JPS6147048B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a transformer protection relay, and more particularly to a protection relay suitable for avoiding malfunctions caused by magnetizing inrush current of a transformer.

Conventional transformer protection relays are ratio differential relays with a suppressing force based on the scalar amount of passing current, and the second harmonic component of the differential output of the transformer input current is used as a suppressing force to prevent malfunctions caused by excitation inrush current. , equipped with a harmonic ratio relay that uses the fundamental wave component as the operating force,
The matching output of both is the final output.

However, even in the event of an internal failure in a transformer, harmonic currents are generated due to the distribution of capacitance and inductance in the system, and the capacitance tends to increase. Wave currents may also occur, and conventional countermeasures against magnetizing inrush current by suppressing second harmonics have the drawback of not operating or prolonging operating time in the event of an internal failure.

An object of the present invention is to provide a transformer protection relay that does not become inoperable due to harmonics when a failure occurs.

In the present invention, the excitation inrush current of a transformer is caused by the excitation impedance of the transformer being asymmetric between positive and negative polarities, and even though the fundamental voltage is applied, the excitation current is large only in the polarity where the magnetic flux is saturated. On the other hand, when an internal fault occurs in a transformer, the free oscillating harmonic current and harmonic voltage generated by the system capacitance and inductance become symmetrical, with positive and negative waves respectively. This is a failure detection method that focuses on the fact that there is a difference in the period in which the ratio of the voltage signal to the current signal exceeds a predetermined value.

FIG. 1 shows an electric power system to which the present invention is applied. 1 is a transformer to be protected, which is shown in a single line diagram in this embodiment. Taking a two-winding primary and secondary transformer as an example, the symbol for the primary side is indicated by the suffix P, and the symbol for the secondary side is indicated by the suffix S. For example, if the primary current is I
_P , the secondary current as _Is , the primary voltage as _Vp , and the secondary voltage as _Vs. CT _P and CT _S are current transformers,
Each I _P and I _S signal is converted appropriately to protect the relay.
Incorporate into RY. In this example, the characteristics of CT _P and CT _S are treated as ideal characteristics with a current transformation ratio of 1, and the output of CT _P ,
It is assumed that the output of CT _S is the same as I _P and I _S .
P _P and P _S are voltage transformers, V _P and V _S respectively.
It is used to take in the signal of
The output of _PPT is treated as V _P and the output of _PTS is treated as V _S.
CB _P and CB _S are breakers. RY is a transformer protection relay according to the present invention, and when an internal failure of the transformer 1 is detected, it gives a disconnection command to CB _P and CB _S ,
1 from the system.

The details of the operation of RY to obtain the above input will be described next with reference to FIG. First, as an arithmetic function for detecting an internal failure of a transformer, for example, the conventionally known ratio differential calculation |I _P -I _S |-K-(|I _P | +|I _S |>I _X ... It is based on equation (1) with (1).

However, I _P : Vector quantity that makes the transformer primary current take the direction of the arrow in Figure 1 positive I _S : Vector quantity that makes the transformer secondary current take the positive arrow direction in Figure 1 K: Suppression coefficient I _X : Judgment level It is.

Reference numerals 50 to 56 in FIG. 6 indicate circuit parts that execute equation (1). First, 50 is a subtracter that receives I _P and I _S as input, and obtains I _P -I _S as an output. .
51, 52 _, and 53 are absolute value circuits that receive _the output of 50, input I _P , _and input _IS , respectively, _and output do. 5
4 is an adder that calculates |I _P |+|I _S |, and 55 is an amplifier that multiplies by coefficient K. 56 was obtained as above |I _P −I _S | and K(|I _P |+|I _S
|) and the bias amount _I.sub.X , respectively, with the polarities shown in the figure. The output of the adder 56 is obtained when equation (1) is satisfied, and this output means that an internal fault has occurred in the transformer.

However, it is well known that equation (1) also holds true for the magnetizing inrush current of a transformer. A solution to this problem is to increase _IX , but the detection sensitivity decreases and there is a risk of malfunction in the event of an internal accident that should be protected. It is possible to suppress the excitation inrush current by taking advantage of the fact that it contains many harmonics, but with recent improvements in transformer core materials, it now contains harmonics even in the event of an internal accident, so it is necessary to clearly distinguish between the two. It's becoming difficult.

In the present invention, a suppression section composed of 23 to 29 is added to the differential calculation section composed of the above-mentioned 50 to 56, and is configured as follows. The input signals of this suppressor are the terminal voltage V of the transformer and the output (I _P -I _P ) of the subtracter 50. These two inputs are led to a divider 24 and their ratio is determined.
This ratio may be based on either one, but here, the calculation of impedance (voltage/current) Z will be explained as an example. 26 is an absolute value circuit for determining |Z|. 23, 25, and 27 are added as necessary to realize this determination at high speed or with high accuracy, and 25 is the same as 24.
27 is an absolute value circuit like 26, and 23 is a π/2 (rad) phase shift circuit. For convenience, let the output of 25 be Z' and the output of 27 be |Z'|.
28 is a low value selection circuit which selects the lower value of the outputs of 26 and 27, and is used as a high value selection circuit when the ratio of 24 and 25 is determined as admittance (current/voltage). The determination circuit 29 operates and provides an output when T _L -T _H >T _X is established between a period T _H in which the output of 28 is greater than the set level Z _S and a period T _L in which it is smaller. However, T _X is the detection level. Reference numeral 21 denotes an inhibit circuit, which provides a signal for tripping the circuit breaker or circuit breaker when 56 has an output and 29 has no output.

The determination concept of this determination unit utilizes the fact that the transformer characteristics are non-linear during the excitation inrush current.
In other words, v is the instantaneous value of the transformer primary terminal voltage, i is the instantaneous value of the difference current between the transformer primary and secondary, and resistance R is the impedance from the primary terminal to the failure point when there is an internal failure in the transformer. , inductance L, then
Generally, v=Ri+Ldi/dt (2) holds true. Here, the resistance R is a fixed value, and there is no difference between the current i and the terminal voltage Ri of the resistance in any case of various current waveforms, such as during normal conditions, internal and external accidents, or when a circuit breaker is closed. Linearity is established. However, when looking at the inductance L, it may not be a fixed value. There is no linearity between the differential value di/dt of the excitation inrush current when the circuit breaker is turned on and the terminal voltage Ldi/dt of the inductance L, and it is nonlinear. For example, when the voltage applied to a transformer is applied in the positive direction, if the residual magnetism of the transformer is also of the same polarity, the magnetizing inductance is low during part of the positive half cycle and is low during part of the negative half cycle. This results in high inductance. Note that L is fixed for currents other than the excitation inrush current.

Therefore, if the inductance or the amount of electricity determined according to the inductance is monitored over at least one cycle and the change is observed, it should be possible to determine whether there is an excitation inrush current or not. In the present invention, impedance, for example, is determined from current i and voltage V as the quantity of electricity determined according to inductance.

Figures 2 to 5 are explanatory diagrams of the operation of the device of the present invention shown in Figure 6. Figure 2 shows the normal operation, Figure 3 shows the operation with respect to the excitation inrush current, and Figure 4 shows the DC component. FIG. 5 shows the operation for fault currents including harmonics. In these figures, a is the divider 24, 25
The input v' is the input v which is phase-shifted by π/2 (rad) by the phase shift circuit 23. b is the differential current i (=I _P -I _S ) of the transformer. c indicates the output Z (=v/i) of the divider 24 and the output Z'(=v'/i) of the divider 25, and d is the output |Z|, |Z' of the absolute value circuits 26 and 27. | is shown. e is the output of the determination circuit 29.

In the normal case of FIG. 2, the difference current i is zero.
Therefore, impedances Z and Z' are positive or negative infinity, and absolute values |Z| and |Z'| are positive infinity. Since the detection level Z _S in d of the figure is a finite value, |Z|>Z _S and |Z'|>Z _S always hold true. The determination circuit 29 determines whether Z _S >|Z| or Z _S >
Assuming that the period in which |Z'| holds true is T _L and the period in which it does not hold is T _H , an output of "1" is given when T _L > _TH . However, T _x =0. Therefore, under normal conditions, the output of the determination circuit 29 is "0", and the output of the AND circuit 29 is "0".
1 is not output. Under normal conditions, the breaker should not be tripped, and the device according to the invention provides correct operating results. It should be noted that the differential calculation section composed of 50 to 56 does not output any output during normal times.

As is well known, the excitation inrush current when the circuit breaker is closed does not form a sine wave, and only flows during a partial period. Again, a large current flows through the closing end of the breaker, but almost no current flows through the other end. Therefore, the difference current i is observed as shown in FIG. 3b, and has a finite impedance value during the period when the difference current i is obtained, and takes a positive or negative infinite value during the period when i=0. The period during which the excitation inrush current flows is usually less than half a cycle, and the period T _L during which the excitation inrush current is below the detection level Z _S is shorter than the period T _H during which the excitation inrush current is below the detection level Z _S . Since the determination device 29 gives an output of "0", even if the adder 56 gives an output of "1" in response to the magnetizing inrush current, the AND circuit 21 does not output and the circuit breaker tripping signal is not obtained. I can't. It is the correct procedure for a transformer protective relay device to not give a tripping signal to the insulator breaker when the insulator breaker is turned on.

As is clear from FIGS. 2 and 3 above, when i=0, the impedance becomes infinite, and this period almost determines the length of T _H.
In the cases shown in Figures 2 and 3, i = 0 for a period longer than half a cycle of the sine wave, but in the case of an internal fault, the difference current i is basically a sine wave, and as shown in Figure 4, the difference current i is a sine wave. Even if a component i _D is mixed in, even if harmonics are mixed in as shown in FIG. 5, i=0 will be achieved extremely instantaneously. Therefore, the absolute value of impedance is Z _S
The lesser period T _L is longer than the greater period T _H ;
The determination circuit 29 in FIG. 6 gives an output "1". In the case of an internal fault, the adder 56 of the differential calculation section also gives an output of "1", so via the AND circuit 21,
The breaker is opened correctly. The case of an external accident can be considered in the same way as in Figure 2.

As described above, according to the transformer protection device of the present invention,
It can give the correct protection output in all cases and is not affected by second harmonics.

As a concrete implementation of the circuit 29 for determining T _L > T _H , an integrating circuit that can charge and discharge in the positive and negative directions is used to integrate in the positive voltage direction during the time period T _L , One method is to integrate in the negative voltage direction during the time period T _H and generate an operational output when the voltage corresponds to a predetermined detection level T _X .

FIG. 7 shows a flowchart in which parts 23 to 29 of the present invention are implemented by performing digital calculations using a computer or the like. FIG. 7 31 shows a digital signal acquisition function obtained by sampling input signals v and i at the same time and at regular intervals. 32 is a memory for storing sample values of v, for example, for π/2 radians in order to obtain v'.

In 33, |Z|=|v/i| is calculated based on the signals v and i sampled at the same time, and in 34, for the signal v' which is v delayed by π/2 radians, |Z'|
Calculate = |v'/i|. 35, |Z| and |Z′|
This is a minimum value detection function that compares and outputs the lower level.

36 is a function to judge the level of the output of 35, and Z
This is a comparison calculation unit that determines whether or not it is smaller than _S.

37 is a counter, and when the value of |Z| or |Z'| is smaller than Z _S , the counter 37 is incremented by the output YES of 36, and conversely, when the output of 36 is NO Add a minus signal to counter 37. Reference numeral 38 denotes a comparison calculation unit that determines whether or not the output of the counter becomes positive for a time width corresponding to a predetermined level _TX , and when the output of 37 becomes equal to or higher than a predetermined level _T It is determined that this is the case, and an output “1” is given.

In addition, as an example of the practical application of the present invention, Z was obtained as v/i, but as the reciprocal Z=i/v, Z'=i/v',
It is also possible to set the high level to the active side and the low level to the non-active side.

Alternatively, detection may be performed using one signal of either Z or Z', or by making v and v' more plural, Z,
You can also find Z′... Further, the above-mentioned determination is made based on the primary terminal voltage, or the same determination is made based on the secondary terminal voltage. OR output of both,
Alternatively, it may be determined whether the current is an excitation inrush current or an internal fault current by AND output.

Further, in a multi-phase multi-winding transformer, a similar determination can be made using differential output current signals for each phase and voltage signals for each terminal.

Furthermore, the scope of application of the present invention is to simplify the determination unit for differential currents of all three phases.

Furthermore, depending on the scalar amount of current passing through each terminal passing through the transformer, the judgment level Z _S and the time width T _L >T
By controlling _H , for example, the larger the passing current, the smaller Z _S becomes smaller, or by adding a sensitivity control constant T _X where T _L > T _H + _T .

[Brief explanation of the drawing]

Fig. 1 is a diagram schematically showing a protected transformer and a protective relay device, Figs. 2 to 5 are explanatory diagrams of the operation of the device of the present invention, and Figs. 6 and 7 are diagrams showing an embodiment of the present invention. Give an example. 24, 25...Divider, 23...Phase shift circuit, 2
6, 27... Absolute value circuit, 28... Low value selection circuit, 29... Judgment circuit.

Claims (1)

[Claims] 1. A first relay that calculates the ratio between the transformer terminal current difference current and the transformer terminal voltage, and the transformer terminal voltage after a phase shift of π/2 (rad). A second relay that calculates the ratio of the two outputs of the first and second relays, outputs a smaller value when the ratio is impedance, and outputs a larger value when the ratio is admittance. A transformer protection relay comprising a selection means and a determination means for detecting an accident occurring in the transformer based on an output value of the selection means. 2. The first relay calculates the ratio between the transformer terminal current difference current and the transformer terminal voltage, and the second relay calculates the ratio between the transformer terminal current difference current and the transformer terminal voltage after a phase shift of π/2 (rad). relay, a selection means for outputting a smaller value when the ratio is an impedance and a larger value when the ratio is an admittance among the two outputs of the first and second relays; A determination means for detecting an accident occurring in the transformer based on an output value, and a third relay for detecting and outputting an internal accident of the transformer according to the difference current, the third relay and the determination means A transformer protection relay is characterized in that it protects a transformer when both detect an internal fault.