target relay 83 after
such operation to its position of Fig. 1, it is necessary
to operate a manual reset switch 88, which interrupts
the energizing circuit 82 for target relay 83 and allows
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1~813ZS
the target relay to drop out.
~ he illustrated target relay also includes a
set of contacts 87 in the closing circuit 94 of the circuit
breaker. The contacts 87 are shown in series with the
closing operator 92 of the circuit breaker. Contacts
87 are opened when the target relay operates, thus
preventing another closing of the circuit breaker
until the target relay is reset.
Although I prefer to directly operate the back-
up circuit breaker in response to operation of the loss-of-
vacuum relay 73, I can instead immediately reclose the
principal circuit breaker 14 and hold it closed. It the
condition that initiated opening of circuit breaker 14 is
still present at that time, the back-up circuit breaker
will trip open following the circuit breaker's 14 closing and
remaining closed. The circuit modifications relied upon
to achieve this modified mode of operation are shown in
Fig. 2. The other components remain the same as in
Fig. 1 (except that a testing circuit, soon to be
described, has been added to Fig. 2).
In Fig. 2, the contacts 80 of the loss-of-
vacuum relay 73 are used to complete an energizing
circuit 79 for a circuit breaker reclosing relay 90. This
relay 90 responds by picking up to close its contacts 91,
thereby completing an energizing circuit 94 for the
circuit-breaker closing device 92. The closing device 92
responds by carrying out a closing operation, following
which it is deenergized by suitable control circuitry
93 of a conventional design.
The principal circuit breaker 14 is prevented
from again tripping open after such a closing operation
by holding open its tripping circuit 50. As indicated in
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~08~325 llSW 04245
Fig. 2, this is accomplished by connecting the contacts 87
of the target relay 83 in the tripping circuit 50 and
in series with the tripping solenoid 25. Once the target
relay 83 picks up in response to operation of the loss-
of-vacuum relay 73, it remains picked-up until manually
reset, thereby preventing another tripping operation
until manually reset.
Closing can be effected independently of the
loss-of-vacuum relay by closing a manually-controlled
switch 95 in parallel with the contacts 80 of the loss-
of-vacuum relay.
As mentioned hereinabove, the circuit of Fig.
2 includes ~eans for testing the loss-of-vacuum relay 73
to determine at any given time whether it is operative.
This test means comprises a transformer 100 having a
primary winding 102 and a secondary winding 104. Primary
winding 102 is connected in series with a normally-open
test switch 105 across a source 103 of a.c. test voltage,
e.g., 120 volts. The secondary winding 104 is connected
in series with a diode 106 and a resistor 107, and this
series combination is connected across the d.c. terminals
70, 71 of the bridging rectifier 62. When the test
switch 105 is closed, a voltage comparable to that
derived from the current transformers of Fig. 1 is
applied by test transformer 100 and rectifier 106 across
the terminals 70, 71. If the relay 73 is in good
w~rking condition, it will pick up in response to
establishment of this voltage. This will cause the con-
tacts 82 of the loss-of-vacuum relay 73 to close, thereby
picking up the target relay 83, thus providing an indication
that the loss-of-vacuum relay has operated.
In the illustrated embodiment of Fig. 2, the
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1081325
loss-of-vacuum relay will remain picked up by the test
voltage even though the test switch 105 is released and
allowed to open. This is the case because the loss-
- of-vacuum relay has an extra set of contacts 110 that, upon
closing in response to operation of the loss-of-vacuum
relay, establish a- shunting seal-in circuit around the
test switch 105. Resetting of the loss-of-vacuum
relay 73 can be effected when desired by opening reset
switch 112 in series with the primary winding 102,
thereby deenergizing the transformer 100 and allowing
relay 73 to drop out.
The above-described testing operation can be
carried out whether the principal circuit breaker 14 is
open or closed. Even if the "a" switch contacts 60 are
closed, as they would be when the circuit breaker is
closed, the completed shorting path 35 is electrically
isloated from the voltage applied by the testing trans~
former 100 because of the presence of bridging rectifier
62. The polarity of the testing voltage is such that
bridging rectifier 62 blocks current through shorting
:
path 35. -
If it is desired to prevent the closing operator
92 from operating in response to pick-up of the loss-
of-vacuum relay 72 during a testing operation, such
result can be effected by providing a suitable manually-
controlled switch (not shown) in the energizing circuit
70, which switch will be appropriately operated to hold
open the energizing circuit 79 during this testing operation.
~; The testing circuit can be used not only to
seal-in the loss-of-vacuum relay 73 during testing but
, i .
-, also following a normal operation of the loss-of-vacuum
, relay. For example, assume that the loss-of-vacuum relay
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llsw 04245
1081325
73 is picked up in response to a voltage developed by
one of the current transformer secondary windings 31, 32,
or 33 of Fig. 1. Such pick-up would close contacts 110,
thereby causing the test source 103 to apply voltage
across terminals 70, 71 of the bridging rectifier. This -
latter voltage would continue to be present, event if
the circuit breaker reclosed and "a" switch 60 was
reclosed.
Although the testing circuit is illustrated as
applied to the circuit of Fig. 2, it is to be understood
that it can equally well be applied to the circuit of
Fig. 1. This would be done in generally the same manner as
disclosed with respect to Fig. 2.
; Whether the loss-of-vacuum relay 73, in operating,
, trips a back-up circuit breaker, as in Fig. 1, or causes
reclosing of the principal circuit breaker 14, as in Fig.
2, the effect of such relay operation is to initiate
protective action that limits damage to the circuit -
I breaker 14 from the condition (e.g., loss of vacuum) which
; 20 initiated operation of the relay 73.
It is to be noted that my loss-of-vacuum detecting
means utilizes only simple components of proven reliability.
In the illustrated embodiment of Fig. 1, the only electronic
l componenets used are the diodes 79 and those present in
-' bridge 62. In a typical example, these diodes are power-
I type diodes rated at 25 amperes continuous current and
capable of passing currents exceeding 100 amperes for
~1~ brief periods. In this example, the current transformers
have a 2000 ampere primary rating and a 5 ampere secondary
rating.
Although the illustrated time-delay relay
means 64 comprises an R-C circuit, it is to be understood
that other forms of time-delay relays could instead be
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10813~5 llsw 04245
used, such as a slugged telephone-type relay having
built-in time delay pick-up provisions. .
While I have shown and described particular
embodiments of my invention, it will be obvious to those :
skilled in the art that various changes and modifications ~. :
may be made without departing from the invention in its
broader aspects; and I, therefore, intend herein to
cover all such changes and modifications as fall within
the true spirit and scope of my invention.
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