GB2098417A - Pilot wire line protection systems - Google Patents

Abstract

A circulatory current system for the protection of an electricity distribution feeder 10-14 comprises two similar circuit arrangements, one at each of two stations on the feeder, the arrangements being connected by two pilot wires 16, 18. Each arrangement has a summation transformer 28 with its primary winding 30 fed from a current transformer 32-36 inductively coupled to the feeder. The secondary winding 44 of the summation transformer is arranged across the pilot wire terminals and across two semi-conductor rectifiers 48, 50 in opposed series relationship. A protection relay coil 54 is connected between a tap on the secondary winding and a point intermediate the two rectifiers. The latter are connected to the secondary winding by respective semiconductor rectifiers and resistances 40, 42 in parallel with one another. The opposed rectifiers 48, 50 are either directly connected to the pilot wires or are indirectly connected by an isolation transformer. Direct current supervision of pilot wire integrity may also be provided optionally. <IMAGE>

Description

SPECIFICATION Circuit arrangement The invention relates to circuit arrangements.

The invention particularly relates to a circuit arrangement which can be used at each of two stations between which an electricity distribution feeder extends. The circuit arrangements are interconnectable by two pilot conductors to form a circulating current type of system by which the feeder can be protected against the effects of various internal fault conditions.

An example of such a system is described in British Patent specification No. 723555.

In known systems of the kind described in British Patent specification No. 723555 a nonsymmetrical mode of operation is adopted with the result that the system can respond to fault conditions only in one half of any alternating current cycle.

The object of the invention is to provide a circuit arrangement by which the maximum time required for the system to respond to fault conditions is reduced, the arrangement being such that the system operates in a "full wave" mode: that is, the system can respond to fault conditions in either half of any alternating current cycle.

A circuit arrangement according to the invention is connectable at two points to respective pilot conductors and comprises a summation transformer having a secondary winding which is arranged across said points and across two opposed semiconductor rectifier elements in series with one another, a protection relay coil being connected between a tap on the secondary winding and a point intermediate said opposed rectifier elements, which elements are connected to the secondary winding by respective semiconductor rectifier and resistive elements in parallel with one another.

Preferably, said circuit arrangement includes an isolating transformer having a first winding connected across said opposed rectifier elements and having a second winding connected across said two points.

A source of direct current may be provided for the purpose of supervision of the integrity of the pilot conductors, a capacitive element being provided to block the flow of said direct current except through said pilot conductors.

The invention includes a complete protection system in which two similar circuit arrangements are provided at respective locations on a feeder.

Each summation transformer may have its primary winding connected to current transformer means inductively coupled to the feeder.

Protection systems will now be described by way of example to illustrate the invention with reference to the accompanying drawings in which: Figures 1 and 2 are circuit diagrams schematically showing first, second and third forms of system; and Figures 3 and 4 are diagrams showing equivalent circuits and voltage conditions for the system of Figure 1 under various operating conditions.

Figure 1 shows a feeder comprising three phases 1 0, 1 2, 1 4 extending between two stations at each of which there is a protective circuit arrangement, with two pilot wires 1 6, 1 8 extending between the circuit arrangements and connected respectively between two points 20, 22 of one arrangement and points 24, 26 of the other.

Each circuit arrangement is fed by a respective summation transformer 28, in which the primary winding 30 has taps connected to like ends of current transformers 32, 34, 36 one for each of the three phases 10, 12, 14 of the feeder, the remote end or another tap (not shown) of the primary winding 30 being connected to the commoned opposite ends of the current transformers 32, 34 and 36.

The summation transformer is a known device of a kind similar to those used for many years in protection systems of the circulating current class to enable either balanced or unbalanced currents in the feeder phases to be reproduced as single currents in the transformer secondary. The summation transformer can be designed to minimise the burden imposed on the current transformers 32, 34, 36 by the pilot circuit by choice of impedance levels and the summation transformer isolates the current transformers from the pilot circuit so that the former may be earthed while the pilot conductors are not earthed. The summation transformer 28 has relevant 5 kilovolt insulation at 38.

The circuit arrangements at each station are similar and only one need be described. The arrangement in each case comprises two elements, a semiconductor rectifier 40 and a resistor 42, in parallel, connecting one end of the secondary winding 44 of the summation transformer 28 to the point 20 and two similar elements 40 and 42 connecting the other end of the secondary to the point 22.

The points 20, 22 are interconnected by mutually similar, opposed semiconductor rectifiers 48, 50 in series so that the winding 44 is connected across the points 20, 22 and across the two rectifiers 48, 50.

The winding 44 has a centre tap 52 and an attracted-armature relay has its coil 54 connected between the tap 52 and a point 56 intermediate the rectifiers 48, 50. The relay has contacts at 58 which are connected to a trip coil at 59 of a circuit breaker arranged to open contacts 61 to disconnect the feeder in response to detection of internal fault conditions.

A non-linear resistor 62 is connected across the relay coil 54.

Similar non-linear resistors 64, 66 are connected in parallel with respective halves of the 'secondary winding 44.

Figure 2 shows a second form of system in which the circuit arrangement at each station includes an isolation transformer 70 having a primary winding 72 connected across the opposed rectifiers 48, 50. The secondary winding 74 of the isolation transformer 70 is tapped so as to provide a choice of turns on the secondary winding, which is connected across the points 20, 22 (or 24, 26).

This form of system has the advantages over the form of system shown in Figure 1 that: (i) an increased choice of combination of pilot wire resistance and capacitance is made available by the use of the tappings on the transformer secondary winding 74; and (ii) the necessary level of insulation of the pilot wires need extend only as far as the transformer 70 so that the remainder of the circuit arrangement at each station can be of standard constructions regardless of the level of pilot conductor insulation to earth, which may typically be 5 kilovolts or 1 5 kV, or higher.

Figure 2 also shows a further feature which is optional but which is preferred. That feature is supervision of the integrity of the pilot conductors to ensure that, should the pilot conductors become open circuited, short-circuited or crossed, for example, an alarm is operated.

The supervision uses a direct current source at one station (the "send" station) and an alarm relay at the other station (the "receive" station). Before the supervision circuitry can be made effective, it is necessary to remove two links 75, one bridging the terminal 20 and a series terminal 21 and the other bridging the terminal 24 and a series terminal 25. The supervision circuitry, is then connected as follows: at the "send" station a source 80 of direct current, is connected by conductors 82, 84 across the terminals 20, 22 for example, and a capacitor 76 is connected across the terminals 20, 21; at the "receive" station, an alarm relay 81 is connected by conductors 83, 85 across the terminals 24, 26 and a capacitor 77 is connected across the terminals 24, 25.

In Figure 2 arrowheads indicate the points at which the conductors 82, 84 and 83, 85 and the capacitors 76, 77 are connected to the terminals 20, 21,22, 24, 25 and 26. The circuits and means by which the supervising direct current is monitored are not shown.

Where direct current is used as shown in Figure 2, the capacitors 76 and 77, which are an integral part of the pilot supervision circuitry, block the flow of direct current to the secondary windings 74.

The remainder of each circuit arrangement in Figure 2 is similar to that described in relation to Figure 1 and need not be repeated. The primary windings of the summation transformers, the line current transformers and the circuit breaker contacts shown in Figure 1 are omitted from Figure 2 for simplicity.

In the examples of systems described above the effective resistance of the pilot wire loop is preferably made equal to a predetermined value (for example 2000 ohms) bythe provision of padding resistance between the terminal 20 (Figure 1) and the rest of the system to the left of the terminal 20 and an equal amount of padding resistance between the terminal 24 and the rest of the system to the right of the terminal.

In Figure 2 the padding resistance would be provided between the terminal to which the upper end of the left-hand winding 72 is connected and the rest of the system to the left, and between the terminal to which the upper end of the right-hand winding 72 is connected and the rest of the system to the right.

The padding resistances are thus in series with the pilot wire 1 6 and may take the form of variable resistors or chains of resistances with removable links connected in parallel across the resistances, the value of the padding resistance being changeable by selective removal of links.

Figure 3 shows that the polarity of the voltage circuits equivalent to the pilot circuit shown in Figure 1 during successive half-cycles of alternating current in the secondary winding 44, under fault conditions external to the feeder. The figure also shows the corresponding voltage distribution at (a) and (b), the resistance Ra of the resistors 42 and 42 being equal and each being greater than half the value of the sum of the padding resistances at the local and remote ends and the resistances of the pilot wires.

At (c), Figure 3 shows the voltage across the pilot conductors at X1-X and Y1-Y, the relay voltages at X,--C and Y1-C; and the relay voltages at X-C and Y-C all for successive halfcycles.

Figure 3 shows that the polarity of the voltage across each relay coil 54 in each half-cycle is opposite to that required for operation, so that the relays remain the unoperated condition. This voltage must be overcome before the relay can operate, so that there is inherent stability in the system.

Figure 4 shows the voltages arising in successive half-cycles in the effective circuit equivalent to Figure 1 under internal fault conditions, the fault being fed from both ends.

The polarities of the voltages are now such that, in either half-cycle, current can flow through the coils 54 to operate the relays to trip the protective circuit-breakers of the feeder. The response of the system to fault conditions is thus not dependent on the polarity of the half-cycle in which the fault condition occurred. Either relay, in responding to a fault condition receives current through every full cycle of current. The maximum time for the system to respond is thus less than that required in systems of the known kind in which, because of the assymmetric, half-wave circuit form of arrangements used, the response of the system to fault conditions was inhibited during alternate half-cycles (for example during negative half-cycles), response being permitted only during the intervening (positive) half-cycles. Thus if a fault condition occurred at any point in a negative half-cycle, the commencement of current flow in the relays was always delayed until some point in the next, positive half-cycle. In any complete cycle of current the relay received current for a period of only half a cycle. The response time of the systems was therefore considerably greater than that arising in systems using the present invention.

The operation of the system shown in Figures 2 and 3 is similar to that just described.

The non-linear resistors 62, 64 and 66 limit the voltages across the relay coil 54 and across the pilot conductors 1 6, 1 8, the resistances of the resistors decreasing as the voltage across them increases.

By suitable choice of component values and transformer tappings the protection system described above can respond to the following fault conditions: any phase 1 0, 12 or 14 to earth; any short circuit between any two phases; and any three-phase fault.

The supervisory circuitry shown in each protective circuit arrangement in Figure 2 may, if preferred, be used in the circuit arrangements shown in Figure 1. The capacitors 76 would be connected between respective points 20 or 24 and rectifiers 48.

Claims (14)

1. A circuit arrangement connectable at two points to respective pilot conductors comprising a summation transformer having a secondary winding which is arranged across said points and across two opposed semiconductor rectifier elements in series with one another, a protection relay coil being connected between a tap on the secondary winding and a point intermediate said opposed rectifier elements, which elements are connected to the secondary winding by respective semiconductor rectifier and resistive elements in parallel with one another.

2. An arrangement according to claim 1, in which the opposed rectifiers are directly connected across said two points.

3. An arrangement according to claim 1, in which an isolating transformer has a first winding connected across said opposed rectifier elements and has a second winding connected across said two points.

4. An arrangement according to claim 3, in which a choice of tappings is provided on said second winding to allow the effective number of turns thereof to be changed.

5. An arrangement according to any preceding claim in which part of supervision circuitry is connected across said two points for the purpose of DC supervision of the integrity of pilot wires and a capacitive element is provided to block the flow of direct current except through said pilot conductors.

6. An arrangement according to any preceding claim, in which a non-linear resistor is connected in parallel with said protective relay coil.

7. An arrangement according to any preceding claim, in which two non-linear resistors in series with one another are connected across said secondary winding and in which said centre tap thereof is connected to a point intermediate said two non-linear resistors.

8. A protection system comprising two similar arrangements each according to any preceding claim, the arrangements being positioned at respective locations on a feeder and being interconnected by two pilot wires connected one to each of said two points at each station.

9. A system according to claim 8, in which each summation transformer has a primary winding which is connected to current transformer means inductively coupled to the feeder.

10. An arrangement according to claim 1 substantially as described with reference to Figures 1, 3 and 4 of the accompanying drawings.

11. An arrangement according to claim 1 substantially as herein described with reference to Figures 2, 3 and 4 of the accompanying drawings including "send" or "receive" supervision circuitry shown in Figure 2.

12. An arrangement according to claim 1 substantially as described herein with reference to Figures 2, 3 and 4 of the accompanying drawings but omitting all supervision circuitry.

13. An arrangement according to claim 10, in which "send" or "receive" supervision circuitry is provided for the purpose of DC supervision of the integrity of pilot wires, substantially as described.

14. A protection system comprising two similar arrangements each according to any claim of claims 1 to 7 or 10 to 13.

1 5. A protective system substantially as described herein with reference to Figures 1, 3 and 4 of the accompanying drawings.

1 6. A protection system substantially as herein described with reference to Figures 2,3 and 4 but omitting all the supervision circuitry shown in Figure 2.

1 7. A protection system according to claim 15, in which "send" and "receive" supervision circuitry is provided for the purpose of supervision of the integrity of pilot wires substantially as described.

1 8. A protection system substantially as described herein with reference to Figures 2, 3 and 4 including the "send" and "receive" supervision circuitry as shown in Figure 2.