FR2464586A1 - STABILIZED OVERCURRENT RELAY FOR ELECTRICAL EQUIPMENT - Google Patents

Abstract

THE INVENTION CONCERNS ELECTRICAL APPARATUS. A STABILIZED OVERCURRENT RELAY INCLUDES A REFERENCE 9 DEVICE THAT INCREASES THE REFERENCE VALUE WHEN CONNECTING ELECTRICAL EQUIPMENT, SUCH AS FOR EXAMPLE CABLES OR POWER TRANSFORMERS. THE REFERENCE DEVICE IS DESIGNED SO THAT THE INCREASE IN THE REFERENCE VALUE OCCURS INSTANTLY AT THE TIME OF CONNECTION, AND SO THAT THE REFERENCE VALUE THEN DECREASES ACCORDING TO AN EXPONENTIAL CURVE TO THE NORMAL LEVEL. APPLICATION TO THE PROTECTION OF ELECTRICAL NETWORKS.

Description

i

  The present invention relates to a device for stabilizing

be overcurrent relays.

  It is known that transients occur in phase currents when power transformers are connected. These transients can be 10 to 20 times the rated current and they

  may have the appearance shown in Figure lb (see above).

  after). If this is not considered in an overcurrent relay, the relay will trip when the power transformer is connected. The same problem arises

  for connecting certain other cables and electrical devices.

  To stabilize against calls for coverage

  magnetising, normally a filtering or an increase is used.

  tation of the reference value. In many cases, filtering

is too slow and complex.

  In the case of an increase of the reference value, the relay must receive a pulse before the closing of the line circuit breaker, so as to increase the reference value for the entire duration during which it is estimated that the closing transient will be greater than to the normal reference value. If we use the contact

  auxiliary circuit breaker to provide this impulse, the

  line is becoming more expensive. The trigger value

  is also increased up to 10 to 20 times the rated current

  for a relatively long period. If the closing control contact is used instead to obtain the pulse, there is no increase in wiring cost. However, the duration of the increase in the reference value will be even longer, because of the uncertainty of the delay that runs until closure.

  circuit breaker, for example because of the synchronization. If it appears

  now a fault with a fault current lower than about 10 to 20 times the nominal current, the protection device does not disappear this defect, until the passage of time

  during which the reference is increased.

  The object of the invention is to provide a solution to the aforementioned problems as well as other related problems, and is characterized in that the reference value is designed to increase instantly at the precise moment when a device , etc. is connected, after which the output signal which corresponds to the reference value decreases exponentially until

normal level.

  The advantage of the invention is that it makes it possible

  hold the increase of the reference value at the correct moment and that the increase does not last as long. By detecting the homopolar current, a criterion is obtained indicating that a connection has been made, which makes it possible to increase the reference value of the relay. The reference value then decreases to the normal level according to an exponential function. Thus, the considerable increase

  the reference value takes place for a relatively short period.

  The moment of increase of the reference value is controlled by a

  flip-flop whose inputs receive a signal that comes from a detection

  current generator and a signal that comes from the contact of

  closing the circuit breaker or the auxiliary contact of the circuit breaker

line.

  The invention will be better understood on reading the description

  which follows of an embodiment and with reference to the accompanying drawings, in which: Figures la, lb and lc represent some configurations

of curves that appear.

  Fig. 2 is a diagram which includes a circuit breaker, a line breaker, a control contact and conductors for a three-phase system.

  FIG. 3 represents a device corresponding to the invention

tion.

  Figure 4 shows some of the signals that appear

  in the circuit of Figure 3, with corresponding designations

to those of Figure 3.

  FIG. 1a shows a typical fault current curve.

  risk, with a complete direct component. Figure 1b shows the magnetizing current draw, for example for a power transformer. Figure lc represents the reference value with the normal low level, the instantaneous increase and the exponential decrease

to return to the normal level.

  FIG. 2 shows a three-phase line with R, S and T phases. A line circuit breaker 1 equipped with an auxiliary contact 2 is connected to this line. In this case, the auxiliary contact must be of the spring-return type, as well as the additional contact which

240ô4586

  is associated with the closing control contact, 3. These contacts provide a signal A which is used as an input signal for relay 4. The reference C designates the current in one of the phases and

  the reference B designates the homopolar current. These signals are also

  relay 4 input signals.

  FIG. 3 represents a device corresponding to the invention

  tion. The signal A corresponds to the signal A of FIG. 2. This signal serves to prepare the protection device for a change of the reference value. The signal B corresponds to the signal B of FIG.

  This signal triggers the increase of the reference value.

  The signal C corresponds to the signal C of FIG.

  portionnal current in the line that monitors the relay. The signal

  A is applied via a pulse extension circuit.

  6, at an inverted input R of an R-S flip-flop.

  the flip-flop 8 receives a logic signal from the comparator

  amplitude sensor 7 and the input signal thereof is a signal which is proportional to the zero sequence current. The exit of the rocker

  connected to the input of a reference device 9 whose output

  tie is itself connected to the reference input of the comparator

  amplitude 5 of the intensity relay.

  In the home position, the signal A is set to zero and the flip-flop 8 can not provide any output signal. There is thus no increase in the reference value, although the comparator 7

  detects a homopolar current. When activating the iron contact

  Closure of the control unit or the auxiliary contact of the circuit breaker

  signal A is set to 1. This releases the flip-flop.

  In this case, the output of the flip-flop applies a 1 to the input of the reference device 9, and the reference value is momentarily increased. We get the configuration of

  curve of the reference value (Figure 1c) by loading a condensa-

tor (not shown).

  Figure 4 shows the signal that comes from the contact

  closure circuit or the auxiliary contact of the line circuit breaker A. The signal D represents the same pulse after passing through the

  Pulse extension circuit 6. Signal E is the pulse that pro-

  The signal F is the output signal of the flip-flop 8. The curve G shows how the reference value changes when the criterion of increasing the value of

  reference is satisfied, at the moment.

  It goes without saying that many modifications can be made to the device described and shown, without departing from the scope of the invention.

Claims (4)

REVENDI CATIONS   1. Overcurrent relay for electrical devices and apparatus, such as cables and power transformers, comprising a reference device (9) for the current (C), characterized in that the reference value is designed in such a way to be temporarily increased when a device is connected, and in that the reference device is designed so that its   The output signal then decreases exponentially to the normal level.   wrong. 2. Device according to claim 1, characterized in that the increase of the reference value is carried out using a   logic circuit which provides an output signal when it receives simul-   a signal from a closing control contact of a line circuit breaker (A) and a signal from a homopolar current P).   3. Device according to claim 1, characterized in that the increase of the reference value is carried out using a   logic circuit which provides an output signal when it receives simul-   a signal from an auxiliary contact of the circuit-breaker   line (A) and a signal from a homogeneous current detector polar (B).   4. Device according to claim 1, characterized in that   that the reference value is obtained by charging a capacitor.