GB2272119A - Circuit breaker trip circuit supervision - Google Patents

Abstract

A trip circuit, for a three phase transmission circuit breaker, has the trip coil TCA, TCB, TCC for each phase in one of three parallel-connected branches, and a trip relay initiating contact TRC in a common line in series with the three branches. A solid-state supervision relay SA, SB, SC in each of the parallel branches includes a zener diode ZD1 in series with the respective trip coil which provides a substantially constant voltage in normal operation independent of whether it carries a low current with the circuit breaker closed or open and its associated trip coil energised, or a high current pulse with its trip coil energised in the circuit breaker "preclosed" condition. A circuit failure in any of the branches or the common line in any of these conditions results in the absence of this constant voltage being detected by a light source Land switch DA in at least one of the supervision relays and hence alarm activation via an alarm relay coil OA. The light sources L may be protected by further zener diodes and fuses. <IMAGE>

Description

CIRCUIT BREAKER TRIP CIRCUIT SUPERVISION This invention relates to trip circuits for circuit breakers used in three phase high voltage electrical power transmission systems, and particularly to arrangements for supervision of such trip circuits.

It is essential that a circuit breaker of this type is tripped, i.e. opened, on the occurrence of a fault in the system in order to isolate the fault and thereby protect the system from damage. The trip circuit, which includes the series connection of a trip relay initiating contact, a circuit-breaker auxiliary switch contact and a trip coil, needs to be continuously supervised so that an alarm is given in the event of a failure of the circuit. Such failure may be for example due to a loss of power supply to the circuit, or a failure of the auxiliary switch contact to conduct following mechanical closure, or an open circuit coil or resistor or a break in the wiring.

Figure 1 of the accompanying drawings shows a trip circuit of the above type with a known supervision arrangement.

Referring now to Figure 1, the trip circuit includes the series connection of a trip relay initiating contact TRC, a circuit-breaker auxiliary switch contact AS1 and a trip coil TC across a trip circuit auxiliary power supply AUX1. Supervision is provided by three electromagnetic relay units, as in the GEC ALSTHOM Type MVAX31 relay. A first relay unit coil RL1 is connected in series with an external resistor R1 across an alarm auxiliary power supply AUX 2, a second relay unit coil RL2 is connected in series with an external resistor R2 across the initiating contact TRC and a third relay unit coil RL3 is connected in series with an external resistor R3 across the auxiliary switch contact AS1. Contacts RL2-1 and RL3-1, respectively associated with the coils RL2 and RL3, are parallel connected in series with this coil RL1.A second circuit breaker auxiliary switch contact AS2 is connected in series with the coil RL3.

In the normal condition of the trip circuit when the circuit breaker (not shown) is closed, the contact TRC is open, the contact AS1 is closed and the contact AS2 is open. In this case the coil RL3 is de-energised and the contact RL3-1 is open, the coil RL2 is energised and the coil RL1 is energised via the closed contact RL2-1.

Contacts (not shown) associated with the coil RL1 prevent operation of visible and audible alarms when the coil RL1 is energised. Failure of the trip circuit, for example by a break in the trip circuit wiring, when the circuit breaker is closed de-energises the coil RL2 which opens the contact RL2-1 and de-energises the coil RL1 causing operation of the alarms.

In the normal condition of the trip circuit when the circuit breaker is open, the contact TRC is closed, the contact AS1 is open and the contact AS2 is closed. In this case the coil RL2 is de-energised and the contact RL2-1 is open. the coil RL3 is energised and the coil RL1 is energised via the closed contact RL3-1 preventing operation of the alarms. Failure of the trip circuit when the circuit breaker is open deenergises the coil RL3 which opens the contact RL3-1 and de-energises the coil RL1 causing operation of the alarms.

The above described GEC ALSTHOM Type MVAX31 supervision relay is suitable for circuit breakers with "ganged" operation, ie. where one trip coil is provided for all three poles (phases) of the system or three such trip coils are provided and connected in series. There are however system applications where it is desirable to have "individual pole" circuit breakers, that is where the trip circuit includes three parallel connected branches, each branch having a trip coil for a respective one of the three phases in series with a circuit breaker auxiliary switch contact for that phase, and the trip circuit includes a trip. relay initiating contact in a common line in series with the three branches.The MVAX31 supervision relay cannot be adapted in a straighfforward manner to provide adequate supervision for the trip circuit of such an "individual pole" circuit breaker for the following reason. If a relay unit coil RL3 and a second auxiliary switch contact AS2 were provided across the first auxiliary switch contact AS1 in each branch, then with the circuit breaker open a circuit failure in one of the three branches will de-energise all three relay coils RL3, thereby causing alarm operation. However, with the circuit breaker closed a circuit failure in one of the three branches would be shunted by the other branches and not result in a sufficient change in impedance to de-energise the relay coil RL2 and the alarms would therefore not be operated.This problem would not arise if the trip circuit had three separate trip relay initiating contacts, one for each phase and in series with a trip coil for that phase, but present standardisation of circuit breaker equipment would make it difficult to alter that equipment to provide three such initiating contacts and furthermore there are system applications which require there not to be separate initiating contacts for the three phases.

An object of the present invention is to overcome the above described problem for an "individual pole" circuit breaker trip circuit having a trip relay initiating contact common to the three parallel trip coil branches.

The basic idea of the invention is to detect circuit failure in the parallel branches or the common line by monitoring the current in the branches. The inventive arrangement enables supervision of the comparatively low continuous current normally carried by each branch when the circuit breaker is closed or open and the trip coils are not energised and the comparatively high current pulse carried by each branch when its trip coil is energised with the circuit breaker in the closed condition to change it to the open condition.

According to the invention there is provided a trip circuit for a circuit breaker used in a three-phase high voltage electrical power transmission system, the trip circuit including three parallel connected branches, each branch having a trip coil for a respective one of the three phases in series with a circuit breaker auxiliary switch contact for that phase, and the trip circuit including a trip relay initiating contact in a common line in series with the three branches, wherein each branch normally carries a comparatively low continuous current when the circuit breaker is closed or open and the trip coils are not energised and each branch carries a comparatively high current pulse when its trip coil is energised with the circuit breaker in the closed condition to change it to the open condition, and wherein the trip circuit also includes a respective solid-state supervision relay in each branch, each supervision relay including constant voltage means in series with the trip coil in that branch, each constant voltage means having a substantially constant voltage across it independent of whether there is the comparatively high or low current through that branch, each supervision relay including detection means for detecting both the normal presence of said constant voltage and the absence of said constant voltage upon a circuit failure in said branch or in said common line, and each supervision relay including output means responsive to said detection means to provide an alarm operation signal upon a said circuit failure.

In the just-described trip circuit, each constant voltage means may be provided by one or more zener diodes.

Each said detection means may include a light source which is connected across the constant voltage means and is turned on in the normal presence of said constant voltage or is respectively turned off in the absence of said constant voltage, with each said output means including a light-activated electrical switch which is controlled by the light source so as to be in a first or a second state when the light source is respectively turned on or off, the second state of the switch providing said alarm operation signal.

The three light-activated electrical switches may be connected in series with the operating coil of an electromagnetic relay such that an alarm is activated by said coil being de-energised responsive to any one or more of the switches being in its second state.

With the light source and light-activated switch arrangement as described in either of the two previous paragraphs, each said light source may be in series with a fuse with each supervision relay including monitoring means responsive to an opencircuit condition of the respective constant voltage means to blow the fuse whereby the respective light source is turned off and a said alarm operation signal is thereby provided.

This just described arrangement provides protection for the light source in each supervision relay against a potentially damaging excess voltage to which it wil be subjected if the constant voltage means across which it is connected goes opencircuit. In this case, each supervision relay may furthermore have an indicator light connected in series with the respective said light source such that an alarm operation which is due to the open-circuit condition of one of said constant voltage respective indicatior light.

Examples of a trip circuit according to the invention will now be described with reference to Figures 2 and 3 of the accompanying drawings, in which Figure 2 shows a trip circuit including schmatic diagrams of the three solid-state supervision relays; and Figure 3 shows more details of the supervision relays with these relays furthermore including monitoring means for the constant voltage means and these relays being differently connected to an alarm circuit.

Referring now to Figure 2, a trip circuit is shown for a circuit breaker (not shown) used in a three-phase high voltage electrical power transmission system. The circuit includes three parallel connected branches, each branch having a trip coil TCA, TCB, TCC for a respective one of the three phases in series with a circuit breaker auxiliary switch contact ASA, ASB, ASC, each respectively shunted by a large value resistor RA, RB, RC, and each branch having a respective solid-state supervision relay SA, SB, SC which includes a zener diode ZD1 in series with the respective trip coil.

The trip circuit also includes a relay initiating contact TRC, shunted by a large value resistor RT, in a common line in series with the three branches and connected across a trip circuit auxiliary d.c. power supply + V1, - V1. There may be further trip relay initiating contacts, providing respective protection functions, connected in parallel with the contact TRC across the resistor RT.

In each supervision relay SA, SB, SC a resistor RL in series with a light source L, such as a light-emitting diode, is connected across the zener diode ZD. This arrangement is such that the normal substantially constant zener voltage is chosen to be sufficient to energise the light source L. Also in each supervision relay SA, SB, SC there is a two-state light-activated electrical switch DA connected to a d.c. power supply +V2, -V2. The switch DA, via an optical coupling OC, has its control input galvanically isolated from but optically coupled to the light source L. Each electrical switch DA may include a Darlington type amplifier arrangement providing at least unity current gain with respect to the current through the light source L and required power gain is provided by choice of the voltage +V2, -V2 with respect to the voltage across the light source L.

Each switch DA is associated with a respective one of three contacts CA, CB, CC which are connected in series with each other and with the operating coil OA of an alarm relay across a d.c. alarm power supply +V3, -V3. The dotted line connections shown from the switches DA to the contacts CA, CB, CC will each include a relay.

In normal operation of the trip circuit when the circuit breaker is closed and the trip coils TCA, TCB, TCC are not energised, the auxiliary contacts ASA, ASB, ASC are closed, the initiating contact TRC is open and, via the resistor RT, the three parallel branches each carry a comparatively low continous current. This low current in each branch may typically be in the range 1 0-30mA which is low enough to cause only a low energy drain on the auxiliary power supply +V1, -V1 while at the same time being high enough for the respective zener diodes ZD to provide their normal zener voltage, which may typically be in the range 3-7 volts.In this normal operation, the presence of each normal zener voltage is detected by the respective light source L and the respective switches DA are responsive to that detection each to provide a high output state so that the three contacts CA, CB, CC are maintained closed and hence the alarm operating coil OA energised and the alarms de-activated.Upon the occurrence of a circuit failure in one of the three parallel branches, or in the common line of the trip circuit, the absence of the low continuous current will cause the absence of the corresponding zener voltage, or the absence of all three zener voltages as the case maybe, which will be detected by the respective, or all three, light source(s) L switching off to which the respective, or all three, switches DA will respond by switching to their low output state(s) which provides an alarm operation signal, that is to say the respective, or all three, contact(s) CA, CB, CC will open and the alarm relay operating coil OA will be de-energised causing audible and/or visible alarms to be activated.

In normal operation of the trip circuit when the circuit breaker is open and the trip coils TCA, TCB, TCC are not energised, the auxiliary outputs ASA, ASB, ASC are open, the iniciating contact TRC is open and, via the registor RT and respectively the transistors RA, RB, RC, the three parallel branches, similarly to the previously described situation with the circuit breaker closed, carry a comparatively low continuos current. This current will be lower, done to the resistors RA, RB, RC being in circuit, than for the case with the circuit breaker closed but of the same order of magnitude and typically again may be in the same range of 1 0-30mA.Thus, as for the case in which the circuit breaker is closed, in normal operation the presence of the normal zener voltage across the zener diode ZD1 in each supervision relay SA, SB, SC is detected by the respective light source L and in consequence the operating coil OA is energised and the alarms de-activated. Also, as for the case in which the circuit breaker is closed, upon the occurence of a circuit failure in one of the branches or in the common line of the trip circuit the resultant absence of one or all zener voltages will be detected by one or all of the light sources (L) switching off, responsive to which one or all switches DA will provide an alarm operation signal to open the respective contact(s) CA, CB, CC and de-energise the alarm relay operating coil OA.

Also in normal operation, the circuit breaker is changed from the closed condition to the open condition by energising the trip coils TCA, TCB, TCC with a pulse of high current through each of the three parallel branches of the trip circuit.

The trip relay initiating contact TRC is closed for the duration of this puse, known as the "pre-closed" condition of the trip circuits, and then opens again when the auxiliary switch contacts ASA, ASB. ASC open. The duration of this current pulse is typically not greater than 1 Oms and the value of the high current, which is limited only by the resistance of the trip coils, may typically be about 10A. The normal zener voltages across the zener diodes ZD1 are maintained during this high trip coil energising current pulse, typically towards the higher end of a range 7-10 volts.The zener diodes ZD may therefore each be considered as constant voltage means having a substantially constant voltage across it independent of whether there is the comparatively high current, typically 1 OA, or the comparatively low cirrent, typically 1030mA, through the respective parallel branch of the trip circuit. Thus in normal operation during the high current pulse "pre-closed" condition of the trip circuit, the presence of the normal zener voltages across the xener diodes ZD1 has the effect of maintaining the coil OA energised and the alarms de-activated. If there is a circuit failure in one of the branches or in the common line of the trip circuit during this "preclosed" condition, the resultant absence of one or all zener voltages will de-energize the coil OA and activate the alarms.

Referring now to Figure 3, more detail of the supervision relays SA, SB, SC is shown than in Figure 2, with these relays including the additional provision of monitoring means for the constant voltage means, and also with, the output switches of these relays differently connected to an alarm circuit compared with the arrangement shown in Figure 2. Full details of the components within each supervision relay is shown only for the relay SC, the details for the relays SB and SA being similar.

The constant voltage means in each supervision relay which was shown as provided by a single zener diode ZD1 in Figure 2 is shown in Figure 3 as provided by two parallel connected zener diodes ZD11 and ZD12 which each perform the same function and provide back-up in the event of failure of either one of them.

The light-emitting diode light source L within each supervision relay SA, SB, SC is shown in Figure 3 as having its light transmission directed on to a light activated junction of one of a pair of Darlington amplifier arranged transistors forming the electrical switch DA. The dotted outline around the light source L and the switch DA in each supervision relay indicates that these two components are in practice provided in a single commercially available device known as an opto-isolator.

The output transistors of the three switches DA in each supervision relay indicates that these two components are in practice provided in a single commercially available device known as an opto-isolator.

The output transistors of the three switches DA are connnected in series with each other and with an electromagnetic output relay operating coil OD which operates a contact CD. This contact CD is in series with the operating coil OA of an alarm relay which is connected to a d.c. alarm power supply +V3, -V3. A zener diode ZD3, whose zener voltage may typically be approximately 30 volts, is connnected across the output transistors of these three switches DA and the coil OD and is connected in series with a high value resistor RV across a d.c. power supply +V21, - V21 which may typically be approximately 100 volts.

Whereas each light-emitting diode light source L was shown in Figure 2 as being in series with a single resistor RL across the respective constant voltage means ZD1, it is shown in Figure 3 as being in series with a resistor RL1, an indicator light LI, a resistor RL2 and a fuse RF across the constant voltage means ZD11, ZD12. A further zener diode ZD2 is connected across the part of this series arrangement which consists of the components L, RL1 and LI. This arrangement monitors the constant voltage means ZD11, ZD12 as will be explained below.

In normal operation of the zener diodes ZD11, ZD12 their zener voltages of typically 7-10 volts provide, in the normal circuit breaker open, or closed or pre-closed conditions, a low current of typically about 1 0my through the fuse RF, the resistor RL2, the indicator light LI, the resistor RL1 and the light-emitting diode L.

In this normal operation the fuse RF, which may be rated typically at 50mA, conducts: the resistors RL1 and RL2 provide a low voltage across the zener diode ZD2 such that it does not conduct, typically a voltage of about 3 volts compared with a zener voltage for ZD2 of about 5 volts; and the indicator light LI is turned on which provides an indication of the normal constant voltage across the zener diodes ZD11, ZD12.

Further in this normal operation of the zener diodes ZD11, ZD12, and in the absence of a circuit failure in the common line or any of the three parallel branches of the trip circuit, the light-emitting diodes L in each of the relays SA, SB, SC pass the low current of typically about 1 OmA with about 1.5 to 2 volts across them and are turned on. The light-activated switch DA in each of the relays is thus in a first, conducting, state. The corrent gain of each of the output transistors of the switches DA with respect to its associated light-emitting diode L will be at least unity, that is unity or perhaps a little more, and this current together with the voltage provided by the zener diode ZD3 will provide sufficient power to the relay operating coil OD to keep it energised and hence the coil OA energised and the alarm(s) de-activated.

A circuit failure in the common line or any of the three parallel branches of the trip circuit will result in one or more of the switches DA being in a second, nonconducting, state which will de-energise the coil OD and activate the alarm(s).

If, in the absence of any other circuit failure in the common line or any of the three branches of the trip circuit, a pair of zener diodes ZD11, ZD12 in one of the supervision relays SA, SB, SC' becomes open-circuit and fails to provide the normal substantially constant zener voltage then, in the absence of the monitoring arrangement including the zener diode ZD2, substantially the trip circuit supply voltage, which may typically be about 100 volts, would be applied to the light-emitting diode L in that supervision relay which would damage the opto-isolator device consisting of that light-emitting diode L and its associated light activated switch DA.

However, with the arrangement shown in Figure 3, such a failure of the constant voltage means ZD11, ZD12 is monitored by the by-pass zener diode ZD2 which responds by conducting and clamping the the voltage across the components LI, RL1 and L to its zener voltage, typically about 5 volts, which safeguards the opto-isolator L, DA. The current through the zener diode ZD2, resistor RL2 and fuse RF in this open-circuit failure condition of the zener diodes ZD11, ZD12 then exceeds the rating of the, typically 50 volt fuse RF which therefore blows. As a consequance of the fuse RF blowing the light source L turns off which results in the alarm(s) being activated and also the indicator light LI turns off which identifies the cause of the alarm operation as being the failure of the constant voltage means ZD11, ZD12 in that particular supervision relay.

Claims (7)

1. A trip circuit for a circuit breaker used in a three-phase high voltage electrical power transmission system, the trip circuit including three parallel connected branches, each branch having a trip coil for a respective one of the three phases in series with a circuit breaker auxiliary switch contact for that phase, and the trip circuit including a trip relay initiating contact in a common line in series with the three branches, wherein each branch normally carries a comparatively low continuous current when the circuit breaker is closed or open and the trip coils are not energised and each branch carries a comparatively high current pulse when its trip coil is energised with the circuit breaker in the closed condition to change it to the open condition, and wherein the trip circuit also includes a respective solid-state supervision relay in each branch, each supervision relay including constant voltage means in series with the trip coil in that branch, each constant voltage means having a substantially constant voltage across it independent of whether there is the comparatively high or low current through that branch, each supervision relay including detection means for detecting both the normal presence of said constant voltage and the absence of said constant voltage upon a circuit failure in said branch or in said common line, and each supervision relay including output means responsive to said detection means to provide an alarm operation signal upon a said circuit failure.

2. A trip circuit as claimed in Claim 1, in which each constant voltage means is provided by one or more zener diodes.

3. A trip circuit as claimed in Claim 1 or Claim 2, in which each said detection means includes a light source which is connected across the constant voltage means and is turned on in the normal presence of said constant voltage or is respectively turned off in the absence of said constant voltage, and in which each said output means includes a light-activated electrical switch which is controlled by the light source so as to be in a first or a second state when the light source is respectively turned on or off, the second state of the switch providing said alarm operation signal.

4. A trip circuit as claimed in Claim 3, in which the three light-activated electrical switches are connected in series with the operating coil of an electromagnetic relay such that an alarm is activated by said coil being de-energised responsive to any one or more of the switches being in its second state.

5. A trip circuit as claimed in Claim 3 or Claim 4, in which each said light source is in series with a fuse and in which each supervision relay includes monitoring means responsive to an open-circuit condition of the respective constant voltage means to blow the fuse whereby the respective light source is turned off and a said alarm operation signal is thereby provided.

6. A trip circuit as claimed in Claim 5, in which each supervision relay has an indicator light connected in series with the respective said light source such that an alarm operation which is due to the open-circuit condition of one of said constant voltage means is identified by a change of state of the respective indicator light.

7. A trip circuit substantially as herein described with reference to and as shown in Figure 2 or in Figure 2 as modified by Figure 3 of the accompanying drawings.