JPH0118646B2 - - Google Patents

Description

[Detailed description of the invention]

This invention relates to a differential relay. Electrical equipment connected to the power grid (such as transformers)
Differential relays are widely used to detect internal faults in buses and buses. In particular, transformers and busbars are energy passing means, and a current transformer CT is installed at the input and exit of the transformer, the currents are combined to obtain a differential current, and the presence or absence of the differential current is used to detect the presence or absence of a fault. This is considered the most rational method. Regarding differential protection for transformers, in order to establish the voltage (magnetic flux) of the transformer, no differential current flows that is difficult to understand except for the excitation current that appears as a slight error, but when the transformer is turned on, Until the magnetic flux (voltage) of the transformer is established, the transient excitation current (excitation inrush current) appears at a considerable level and becomes indistinguishable from the fault current that would normally flow during an accident. For this reason, a so-called harmonic suppression method that focuses on the waveform of the transient excitation current has been used. In other words, if we analyze the transient excitation current when the transformer is turned on, as shown in Figure 1b, we will get the harmonic component ratio as shown in Figure 1c, for example, and the harmonic components, especially the second harmonic component, will be I understand that there are many things. FIG. 2 shows the configuration of a conventional ratio differential relay for protecting a transformer, which focuses on this fact. In Figure 2, current transformers installed on both sides of the inlet and outlet of transformer T
The primary and secondary currents introduced from CT ₁ and CT ₂ are applied to auxiliary transformers 1 and 2, respectively, and their algebraic sum is introduced to auxiliary transformer 3 as a differential current. After these currents are rectified by rectifiers 4, 5, and 6 and applied to filters 7, 8, and 9, a voltage that operates on the suppression side is generated by the suppression circuit 10, and this is related (proportional) to the differential current. The output of the filter 9 is compared with the output of the filter 9 by a comparator 11, and a relay having the same function as a conventional ratio differential relay is constructed. On the other hand, the differential current generates a voltage secondarily through an auxiliary transformer 12, and a two-terminal frequency selection filter (second harmonic blocking) 1 consisting of a capacitor reactor, etc.
3, and 2, which also consists of a reactor and a capacitor.
Terminal frequency selection filter 14 (passing second harmonic)
are applied to each. The current that has passed through the filter 13 passes through the rectifier 15 and the high-pass filter 17 as a voltage that operates on the operating side, while the current that has passed through the filter 14 passes through the rectifier 16 and the high-pass filter 18 that operates on the suppressing side. The fundamental wave component and the second harmonic component are mutually compared. The resulting output is given to the final determination circuit 20. Even if a large differential current is generated due to the transient excitation current at the time of turning on, and the comparator 11 operates to produce an output of "1", the above-mentioned comparator 19 will detect the second harmonic component in the transient excitation current and the fundamental wave. The final judgment circuit 20, which detects that the ratio with the component is large and does not operate, and ultimately generates an AND output, becomes inoperative, thereby preventing malfunction of the entire system. As seen in this configuration example, the method of detecting the second harmonic using an analog filter is effective and is in practical use. What I have,
Due to the fact that the filter comparator itself is provided with a time delay before the comparator 19,
At this time, there is a problem that the response to the fundamental wave is delayed and the operation of the entire relay is delayed. This invention eliminates the above-mentioned conventional drawbacks,
The present invention provides a differential relay with excellent responsiveness and high reliability. Hereinafter, a differential relay according to the present invention will be explained with reference to FIG. In the figure, T is the transformer to be protected,
CT ₁ and CT ₂ are transformers placed on the high and low voltage sides of the transformer T, respectively, 23, 24, and 25 are auxiliary transformers, 23 and 24 are auxiliary transformers that receive the above input, and 25 is a transformer that receives the above input. Auxiliary transformers that combine inputs, 26, 27, and 28 are auxiliary transformers 2
This is a level converter that rectifies and converts the level of signals from 3, 24, and 25. 29,30,3
Reference numeral 1 designates a sample-and-hold amplifier that receives the DC outputs of the level converters 26, 27, and 28, and samples and holds the instantaneous values thereof.The output of the sample-and-hold amplifier 31 is simultaneously sent to a circuit for performing harmonic analysis, which will be described later. is also applied. 32 is a multiplexer, sample and hold amplifier 2
Three amplifier outputs 9, 30, and 31 are switched according to a command from the microprocessor 35 and applied to the A/D converter 33. A/D converter 33
is the sample hold amplifier 29, 30, 3
The output of the multiplexer 32 which has switched the output of 1 is taken as an input, converted into a digital value, and can be set to the next input register 34. The microprocessor 35 is connected to the sample and hold amplifier 2.
The inputs applied to 9, 30, and 31 are subjected to calculation/judgment processing and outputted to the output register 36, as well as the multiplexer and sample/hold amplifiers 29, 30, and 31.
It also controls registers, etc. Output register 3
6 holds the determination output "1" or "0" from the microprocessor 35. A level converter 37 amplifies the output of the register 36 so that it can drive an external circuit. Note that various timing signals are outputted from the microprocessor 35 as control signals for the sample-and-hold amplifier, multiplexer, A/D converter, and register. The above constitutes a portion having the function of a ratio differential relay. The operation of this portion corresponding to a conventional ratio differential relay will be described below. First, the current flowing in the primary and secondary main circuits of the transformer
I ₁ and I ₂ are transformed by CT ₁ and CT ₂ , and terminal 2
1 and 22 as aI ₁ and aI ₂ , respectively. This current is transformed by auxiliary transformers 23, 24 to generate a secondary voltage proportional to the passing current input. Also, the algebraic sum a ₍ I _{1 +}
An auxiliary transformer 25 with I ₂ ) as input generates a voltage proportional to the differential current to the secondary. These voltages are converted to level converters 26, 27,
28, output voltages cI ₁ , cI ₂ , c (I 1 + I ₂ ) proportional to the currents I ₁ , I ₂ passing through the primary side and their differential current I _D ( _{≡I 1 +} I ₂ ) are converted. obtain. Although this output changes from moment to moment, it is sampled and held at appropriate timings, for example, every π/12 radians. Next, these three voltages are switched by the multiplexer 32 and applied to the A/D converter 33, and the A/D
It is held in the input register 34 after conversion. The microprocessor 35 inputs the data held in the input register and processes this input using an appropriate algorithm to determine whether there is an accident inside the device. For example, (I ₁ +I ₂ )−Rf(I ₁ , I ₂ )≦0, it is “0” (I ₁ +I ₂ )−Rf(I ₁ , I ₂ )＞0, it is “1” (1) When the relationship is satisfied, it is a kind of ratio differential relay, and furthermore, Rf (I ₁ , I ₂ ) = R・Max (|I ₁ |, |I ₂ |) is equivalent to the maximum value suppression method using diodes. It is. Here, a discussion of the ratio differential function of the ratio differential relay described above is not the purpose of the present invention, and will therefore be omitted. In any case, the above-mentioned currents I ₁ , I ₂ , I ₁ +I ₂ are a set of several current values sampled and held at certain points during one cycle of the grid frequency, and the above relational expression ( It is sufficient if 1) holds true. The second harmonic discriminating circuit, which is a feature of the present invention, will be described below, but as a preparation, an algorithm for harmonic analysis will be described. Generally, if one period of the distorted wave alternating current y is divided into m equal parts, and the instantaneous values of y at each time point 0, 1, 2, m-1 are y ₀ , y ₁ , y ₂ ...ym-1, respectively,
The strain wave y is expressed as y=a ₀ +a ₁ cosωt+a ₂ cos2ωt+... +b ₁ sinωt+b ₂ sin2ωt+... Here a ₀ , a ₁ , a ₂ , ...b ₁ , b ₂ ...
Finding... is harmonic analysis. In general, in a commercial power system, the order of harmonics that is of interest in the operational control of the system is about the 5th order, and higher harmonics can be ignored except in special cases. Unlike power transmission line protection relays, which are subtly affected by transient phenomena during accidents, in this case, the second harmonic component in the excitation current when the transformer is turned on is handled, so the fifth harmonic must be taken into account. It is sufficient. To find up to the 5th harmonic, you only need to find 11 coefficients and 12 samples: y ₀ , y ₁ , y ₂ ,...
…y ₁₁ is enough. The harmonic components after measuring the 12 y values y ₀ , y ₁ , y ₂ , . . . y ₁₁ are given below.

[Table] 1 〓3

b ₂ =

Claims (1)

[Claims] 1. A sampling circuit that samples the differential current of a differential relay that protects an electrical device or a bus bar; A plurality of analog constant coefficient multiplication circuits that multiply the output of this sampling circuit by different constant coefficients; A switching circuit that switches the outputs of a plurality of analog constant coefficient multiplier circuits under control based on a predetermined algorithm;
The outputs of the D conversion circuit and the A/D conversion circuit are integrated for a predetermined period of time for each fundamental wave component and second harmonic component, and are calculated based on a predetermined algorithm to calculate the above fundamental wave component and second harmonic component. A differential relay comprising a digital calculation control circuit that suppresses the output of a control signal for interrupting a fault current when the components have a certain relationship.