GB2059695A

GB2059695A - Overcurrent protection relay

Info

Publication number: GB2059695A
Application number: GB8028298A
Authority: GB
Original assignee: ASEA AB
Current assignee: ABB Norden Holding AB
Priority date: 1979-09-03
Filing date: 1980-09-02
Publication date: 1981-04-23
Also published as: SE7907301L; DE3031509A1; YU216480A; SE418554B; BR8005572A; FR2464586A1

Abstract

The invention relates to an overcurrent relay 5 with means to increase the threshold value @ for tripping of the relay when electrical apparatus, for example power transformers and cables, are connected to a network. The increase in the threshold value @ is arranged to take place instantaneously upon connection of the apparatus and a threshold value control device (9) is provided to ensure that after the instantaneous increase, the threshold value decreases exponentially to a normal threshold value. <IMAGE>

Description

SPECIFICATION Stabilized overcurrent protection relay and improved method of stabilizing such a relay Technical Field The present invention relates to an improved overcurrent protection relay of the kind used to protect electrical apparatus against current overload and in particular is concerned with improving the stabilization of such protection relays against tripping in the face of anticipated current surges such as arise when electrical equipment is connected to one or more lines in an electrical network.

Background Art It is well known that transients appear in the phase currents of an electrical network when a power transformer is connected to the network. These transients may have a peak value which is 10 to 20 times the rated current. If the possibility of such transients appearing is not taken into consideration in the design of an overcurrent protection relay, there is a risk of the relay being tripped when a power transformer is connected to the network. The same possibility of unnecessary tripping of the relay arises upon the connection of certain other electrical apparatus and/ or cables into the network.

To stabilize a protection relay against such transients, it is known to resort to filtering out the transients or to temporarily increasing the threshold value at which the relay trips. Resorting to filtering is in many cases undesirable since it is a slow and complicated process.

To rely on a temporary increase in the threshold value at which the relay trips, requires that the relay must receive an appropriate triggering signal before the connection into the network is made in order to initiate the increase in the threshold value before the transient appears and to maintain that increased threshold value for as long as the transient is expected to be higher than the threshold value at which the relay normally operates. If an auxiliary contact on the line circuit-breaker is to be used to provide this triggering impulse, the task of laying the lines of the network will become more expensive.

Further, the threshold value will have to be increased to 10 to 20 times the rated current and will have to be held at this high value for a relatively long period. If instead, the operating contact used for connecting the equipment to the network is used to generate the triggering signal, no increase in the cost of installing the lines will arise but the period during which the threshold value is held at its high value will, however, be even longer because of the uncertainty in the time taken for the operating contact to close, for example because of the need to synchronize the closure. Should a fault occur in the network which generates a fault current which is lower than about 10 to 20 times the rated current during the period in which the threshold value is increased, the protection device will not detect this fault until the period of increased threshold value has run out.

Disclosure of Invention The invention aims to provide a solution to the above-mentioned problems and other problems associated therewith.

According to one aspect of the invention an overcurrent protection relay for electrical apparatus, comprising a threshold value control device for varying the threshold value at which the relay is tripped, is characterized in that the control device is adapted to increase the threshold value momentarily when the apparatus is connected into an electrical line, and thereafter to decrease the threshold value exponentially to return to a normal value.

According to a further feature of the invention, a method of stabilizing an overcurrent protection relay against tripping by anticipated transient current surges which method comprises increasing the theshold value at which the relay is set to trip during the period in which appearance of the transient current surge is anticipated, is characterized in that said threshold value increase is tailored substantially to match the exponential decay in the amplitude of the current surge.

An important advantage of the invention is that the initial increase in the threshold value occurs at the correct moment and that the initial increase does not last for such a long time. Preferably the zero sequence current is detected and is used to trigger the initial fast increase of the threshold value. The threshold value then decreases exponentially to the normal level. Thus, the considerable initial threshold value increase arises for only a relatively short period. Timing of the threshold value increase can be controlled by a flip-flop which, on its inputs, receives a signal from a zero sequence current detector and a signal from the closing operating contact of the line circuit-breaker or from an auxiliary contact on the line circuit-breaker.

Brief Description of Drawings The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure la shows a typical fault current curve arising in a phase line of a network.

Figure ib shows a typical transient current surge arising, for example, on connection of a power transformer to a line of the network, Figure ic shows the threshold value increase employed in accordance with the invention, with the normal threshold level on the left followed by an instantaneous initial increase and an exponential decrease back to the normal threshold level, Figure 2 is a circuit diagram of a threephase network with a relay, a line circuitbreaker and an operating contact, Figure 3 is a block diagram showing a relay according to the invention, Figure 4 shows some of the signals appearing in the device of Fig. 3, and Figure 5 shows a part of Fig. 3 in more detail.

Description of Preferred Embodiment Fig. 2 shows part of a three-phase network with phase lines R, S and T. A circuit-breaker 1, with an auxiliary contact 2, is connected to each line of the network and an operating co.ntact 3 is used for energizing the circuit-breaker 1. The auxiliary contact 2 is of aspring-back type, as is an additional contact 3a associated with the operating contact 3.

The contact 3a generates one signal A which is fed as an input signal to an overload protection relay 4. Further input signals to the relay 4, RI, SI and TI, are derived from the currents flowing in the conductors connected to the phase lines by the circuit-breaker 1 and one of these (RI) is indicated at C.

Fig. 3 shows the relay 4 adapted to operate according to the invention. Signal A corresponds to the signal A shown in Fig. 2 and is employed to prepare the protection relay for a change in its threshold value. Signal B corresponds to the signal B in Fig. 2 and this signal initiates the increase in the threshold value. Signal C corresponds ta the signal C in Fig. 2 and is proportional to the current in the line which the relay 4 is to monitor. The signal A is fed, uia a pulse extension circuit 6, to an inverted input R' of an SR flipflop 8. Tb the second input S' of the flip-flop 8 there is connected a logic signal from an amplitude comparator 7, the input signal B of which is a signal proportional to the zero sequence current.The output F af the flip-fla,o 8 is connected to the input of a threshold value control device 9, the output G of which in its turn is connected to the threshold value input of an amplitude comparator 5 of the current relay.

Under normal conditions, signal A will be logic zero and the flip-flop 8 cannot deliver any output signal. Thus, no threshold value increase occurs when a zero sequence current is detected by the comparator 7. When the contact 3 of the circuit-breaker 1 is closed, the auxiliary contact 3a also closes, causing.

signal A to become logic one. The appearance of this logic one signal on the input R' of the flip-flop 8 releases the flip-flop on the next detection of zero sequence current, the output of the flip-flop 8 then supplying a logic one signal to the input of the control device 9 causing an immediate increase in the threshold value. The exponential decay in the threshold value which follows the immediate increase- (and which is shown in Fig. 1 c) is suitably obtained by charging a capacitor (not shown).

Graph A in Fig. 4 shows the signal arising from the auxiliary contact 3a on the occasion of a closing operation of the line circuitbreaker 1. This signal could equally well be derived direct from the operating contact 3.

Graph D shows how this signal A is extended by the circuit 6. Graph E shows the pulse generated by the zero sequence current detector 7. Graph F shows the output signal from the flip-flop 8, and graph G shows how the threshold value changes when the conditions for a theshold value increase are fulfilled at the time t.

Fig. 5 shows one possible arrangement for generating the d.esired threshold value. F and G are the same signals as shown in Fig. 3. 10 is a capacitor, 11, 1 2 and 1 3 are resistors and 1 4 is an operational amplifier. The leading edge of the single pulse which constitutes the F-signal, charges up the capacitor 10, to produce a substantially instantaneous increase in the voltage appearing across the resistor 11. There is, therefore, a substantially instantaneous increase in the G-signal. Thereafter, the capacitor 11 discharges through the resistor 11 causing the G-signal to follow an exponentially decreasing curve shape.

Claims

1. An overcurrent protection relay for electrical apparatus, comprising a threshold value control device for varying the threshold value at which the relay is tripped, characterised in that the-control device is adapted to increase the threshold value momentarily when the apparatus is connected into an electrical line, and thereafter to decrease the threshold value exponentially to return to a normal value.

2. A protection relay according to claim 1, in which the threshold value increase is initiated by means of a logic circuit which delivers an output signal when it simultaneously receives a signal from a moving contact associated with a circuit-breaker used to connect the apparatus into the line and a signal from a zero sequence current detector.

3. A protection relay according to claim 2, in which the moving contact is the operating contact of the circuit-breaker.

4. A protection relay according to claim 2, in which the moving contact is an auxiliary contact in the circuit-breaker.

5. A protection relay according to any preceding claim, in which the exponential decrease of the threshold value is obtained by charging a capacitor.

6. An overcurrent protection relay for electrical apparatus, substantially as hereinbefore described with reference ta Figs. 2, 3 and 5 of the accompanying drawings.

7. A method of stabilizing an overcurrent protection relay against tripping by anticipated transient current surges, which method comprises increasing the threshold value at which the relay is set to trip during the period in which appearance of the transient current surge is anticipated, characterised in that said threshold value increase is tailored substantially to match the exponential decay in the amplitude of the current surge.

8. A method as claimed in claim 7, in which the onset of the threshold value increase is controlled by means of a zero sequence current detector.

9. A method of stabilizing an overcurrent protection relay against tripping by anticipated transient current surges, substantially as hereinbefore described with reference to the accompanying drawings.