RU2497256C1 - Device for differential protection of power transformer - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: device includes current sensors that consist of a secondary winding and a core from non-magnetic material, smoothing filters, a signal converter, a differential signal shaping unit, the first shaper of actual value of input signal, an actuation set point shaping unit, the first braking chain that includes a signal delay unit, an adder and the second shaper of actual value of input signal, the second braking chain that includes a braking signal shaping unit and the third shaper of actual value of input signal, the first and the second locking member, as well as a responding member that includes a comparator and a logic unit.

EFFECT: improving quick action at external short circuits, and increasing sensitivity to turn-to-turn short-circuits in a power transformer in maximum loading modes.

9 dwg

Description

The present invention relates to electrical engineering, in particular to devices for relay protection of transformers from short circuits (short circuit), and can be used to protect double-winding and three-winding power transformers.

A device for differential protection of a power transformer is known, which comprises: an intermediate transformer with a load resistor and a rectifier, a first square-wave pulse shaper, to the output of which a first inverter, a first time delay element, a signal memory, a second inverter and a first reacting organ are connected in series. This part of the device works with internal faults that are not accompanied by deep saturation of current transformers (CT). For this purpose, current-free pauses at the input of the first shaper of rectangular pulses are measured, if the pauses turn out to be less than the set value, the first reacting organ is triggered. When current transformers are saturated, currentless pauses are distorted, and the first reacting organ does not work.

The second rectifier pulse shaper is also connected to the output of the rectifier bridge, the second and third time delay elements connected through the third and fourth inverters to the control inputs of the BAN elements, the main inputs of which are connected to the second rectangular pulse shaper, are connected to the output of the rectifier bridge. In this case, the output of the first inhibit element through the first pulse width converter and the memory element is connected to the first input of the comparison unit, and the output of the second inhibit element through the second pulse width converter is connected to the second input of the comparison unit, the output of which is connected to the input of the second reacting organ. This part of the device compares the amplitudes of the triangular pulses of the second period of the transition process with the amplitudes of the triangular pulses of subsequent periods, while the amplitudes of these pulses correspond to the half-wave length of the differential current at a certain measurement level. If at a certain point in time the amplitude of the pulse of one of the subsequent periods exceeds the amplitude of the triangular pulse of the second period, then the protected transformer is turned off [Copyright certificate SU 1095298 A1, 05/30/1984].

The disadvantage of this device is the low speed in the internal fault modes, in which there is a saturation of the CT. Namely, there are possible modes in which the saturation of the CT can begin in the first period of the transition process, and the exit from saturation - much later. At the same time, when the pulse amplitude of one of the subsequent periods exceeds the amplitude of the triangular pulse of the second period, it occurs not earlier than the moment the CT begins to exit saturation, which causes a significant delay in the protection operation.

There is also known a differential protection device for a power transformer, which is a prototype of the invention. The prototype contains: current sensors, consisting of a secondary winding, a core of magnetic material and installed at the ends of the protected object; a working circuit consisting of a transreactor, a rectifier and differential cutoff; the first braking circuit, consisting of a transreactor, a second harmonic filter, a rectifier and a smoothing filter; a second braking circuit, consisting of a block for generating a brake signal, a circuit that implements a braking characteristic, and a smoothing filter; a reacting organ consisting of a rectangular pulse relay driver, two time delays and two inverters.

This device uses a combination of the time-pulse principle of detuning with braking from the second harmonic of the differential current for detuning from inrush currents of magnetization current (BTN). The second harmonic current is extracted using a passive filter, which is part of the first brake circuit. The second brake circuit detaches from steady-state unbalance currents using percent braking by the sum of the modules of the currents of the arms. The differential current in the prototype is formed from the secondary signals of current sensors and rectified in the working circuit [A. Dmitrienko M. Differential protection of transformers and autotransformers. - "Electricity", 1975, No. 2, p.1-9].

The disadvantages of the prototype include the following. To determine the current mode of operation of the power transformer in the device, the duration of dead time pauses at a certain level of measurement is determined. It is obvious that when the current sensor is saturated as a result of the inrush of the magnetizing current passing through it, the pause metering level will increase in order to reliably detune from this mode. A passive filter is designed for this, which extracts the second harmonic from the differential signal. The level of the selected second harmonic is overstated and, accordingly, the offset from the BTN. An emergency mode is also possible in which the current sensor saturates when overcurrents of internal fault flow through it. Saturation of the sensor leads to the fact that the passive filter extracts the second harmonic from the differential current and overestimates the level of measurement of current-free pauses. Obviously, in this case, the magnitude of the measurement may be sufficient to block the reacting organ. The maximum load mode of operation is also possible when a significant unbalance current flows in the differential circuit. An increase in the trip setting for detuning from this current leads to a decrease in the sensitivity of the protection to small internal fault currents, for example, to short circuits (OT). Thus, the disadvantages of the prototype are to reduce the speed of protection when the current sensor saturates when the internal short-circuit current flows through it, as well as to reduce the sensitivity to windings in the power transformer when the maximum load currents flow.

The objective of the invention is to increase the speed of protection with internal short-circuit, as well as increasing sensitivity to windings in a power transformer at maximum load conditions.

This object is achieved in that in the device for differential protection of the power transformer, which contains current sensors consisting of a secondary winding, a core and installed at the ends of the protected object, a reacting organ, a first braking circuit and a second braking circuit, which contains a block for generating a brake signal, cores of current sensors are made of non-magnetic material, and a smoothing filter is connected to the output of each sensor. At the same time, the first and second blocking bodies, a signal converter, a differential signal generation unit, a first driver of the effective value of the input signal and a unit for generating the setpoint of operation are additionally introduced into the device. All outputs of the smoothing filters are connected to the inputs of the signal converter, the second and third outputs of which are connected to the inputs of the second blocking organ, and the first, second and third outputs are connected to the inputs of the differential signal generation unit and to the inputs of the second braking circuit. The input of the first braking circuit and the first driver of the effective value of the input signal are connected to the output of the differential signal generating unit. The output of the second braking circuit is connected to the first input, and the output of the second blocking organ is connected to the second input of the response setting unit, the output of which is connected to the third input of the reacting organ. The output of the first braking circuit is connected to the first input, and the output of the first driver of the effective value of the input signal is connected to the second input of the first blocking organ. The output of the first driver of the effective value of the input signal is also connected to the second input of the reacting organ, the first input of which is connected to the output of the first blocking organ.

The first braking circuit contains a signal delay unit, an adder and a second driver of the effective value of the input signal, the input of the second driver of the effective value of the input signal connected to the output of the adder, the first input of which is connected to the output of the signal delay unit, and the second input is combined with the input of the signal delay unit. A third driver of the effective value of the input signal is introduced into the second braking circuit, the input of which is connected to the output of the brake signal generation unit. The responsive body contains a comparator and a logic unit, the control input of the comparator connected to the output of the first driver of the effective value of the input signal, and the reference input to the output of the unit for generating the setpoint response, the first input of the logic unit is connected to the output of the first locking device, and the second input to the output comparator.

The proposed device is implemented in the form of microprocessor protection, a structural diagram of which is shown in figure 1. Figure 2 shows the structural diagram of the first braking circuit, Figure 3 shows the structural diagram of the second braking circuit, Figure 4 shows the structural diagram of the reacting body, and figure 5 is the braking characteristic of the protection. In addition, Fig.6-7 shows the waveform of the proposed device in the internal short circuit mode, and Fig.8-9 shows the waveform of operation when the inrush magnetization current.

The proposed device (Figure 1) contains:

1. Current sensors with a core of non-magnetic material. Sensors are installed on each side of the protected power transformer;

2. Smoothing filters. The output of each current sensor 1 is connected to the input of the smoothing filter;

3. The signal converter. The outputs of all smoothing filters 2 are connected to the inputs of the signal converter;

4. Block forming a differential signal (BFDS). All inputs of this unit are connected to the outputs of the signal converter 3;

5. The second chain of braking. All inputs of the second braking circuit are connected to the outputs of the signal converter 3;

6. The second body lock. The inputs of this body are connected to the second and third output of the signal Converter 3;

7. The first chain of braking. The input of the first braking circuit is connected to the output of the BFDS 4;

8. The first driver of the effective value of the input signal (FDZ). The input FDZ, like the input of the first braking circuit, is connected to the output of the BFDS 4;

9. The block forming the settings of the response (BFUS). The first input of the BFUS is connected to the output of the second braking circuit 5, and the second input to the output of the second locking member 6;

10. The first blocking authority. The first input of this unit is connected to the output of the first braking circuit 7, and the second input to the output of the first driver of the effective value of the input signal 8;

11. Reacting authority (RO). The first input of the PO is connected to the output of the first blocking organ 10, the second input is connected to the output of the first FDZ 8, and the third input is connected to the output of BFUS 9.

The first braking circuit (Figure 2) contains:

12. Block delay signal (BSS). The input of this unit is connected to the output of BFDS 4;

13. The adder. The first input of the adder is connected to the output, and the second to the input of the BSS 12;

14. The second driver of the effective value of the input signal. The input of the second driver of the effective input signal of the value is connected to the output of the adder 13.

The second braking circuit (Figure 3) contains:

15. The block forming the brake signal (BFTS). All inputs BFTS connected to the outputs of the signal Converter 3;

16. The third driver of the effective value of the input signal. The input of the third driver is connected to the output of the BFTS 15.

The reacting organ (Figure 4) contains:

17. The comparator. The first input of the comparator is connected to the output of the first FDZ 8, and the second input to the output of BFUS 9;

18. Block logic (BL). The first input of the logic unit is connected to the output of the first blocking organ 10, and the second input is connected to the output of the comparator 17.

In various modes, the proposed device operates as follows. Since differential protection is carried out in phases, the further description is made for one phase of protection, and further exposition assumes software implementation on a microprocessor. The above blocks have the following functions:

2. Smoothing filters. Provide a decrease in the level of high-frequency components;

3. The signal converter. Since the device is based on a microprocessor-based element base, the output signals of each smoothing filter are converted to digital values, and the protection already operates with information on currents flowing in the primary circuit in digital form. The digitized signals from the sides of the power transformer are fed to a signal converter, which, if necessary, eliminates the phase shift caused by the connection group of the windings of the transformer to be protected, as well as bringing the amplitudes of the input signals to the base side. The values of the reduction factors are determined through the values of the rated currents of the sides of the protected transformer;

4. Block forming a differential signal. Forms

differential protection signal I _d from the currents of the sides of the transformer I _1A , I _2A , I _{3A by} summing their instantaneous values;

5. The second chain of braking. Generates at its output the effective value of the brake protection signal I _t _ _RMS . Together with the second locking unit 6 and the unit for generating the setpoint of operation 9, it detunes the protection against steady-state unbalance currents;

6. The second body lock. This body is designed to generate a blocking signal, which prohibits block 9 from calculating the set point of the protection by the brake characteristic only in the absence of both signals I _2A and I _3A ;

7. The first chain of braking. Generates at its output the effective value of the brake signal of protection I _f _ _RMS - Together with the first blocking device 10, it provides the detuning of protection against BTN;

8. The first driver of the effective value of the input signal. FDZ calculates the effective value of the differential signal I _d and sets the output signal of the corresponding level I _d _ _RMS

9. The block forming the settings of the response. This unit calculates the setting of the protection in accordance with the braking characteristic (Figure 5) depending on the magnitude of the brake signal I _t _ _RMS and the presence of a inhibit signal from the second locking element;

10. The first blocking authority. This body is designed to block protection in BTN mode;

11. Responsive body. Identification of the current operating mode of the protected transformer is carried out by a reacting authority, which, when an internal fault is detected, issues a command to turn off the switches of the protected transformer;

12. Block delay signal. It delays the input signal for half a period of industrial frequency;

13. The adder. Summarizes the differential protection signal I _d with the same signal, but delayed by 10 ms. This ensures the absence in the output signal of the adder I _{f the} first harmonic of the industrial frequency;

14. The second driver of the effective value of the input signal. It calculates the effective value of the brake signal I _f and sets the signal of the corresponding level I _f _ _RMS at its output;

15. The block forming the brake signal. Generates a brake protection signal I _t as the sum of the modules of the instantaneous values of the signals I _1A , I _2A and I _3A ;

16. The third driver of the effective value of the input signal. It calculates the effective value of the brake signal I _t , and sets at its output a signal of the corresponding level I _t _ _RMS ;

17. The comparator. Compares the effective value of the differential signal I _d _ _RMS with the trip setpoint I _cf. The comparator for providing a return coefficient less than one has a dead band;

18. Block logic. Identifies the current mode of operation of the power transformer using input logic signals.

I. The internal fault mode (Fig.6-7).

Using the phase and amplitude-corrected signals from signal converter 3, BFDS 4 calculates the differential signal I _d , and BFTS 15 calculates the brake signal. Next, FDZ 8 and FDZ 16 determine their effective values. FDZ works as follows. The input signal is squared and integrated by two integrators, which are alternately reset to zero with an interval of 10 ms. Before resetting each integrator, a square is extracted from the value accumulated by it and a value equal to the obtained result is set at the output of the PDD. This method of calculating the effective value is used in all FDZ, while the zeroing moments in all blocks are synchronized.

In addition to the BFDS 4 and the second braking circuit, the outputs of block 3 are connected to the second blocking element 6. This body is designed to generate a blocking signal, which prevents the block for generating the setpoint of operation 9 from calculating the setpoint of operation of the protection according to the braking characteristic only in the absence of both signals I _2A and I _3A . To do this, the locking unit 6 calculates the effective values of the input signals of the sides of the transformer without power sources, namely I _2A and I _3A . The results obtained are compared with a preset setting selected in such a way that, in the absence of both signals I _2A and I _3A , the interlock does not remove the inhibit signal from its output under the influence of random noise in the measurement channel. If in the primary circuit current flows on at least one of the sides that does not have a power source, then the response setting I _cf is determined in accordance with the braking characteristic (Figure 5) depending on the value of the brake signal I _t _ _RMS . In the absence of both signals I _2A and I _3A , a inhibitory signal from the second blocking element 6 appears, and the pickup setting remains constant and is determined by the initial pickup current I _cf0 . The parameters of the braking characteristic are determined by the initial braking current I _t0 , the braking coefficient K _t , the initial actuation current I _cf0 and are set by the corresponding settings.

With a short circuit in the protected transformer, the signal in the differential circuit is proportional to the fault current. If the effective value of the differential signal I _d exceeds the setpoint of operation I _cf received from block 9, then the comparator 17 is activated and sets the signal of the logical unit at its output. When the differential signal drops below the return setting, the comparator sets a logic zero on its output, thereby returning to its original state.

At the same time, the first braking circuit 7 generates a brake signal for detuning from the BTN. Since sensors with a core of non-magnetic material are used as primary current sensors, which are not saturated during the flow of primary currents with aperiodic components, with an internal short-circuit the differential signal always remains sinusoidal, while with BTN the differential signal, except the main one, contains a second harmonic and harmonics of higher orders. The aperiodic component of the short-circuit current is significantly attenuated by such sensors, and we can assume that it completely attenuates in half a period of industrial frequency. The proposed circuit, which includes a delay unit 12 and an adder 13, eliminates the first harmonic in its output signal I _f , thereby allowing the protection to be blocked in the inrush modes of the magnetization current by the presence of the second harmonic in the differential current. The effective value of the brake signal I _f _ _{RMS is} supplied to the first input of the first blocking device 10, and the effective value of the differential signal to the second input. The first blocking element determines the ratio of input signals by the formula

$$\left(\frac{{\text{I}}_{\text{f\_RMS}}}{{\text{I}}_{\text{d\_RMS}}}\right)\cdot \mathrm{one\; hundred}\%,$$

- действующее значение тормозного сигнала от первой цепи торможения 7,Where $${\text{I}}_{\text{f\_RMS}}$$

- the effective value of the brake signal from the first braking circuit 7,

- действующее значение дифференциального сигнала от блока 8. $${\text{I}}_{\text{d\_RMS}}$$

- the effective value of the differential signal from block 8.

Next, the result obtained is compared by block 10 with a given setting. If the setpoint is greater than the received ratio, then the output of the blocking organ 10 is set to a signal of a logical unit prohibiting the response of the reacting organ 11. If the setpoint is less than the received ratio, then the logic zero is set at the output of the block. Since in the internal short-circuit mode, the signal I _f quickly attenuates and becomes equal to zero (Fig. 6), then at a certain moment after comparison with the setting at the output of block 10, the inhibit signal is removed.

The logic unit 18 identifies the current mode of operation of the power transformer using the input logic signals. In the normal operation mode of the power transformer, the BL is in its initial state and its output signal is zero. When the comparator 17 is triggered and the blocking signal is removed by the first blocking organ 10, block 18 is put into an alarm state and starts a time delay of 10 ms. After the expiration of the time delay, the logic unit again checks the input signals. If at the same time the state of the signals has not changed, then the current mode of operation of the transformer is considered emergency and the logical unit signal is set at the output of the unit. If, however, the comparator 17 returned to its original state or a inhibitory signal from the first blocking device 10 appeared, then the current operating mode of the transformer is considered non-emergency and the logic unit 18 returns to its original state. Since 10 ms after switching to alarm mode, the signals at the BL input do not change (Fig. 7), the current operation mode of the protected transformer is considered emergency and a command is issued to turn off the power transformer circuit breakers.

II. BTN mode (Figs. 8-9).

When the protected transformer is turned on at idle or when the external short circuit is turned off, a unipolar or bipolar magnetization current surge occurs on the supply side. In this mode, the effective value of the differential signal I _d exceeds the setpoint operation I _cf and causes the comparator 17.

оказывается больше уставки первого органа блокировки 10. В связи с этим на выходе блока 10 запрещающий сигнал не снимается и срабатывание защиты не происходит. Таким образом, при протекании в дифференциальной цепи броска тока намагничивания защита надежно блокируется.At the same time, a signal is generated at the output of the adder 13, due to the higher harmonics of the BTN, which leads to the fact that the result of calculating the percentage between the actual values of the signals I _{d_RMS} and $${\text{I}}_{\text{f\_RMS}}$$

it turns out to be more than the setting of the first blocking organ 10. In this regard, the blocking signal is not removed at the output of block 10 and the protection does not work. Thus, when the magnetizing current flows in the differential circuit, the protection is reliably blocked.

The proposed device contains current sensors with a non-magnetic core, which are not saturated during the flow of currents of high multiplicity. The absence of saturation excludes the appearance of a second harmonic in the differential current and increases the speed of protection in internal short-circuit modes of any multiplicity. In addition, the aperiodic component of the short-circuit current is significantly attenuated by such sensors and we can assume that it does not affect the speed of protection. The lack of saturation also eliminates the appearance of a transient unbalance current with external short-circuit, and also reduces both the steady-state unbalance current and its increment with an increase in the through current through the power transformer. Thus, the proposed device can reduce the minimum operating current protection and reduce the slope of the braking characteristic. Under maximum load conditions, the response current of the proposed device increases slightly, thereby increasing the sensitivity of the protection to coil faults in the power transformer.

Claims (1)

A device for differential protection of a power transformer, comprising current sensors installed at the ends of the protected object, which contain a core and a secondary winding, a reactive organ, a first braking circuit and a second braking circuit, which comprises a brake signal generating unit, characterized in that the cores are made in current sensors from non-magnetic material and into the device are additionally introduced smoothing filters, the first and second locking elements, a signal converter, a differential generating unit of the signal, the first driver of the effective value of the input signal and the unit for generating the setpoint of operation, moreover, a smoothing filter is connected to each output of the current sensor, the output of each smoothing filter is connected to the inputs of the signal converter, the second and third outputs of which are connected to the inputs of the second blocking element, and the first, the second and third outputs - to the inputs of the differential signal generation unit and to the inputs of the second braking circuit, the input of the first braking circuit and the first driver of the current The input signal is connected to the output of the differential signal generating unit, the output of the second braking circuit is connected to the first input, and the output of the second blocking device is connected to the second input of the response setting unit, the output of which is connected to the third input of the reacting organ, the output of the first braking circuit is connected to the first input, and the output of the first driver of the effective value of the input signal to the second input of the first blocking body, the output of the first driver of the effective value of the input signal and also connected to the second input of the reacting organ, the first input of which is connected to the output of the first locking element, while the first braking circuit contains a signal delay unit, an adder and a second driver of the effective value of the input signal, and the input of the second driver of the effective value of the input signal is connected to the output of the adder , the first input of which is connected to the output of the signal delay unit, and the second input is combined with the input of the signal delay unit, the third driver is introduced into the second braking circuit The effective value of the input signal, the input of which is connected to the output of the brake signal generation unit, the reacting organ contains a comparator and a logic unit, the control input of the comparator connected to the output of the first driver of the effective value of the input signal, and the reference input to the output of the operation setpoint formation unit, the first input the logic block is connected to the output of the first blocking organ, and the second input to the output of the comparator.

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