JPH0576586B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a protective relay for protecting an electric power system.

[Technical Background of the Invention] Conventionally, this type of relay uses a predetermined value that is the sum of the absolute values of N pieces of data for half a cycle obtained by sampling the system voltage or current at a predetermined sampling period T. Some devices detect overvoltage or overcurrent by comparing with the set value. To explain using a diagram, as shown in FIG. 4, if the grid current is I=Io sin ωt and is sampled at a constant period h (=60°), the amplitude value of the grid current can be obtained as follows. However, K is a correction constant.

K(｜Io sin ω(to−h)｜＋｜Io sin ω _tp ｜+｜Io
sin ω(to+h)｜)……(1) [Problems in the background art] The value obtained by equation (1) is the area sum of the sampling values for half a cycle, and corresponds to the shaded area in the figure, and is not the true value. There was a problem in that a large error occurred between the amplitude value and the amplitude value. This error depends on the sampling phase, and in the example of 60° sampling in equation (1), the maximum error is ±7.7
% error occurs. For example, if the constant K is chosen to be 1/2√3, equation (1) is 0.5, and in the case shown in Figure 4, the error is 0, but the sampling phase is different, and to-h=
In the case of 30°, the value of equation (1) is 0.577, resulting in an error of 7.7%. In order to reduce the error, the cycle h can be shortened and the number of data used can be increased, but the operating time of the relay will be delayed.

[Object of the Invention] The present invention has been made to solve the above problems, and by calculating amplitude values with less calculation error using sampling data similar to the above method, errors due to accurate judgment can be reduced. The purpose is to provide a protective relay that provides functionality.

[Summary of the Invention] In the present invention, the sum of each absolute value in a predetermined sampling period is calculated, and the result is corrected by the absolute value of the difference between the absolute values of the sampling values to obtain the amplitude value of the input electrical quantity. It is something to do.

[Embodiments of the Invention] Examples will be described below with reference to the drawings. FIG. 1 is a block diagram of a protective relay according to an embodiment of the present invention. In FIG. 1, an AC input i detected from a protected object passes through an analog filter 1, and then
The signal is input to the sample and hold circuit 2 and sampled at a predetermined period h. The output of the sample and hold circuit 2 is input to an analog-to-digital converter 3, converted into a digital signal, and then supplied to an input circuit 4. The input circuit 4 converts the level of the input digital signal and supplies it to the arithmetic processing circuit 5. The arithmetic processing circuit 5 performs the arithmetic operations described below, and supplies the result as an operation output to the outside via the output circuit 6, thereby executing a protection operation on the object to be protected.

Next, arithmetic processing in the arithmetic processing circuit 5 will be explained. The AC input i changes as I=Io sin ωt, undergoes conversion processing, and is input to the arithmetic processing circuit 5.

The arithmetic processing circuit 5 samples the amount of electricity from the power system at a predetermined period h, and N pieces of data i _j (j=1, 2, . . . ) for half a cycle obtained from the sampled output signals. The following equation (2) is calculated using N) and a predetermined coefficient k. _N 〓 ^j=1 ｜i _j ｜+k _N-1 〓 ^j=1 _N 〓 ^m=j+1 ｜｜i _j ｜−｜i _n ｜｜ In other words, this calculation calculates the The amplitude value of the input electrical quantity is obtained by calculating the sum and correcting the result by the absolute value of the difference between the absolute values of the sampled values.

Next, as a specific example, a case where the predetermined sampling period h=60° and N=3 will be described.

Since the value of the grid current i sampled at time to-h, to, to+h is the sampling period h=60°, ito-h=Io sin ω(to-60°) ……(3) ito=Io sin ωto...(4) ito+h=Io sin ω(to+60°)...(5) The following calculations are performed using the values of (3) to (5).

｜i _tp −h｜+｜i _tp ｜+｜i _tp+h ｜+K ₁ (‖i _tp-h ｜−
｜i _tp ‖＋‖i _tp ｜−｜i _tp+h ‖＋‖i _tp-h ｜−｜i _tp+h ‖
)
...(6) Equation (6) above is based on three data points separated by 60° from each other, i _tp-h ,
Find the sum of the absolute values of i _tp and i _tp+h , and calculate the result by 3
This shows that the amplitude value of the input amount is obtained by correcting the absolute value of the difference between the absolute values of the data.

The above calculation will be explained using FIG. 2. In Figure 2, a is the absolute value of 3 data |
i _tp-h |, |i _tp |, |i _tp+h |, b is the sum of the three absolute values shown in a |i _tp-h |+|i _tp |+|i _tp+h |, c is Absolute value of the difference between the three absolute values shown in a‖i _tp-h |−
｜i _tp ‖, ‖i _tp ｜−｜i _tp+h ‖, ‖i _tp-h ｜−｜i _tp+h ‖
,
d is the sum of the three quantities shown in c, and e is the waveform of b superimposed with k ₁ times the waveform of d. Comparing the waveforms b and d, we find that when one takes the maximum value, the other takes the minimum value, so by superimposing the two waveforms with an appropriate coefficient _k1 , we can calculate the amplitude as shown in e. The resulting ripple will be smaller. That is, the error caused by the sampling phase becomes smaller. In the case of sampling with a period of 60°, the coefficient to minimize the ripple is (2−√
3)/(√3-1), and in this case, the maximum value of the amplitude value is approximately 2.438, the minimum value is 2.375, and the calculated value will fluctuate between these, (2.438-2.375)/
When 2≒2.4 is adopted as the amplitude value, the calculation error ε is (2.438−2.4)/2.4≒0.016, that is, approximately
It will be 1.6%.

This reduces the error by 6.1% compared to the conventional method expressed by equation (1) using equivalent sampling data.

The amplitude value calculated according to the calculation of equation (6) described above is compared to see if it is larger than a predetermined setting value I, and when a larger result is obtained, the amplitude value is outputted via the output circuit 6. Supply operating output externally.

In addition, in the above embodiment, the case where overcurrent is detected using the grid current as the setting value was explained, but the setting value may also be the grid voltage, which allows detection of overvoltage or undervoltage. You can also do it. Furthermore, the same effect can be expected by similarly determining the magnitude of inputs of a protective relay with multiple inputs, such as a differential protection relay or a distance relay, and performing differential calculation or impedance calculation.

Although the above embodiment describes the case where the sampling period is 60°, the case is not limited to this, and in the above equation (2), when the predetermined sampling period h = 45°, N = 4, It may also be the case that h=30° and N=6.
A similar effect is achieved in the case of a sampling period of 45° and 30°, and the calculation error is 0.8% and 0.4%, respectively, which is the calculation error 4 by the conventional method shown in equation (1).
%, the error is significantly reduced compared to 1.7%.

FIG. 3 shows a comparison of calculation errors between the conventional method expressed by equation (1) and the method according to the present invention when there are N pieces of sampling data in a half cycle.

[Effects of the Invention] As explained above, according to the present invention, a plurality of sampling data in a half cycle is used, and the sum of their absolute values is used, and the absolute value of the difference between the absolute values is used as a correction term. As a result, calculation errors can be reduced compared to conventional calculation methods using equivalent sampling data, and a highly accurate protective relay can be provided.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of a protective relay according to an embodiment of the present invention, Fig. 2 is a waveform diagram explaining the processing of the arithmetic processing circuit of the present invention, and Fig. 3 is the effect of the present invention when compared with the conventional method. A graph to explain,
FIG. 4 is a diagram showing waveform amplitude value calculation. 1...Analog filter, 2...Sample hold circuit, 3...Analog-digital converter, 4
...Input circuit, 5...Arithmetic processing circuit, 6...Output circuit.

Claims (1)

[Claims] 1. The amount of electricity from the power system is sampled at a predetermined period, and N pieces of data i _j (j=1, 2,
..., N) and a predetermined coefficient k, _N 〓 ^j=1 ｜i _j ｜+k _N-1 〓 ^j=1 _N 〓 ^m=j+1 ｜｜i _j ｜−｜i _n ｜｜ A protective relay is characterized in that it derives an output that protects an electric power system by calculating and comparing a value obtained based on the calculation result with a predetermined setting value. 2. The protective relay according to claim 1, characterized in that the predetermined period is 60° and N=3. 3. The protective relay according to claim 1, characterized in that the predetermined period is 45° and N=4. 4. The protective relay according to claim 1, characterized in that the predetermined period is 30° and N=6.