RU1775788C - Three-phase differential protection device for a transformer - Google Patents

Abstract

The inventive known DZ transformers have a slowdown due to the presence of a second harmonic filter in their composition, which is used to tune out the magnetizing current surge (BIT) that occurs when the protected transformer is energized. In order to eliminate the indicated operation delay while maintaining the detuning ability from the BNT, a second comparator, a pulse expander and an And element are additionally introduced in this differential protection device, the second And input connected to the output of the first time delay element, and the output of the And element connected to the second input OR phase. The sensitivity of the differential protection device is 0.1 1H. Response time 20-30 ms. 4 ill. (L C

Description

The invention relates to electrical engineering, in particular to relay protection of power systems, and can be used to protect power transformers with a star-delta connection circuit.

Differential protection of transformers and autotransformers is known, containing in each phase a working circuit, a first and second braking circuit and a reactive organ consisting of a series-connected relay former, a first inverter, a first time delay element, a second inverter and a second time delay element.

In this device, the detuning from the periodic inrush of the magnetizing current (BNT), which occurs when the protected transformer is energized, is performed by braking the second harmonic of the differential current from the current. The second harmonic current is generated by a passive filter included in the first braking circuit.

The method of detuning from the periodic BNT used in the indicated device makes it possible to reliably distinguish the BNT mode from other operating modes of the protected transformer and to exclude false protection trips. The disadvantage of this method is

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ba, there is some deceleration of the protection due to the occurrence of the second harmonic in the transient mode of short circuit (short circuit) in the protected zone.

A three-phase differential relay is also known, which comprises in each phase a working circuit, a relay driver, first and second inverters, a time delay organ, a memory signal for a certain time, an output organ, a brake circuit, first and second pulse duration limiters, and an OR logic element. A first pulse width limiter is connected between the pulse shaper and the first inverter. The outputs of the time delay organs of all three phases are connected in a triangle through the second duration limiters.

The indicated relay does not work when the protected transformer is energized and flows in one of the phases of a periodic BIT. Its disadvantages include low reliability in case of asymmetrical short-circuit in the protected zone, accompanied by the flow of currents with different amplitudes in different phases. of this phase, this will block the adjacent phase of the relay, although the current at its input may be sufficient to trip. Thus, in certain modes, the functioning of the specified relay may fail.

The aim of the invention is to increase the reliability of the operation of the device.

This goal is achieved in that in each phase of the device containing the differential current generating unit, the output of which is connected to a circuit consisting of a half-wave rectifier connected in series, the first comparator, the first inverter, the first time delay element, the OR element, the second inverter and the second a time delay element, an additional circuit is introduced, consisting of a second comparator connected in series, a pulse expander and an element I.

In FIG. 1 shows a functional diagram of a device; Fig.2.3.4 diagrams of work.

The device for differential protection of the transformer comprises in each phase a series-differential current generating unit 1,

a half-wave rectifier 2, the first comparator 3, the first inverter 4, the first time delay element 5, the OR element b, the second inverter 7, the second time delay element 8, as well as a circuit from the second comparator 9 connected in series, the pulse expander 10 and the And 11 element and the inputs of the first 3 and second 9 comparators are connected in parallel, the second input of the And 11 element is connected to the output of the first time delay element 5, and the output of And 11 is connected to the second input of the OR element b of the adjacent phase.

The differential current generating unit 1 sums up the instantaneous values of the currents of the CTs of different protection arms in one phase.

The differential protection device operates as follows. In normal mode, the balance of currents from the higher and lower voltages is performed.

0 of the protected transformer, therefore, the differential current at the output of the differential current generating unit I is negligible. Current at the output of a half-wave rectifier 2

5 is not sufficient to trip comparators 3 and 9, therefore a zero signal is present at their outputs. The same signal is present at the output of the pulse expander 10, which causes the signal O at the output of the element And 11. Thus, in normal mode, the protection phases do not affect each other. Since at the output of the first inverter 4 there is a signal O, then at its output, as well as at the outputs

5 of the first time delay element 5 and AND of the OR element b, there is a signal I. At the outputs of the second inverter 7 and the second time delay element 8 there is a signal O. The protection device does not work.

0 When you turn on the protected transformer under voltage, a transient process begins, accompanied by the leakage of the BIT from the higher voltage side.

5 As can be seen from Fig. 2, the currents in two phases (A and B) containing an aperiodic component have a similar curve shape and differ only in polarity. The current in the third phase (C), let us call it special, does not contain an aperiodic component. The current in the special phase C, in addition to the absence of an aperiodic component, has other significant differences from the currents in the other phases. Firstly, its amplitude is 1.5-2

5 times less than the amplitudes of the currents in other phases, secondly, current-free pauses in the current of a special phase are less than in currents of other phases and their duration can reach a maximum of 4

ms, while in other phases their duration can be 6-7 ms. Timing diagrams of the operation of phase B in the BIT mode are shown in Fig. 3, a, and diagrams of the operation of the special phase C in the same mode are shown in Fig. 3b,

In phase B, under the action of the output signal of a half-wave rectifier 2, both comparators 3 and 9 periodically operate and signals I receive at their outputs. The threshold of operation of the second comparator 9 is chosen to be about 1.5 times greater than the threshold of the first comparator 3, so it will trigger will be somewhat later than the first and the pulse width at its output will be slightly less. The pulse expander 10 increases the duration of the pulses arriving at its input for a time of about 20 ms. therefore, there will be a continuous signal 1 at its output, which permits the transmission of blocking pulses through the AND element 11 to the second input of the OR element 6 of the neighboring phase C. At the output of the first inverter 4, pulses appear in the signal at the output of the first comparator 3. The first time element 5 has a delay of 5 ms; therefore, short pulses appear at its output. If signal O is present at the second input of OR element 6, the pulses from the output of the first time delay element 5 are transmitted without change to the input of the second inverter 7, the output of which is pulses with a repetition rate of 50 Hz, i.e. with a repetition period of 20 ms. The second time delay element 8 has a response delay of more than 20 ms. therefore, in this mode, the signal at its output is equal to O.

Consider the operation of the device in a special phase C with BIT. The main difference from the operation of the device in phase B is that the duration of a longer pause at the outputs of the comparators 3 and 9 is less than 5 ms. Therefore, the signal O appears at the output of the first time delay element 5. However, at the output of the OR element 6 of this phase there are short pulses that come from the output of the OR element 11 of phase B. Further, these pulses are inverted by the second inverter 7 and in the form of a pulse train with a period A 20 ms repetition is applied to the input of the second time delay element 8, which does not operate. In BIT mode, none of the phases of the device will not work.

With the BYT attenuation, the current amplitudes in all phases decrease, and the duration of pauses at the outputs of comparators 3 and 9 of all phases increases. The initial amplitude of the current in the phases A and B is greater than in the special phase C, therefore, by the time when the second comparator 9 in phase B stops working, i.e. the lock will be released from the special phase C, the duration of pauses at the output of the first comparator 3 of phase C will exceed 5 ms and this phase of the device will not work.

Potential diagrams of the operation of one phase of differential protection during short circuit (short circuit) in the zone are shown in Fig.4. Case under consideration when in

At the short circuit current of this phase, an aperiodic component is present. In this case, the differential current will practically follow the shape of the short-circuit current. Under the influence of the rectified signal, the first and second comparators will periodically operate at the outputs of which there are pulses with a pulse repetition rate of 100 Hz at the output of the two-half5 periodic rectifier 2. The duration of the pulses at the output of the first

0 of the comparator is 2 -3 ms, i.e. does not exceed the time delay value of the time element 5, therefore, the signal at the output of the latter becomes equal to 0. From the output of the inverter 7, the signal I is supplied to the input of the second time element 8, which is activated after a time longer than 20 ms. The pulses at the output of the second comparator 9 causes the appearance of a constant signal I at the output of the pulse expander 10, which

0 allows the passage of blocking pulses through the logical element And 11, but since at the second input of this element in this mode there is a signal O, then at the output of the element And 11 there will also be a signal O, i.e.

5 in short-circuit mode in the protection zone, individual phases of the relay will operate independently of each other.

The minimum tripping current of the device for differential protection can reach 50% of the rated current of the protected transformer, the tripping time, depending on the phase of the occurrence of the short circuit, is 20-30 ms.

Claims (1)

The claims 5 A device for three-phase differential protection of a transformer, comprising in each phase a differential current generating unit, the output of which is connected to a circuit consisting of two half-wave rectifiers connected in series, the first comparator, the first inverter, the first time delay element, the OR element, the second inverter and the second time delay element, about t5, characterized in that, in order to increase the reliability of operation, an additional circuit is introduced into each phase, consisting of sequentially connected nennyh second comparator, and the pulses of the dilator element and wherein the input Ft.g