JPS6248449B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a protective relay device that performs digital calculations, and particularly to improvements in its frequency characteristics.

A conventional device of this type is shown in FIG. In the figure, if the solid line is the system current, for example, it is expressed as i=Io sin wt, and this is sampled at a constant period h. The sampled value at time to is i(to)=Io sin wto −(1) The sampled value at time one sampling before (to−h) is i(to−h)=Io sin w(to−h) , h is equivalent to 90° electrical angle, time (to
-h), i (to-h) = Io sin (wto-90゜) = -Io cos wto -(2) (1) and (2) are squared and added to give i ² (to) +i ² (to-h) = Io ² sin ² who + (-Io) ² cos ² wto = Io ² (sin ² wto + cos ² wto) = Io ² -(3), which is the square of the peak value of the grid current. This is a value related to the magnitude of the sine wave current value. In conventional digital protective relays, the sampling period h is selected to be equivalent to 90° determined by the frequency of the grid current, and the calculation shown in equation (3), that is, the time
An overcurrent was detected by comparing a value obtained by squaring and adding the sampling values of to and (to-h) with a set value.

Conventional digital protective relays were equipped with an oscillator with a period of h and sampled the equivalent of 90° of the system frequency, so as shown in Figure 2, the frequency of the current amount changes and becomes 1/2. Then, the electrical angle at (to-h) before one sampling will be 45 degrees ago. Therefore, even if equations (1) and (4) are squared and added, equation (3) will not be obtained. If the frequency fluctuates by a factor other than 1/2, it becomes even more complicated and difficult to express using a mathematical formula, and the value is not related to the magnitude of the grid current. Therefore, conventional digital protective relays have the disadvantage of not being able to protect equipment such as generators whose frequency can change from zero frequency to approximately twice the rated speed (runaway speed).

This invention was made in order to eliminate the drawbacks of the conventional ones as described above, and it samples at a constant period independent of the system frequency and calculates from the sampled values at a certain time, before and after that, and two times before and after. The purpose of this study is to derive values related to the magnitude of the sinusoidal electrical quantity.

An embodiment of the present invention will be described below with reference to FIG. When the sampling period is h, time (to-2h), (to-h), to, (to+h), (to+
2h) are expressed by equations (5) to (9).

i(to−2h)=Io sin w(to−2h) −(5) i(to−h)=Io sin w(to−h) −(6) i(to)=Io sin wto −(7) i(to+h)=Io sin w(to+h) −(8) i(to+2h)=Io sin w(to+2h) −(9) Here, i(to) {i(to+2h)+i(to−2h)}− {i(to+h)−i(to−h)} ² = i(to)Io{sin w(to+2h)+sin w(to−2h)}−Io ² {sin w(to+h)−sin w(to−h )} ² =i(to)Io{2sin wto cos2wh}−Io ² {2cos wto sin wh} ² =i(to)2Io sin wto cos2wh−4Io ² cos ² wto sin ² wh =2i ² (to)(1 −2sin ² wh) −4Io ² (1−sin ² wto) sin ² wh =2i ² (to) (1−sin ² wh)−4Io ² sin ² wh＋4Io ² sin ² wto sin ² wh =2i ² (to) −4i(to) ² sin ² wh−4Io ² sin ² wh＋4i(to) ² sin ² wh =2i ² (to)−4Io ² sin ² wh −(10) Also, {i(to+h)+i(to−h )} ² = Io ² {sin w (to + h) + sin w (to - h)} ² = Io ² {2sin wto cos wh} ² 4Io ² sin ² wto cos ² wh =4i ² (to) (1 - sin ² wh) −(11) Therefore, by eliminating sin ² wh from equations (10) and (11), Io ²
Io ² = 2i ² (to) - i (to) {i (to + 2h) + i (to - 2h)} + {i (to + h) - i (to - h)
} ² /4sin ² wh = i ² (to) [2i ² (to) - i (to) {i (to + 2h) + i (to - 2h)] + {i (to + h) - i
(to-h)} ² }/4i ² (to)-{i(to+h)+i(to-h)} ² -(12), and the square of the peak value of the grid current is obtained, which changes depending on the frequency. It turns out that they are not related at all.

Therefore, one embodiment of the present invention is shown in a block diagram as shown in FIG. 4.

A system current i is introduced into a sample and hold circuit 2 via an analog filter 1, and an analog value sampled and held at sampling synchronization h is inputted into an input circuit 4 as a digital value by an analog-to-digital conversion circuit 3. The arithmetic processing circuit 5 performs calculation based on the above equation (12) from the output of the input circuit 5, and outputs the result via the output circuit 6. Control such as synchronization of each circuit is performed by a control circuit 7.

In addition, in the above embodiment, overcurrent detection that detects when the value of grid current exceeds a set value was explained, but if voltage detection is used, it can also be used as overvoltage detection, or it can be used as overvoltage detection. Therefore, the present invention can also be applied to undervoltage detection, and the same effects as those of the above embodiments can be achieved. The present invention can also be applied to current differential protection relays that operate based on a difference in current, and distance relays that calculate impedance based on voltage and current, and similar effects can be achieved.

As described above, according to the present invention, since equation (12) is subjected to arithmetic processing, it is possible to obtain a digital protective relay device that is completely unaffected by changes in the system frequency.

[Brief explanation of the drawing]

Figure 1 shows the relationship between the waveform and sampling when the grid current is at the rated frequency, and Figure 2 shows the relationship between sampling.
Figure 3 shows the relationship between the waveform and sampling when the grid current becomes 1/2 of the rated frequency.
It is a figure explaining the case where the waveform when the frequency of a system current becomes one of the rated frequency is arithmetic-processed by the principle of this invention. FIG. 4 is a block diagram showing one embodiment of the protective relay device of the present invention.

Claims (1)

[Scope of Claims] 1. In a protective relay device that performs digital calculation based on sampled values obtained by sampling the amount of electricity in a system at an arbitrary fixed period, five sampled values are continuously acquired at a fixed period, and these sampling values are Set the value to i(to-2h), i(to-h), i
(to), i (to + h), i (to + 2h), i ² (to) [2i ² (to) - i (to) {i (to + 2h) + i (to - 2h)} + {i (to + h )−i(t
o-h)} ² ]/4i ² (to)-{i(to+h)+i(to-h)} ² A protective relay device characterized in that it is configured to perform the calculation process of 2.