JPH0210655B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a ratio differential relay for protecting busbars, etc. of a power system. A circuit system shown in FIG. 1 is conventionally known as this type of ratio differential relay. In this Figure 1, 1 is the bus bar of the power system, 2-1 to 2-n are the current detection current transformers (hereinafter abbreviated as CT) of the branch system, and 3-1
3-n is an input device, which is composed of the following elements. 4 and 5 are transformers, 6 is a resistor, and 7 is a full-wave rectifier circuit. Further, 8 is a ratio differential relay, which is composed of the following circuit elements. 9 is transformer, 1
0 is a full-wave rectifier circuit, 11 and 12 are resistors, 13 is an input signal level detection circuit, and 14 is an output relay. Next, the operation of the conventional circuit shown in FIG. 1 will be explained below. First, the transformers 4 built into the input devices 3-1 to 3-n convert the secondary currents of the current detection CTs 2-1 to 2-n, respectively, and the secondary outputs of the transformers 4 are all connected in parallel. Now, the composite current transformation ratio of the transformer 4 in the input device 3-1 which uses the secondary output of CT2-1 as the detection input, and the composite of the transformer 4 of CT2-2 to 2-n of other lines and each input device. If the current transformation ratio is the same for all lines, Kirchhoff's first
According to the law, the transformers 4 of input devices 3-1 to 3-n
The secondary current composite value (hereinafter referred to as differential current) I _D is zero under normal conditions, and even in the event of an external fault on the bus 1, the fault current passes through the bus 1 and becomes zero.
On the other hand, when an internal fault occurs on the bus 1, the current only flows from the power supply end toward the fault point on the bus 1, so the differential current is not zero. The method of detecting the difference in the differential currents in the case of an internal fault on the bus bar, in a normal state, or in the case of an external fault is generally called a differential method. In other words, the principle of this differential method is valid in the area where the current transformation ratio of CT2-1 to 2-n or the transformer 4 of the input device does not cause an error, and in the large current area, CT2-1 to 2-n -n, or the transformer 4 is saturated and an error current is generated, causing a problem in application. In other words, in this case, even though it is an external fault, an error differential current is generated and the relay operates with an error, so
Measures are taken to reduce the detection sensitivity of the relay in the large current range, and to prevent erroneous dynamic current from being detected easily. As this method, CT2-1~
A voltage proportional to the secondary current of 2-n is derived by a transformer 5 and an output resistor 6, and an output voltage (hereinafter referred to as suppression voltage) obtained by parallel combining the output voltages of each line via a full-wave rectifier circuit 7. ) to suppress the relay detection value. This circuit system is called ratio differential system.
An example of the principle structure is a ratio differential relay 8 in FIG. 1 having a circuit structure as shown. That is, the transformer 9 receives the differential current _ID and causes the resistor 11 to generate an output voltage |E _O | via the full-wave rectifier circuit 10. On the other hand, the suppression voltage derived from each input device 3-1 to 3-n is received as a voltage |E _R | across the resistor 12,
The differential output voltage |E _O | and the suppressed output voltage |E _R | are connected so that they are subtracted, so that |E _O |−|E _R |K(K
is a constant) is determined by the level detector 13, and the output relay 14 is operated only in the event of an internal accident. Next, the operating characteristics of the conventional system will be explained below. FIG. 2 is a characteristic diagram of the conventional method shown in FIG. 1, which is generally referred to as a one-terminal ratio characteristic. The horizontal axis shows the suppression amount (I _T is a value proportional to the CT secondary current of one terminal), and the vertical axis shows the differential amount I _D , and the limit differential of relay operation when the suppression amount I _T is changed. It represents the locus of the quantity I _D and the currents I _D and I _T are both values at the primary input of the input device. The straight line 25 represents the conditions for the amount of suppression and the amount of differential when I _T =I _D and only one current end is connected to the bus bar. That is, the amount of suppression is proportional to the secondary power supply of the CT of each line, and the differential amount is the sum of the secondary current vectors of the CT of each line, but since it is a single-end power supply, it ends up being the same as the amount of suppression input. Therefore, the operating characteristics of the ratio differential relay 8 must be such that the slope is smaller than that of the straight line 25. Although it will not be explained in detail here, there are some busbar configurations in which outflow current occurs even though it is an internal fault in the busbar, and in general, it is necessary to be able to detect faults with a ratio of 2 inflows to 1 outflow. Under this condition, the slope of straight line 25 is I _D / _IT = 0.5. Therefore, the operating characteristic 26 of the relay is the slope I _D /I _T
It must be 0.5, which means that the suppressing effect will be weakened. Conventional ratio differential relays were configured as described above, so the suppression force must be I _D /I _T 0.5 to detect a fault inside the bus with outflow current, so it is not recommended to make the suppression force too strong. It is not possible to do so, and the performance is poor as a countermeasure against errors caused by saturation of the current detection CT in the event of an external fault on the bus bar. therefore extreme
It was necessary to keep the capacity of the CT sufficiently large so as not to cause saturation of the CT. However, in recent ultra-high voltage systems, the capacity of the transmission line has increased significantly due to the increase in system capacity, which has increased the decay time of transient DC current in the event of an accident, and relays are required to operate at high speed. There is a need for something that can operate in about one cycle. It is well known that if a current containing a DC component is applied to a CT, the CT becomes extremely susceptible to saturation.
If the CT is left unsaturated, the CT capacity will inevitably become enormous. In addition, if there is residual magnetic flux in the CT and a fault current flows in the direction that is added to the residual magnetic flux, a problem other than the influence of DC component current is that the CT will saturate immediately at the first wave of fault current, so high-speed operation type If so, this would have caused a major drawback in that it would malfunction. The present invention was made in order to eliminate the above-mentioned drawbacks, and as performance requirements for ratio differential relays have become increasingly strict in recent years, conventional relays have reached their performance limits. In view of this, the object of the present invention is to provide a high-performance, inexpensive ratio differential relay that is ultra-high-speed, greatly improves the saturation performance of a current detection CT, and minimizes the increase in CT capacity. Hereinafter, one embodiment of the present invention will be described with reference to the drawings. In FIG. 3, the same parts as in FIG. 1 are designated by the same reference numerals. Reference numeral 15 denotes a ratio differential relay of the present invention, which corresponds to the conventional ratio differential relay 8 shown in FIG. 16 is a transformer, 17 is a full wave rectifier circuit,
Reference numeral 18 denotes a level detection circuit which receives the output of the full-wave rectifier circuit 17 and switches when the input value is above a certain value, and these components are collectively referred to as differential elements hereinafter. 19 is a level detection circuit that switches when the output voltage |E _R | of the resistor 12 is larger than the output voltage |E _O | of the resistor 11 by more than a certain value; 20, when the level detector 19 outputs an output, the signal disappears; OFF to continue the signal for a time T ₁ after
The delay circuit, 21 is a NOT circuit, and 22 is an AND circuit, which will be collectively referred to as a ratio element hereinafter. The input I _D of the transformer 16 is the same as the conventional one, and the input |E _R | of the differential resistor 12 is the suppression amount as the conventional one. The operation of the present invention will be explained below. The method for deriving the differential amount, which is the input to the transformer 16, and the suppression amount, which is the input to the resistor 12, is the same as the conventional method, and therefore will not be described here. First, we will explain each external fault among the busbars in a simple sinusoidal current without CT saturation. First, in the case of an external fault on the busbar, the differential current I _D is zero, as described in the operating principle of the conventional system, and a suppression voltage |E _R | proportional to the fault current occurs. Since the differential current _ID of the duty ratio differential relay 15 is zero, there is no output from the transformer 16, and since the level detector 18 does not switch, the differential element output 23 is in a state of no signal. Also, resistance 1
The output voltage of resistor 1 | E _O | is also zero and the output of resistor 12 | E _R
|E _R |−|E _O |＞K
(K is a constant) is switched, and the output of the OFF delay circuit 20 continues, but since the signal is inverted by the NOT circuit 21, the output signal of the NOT circuit 21 (of the ratio element) is switched. The signal) 24 is in a zero state. Therefore, since both the differential element signal 23 and the ratio element signal 24 are zero, the output of the AND element 22 is also at zero level, and the output relay 14 does not operate. Next, if there is a fault inside the bus, a differential current I _D and a suppression voltage |E _R | proportional to the fault current will occur, and if the differential current I _D exceeds the specified value, a level detector will be detected. 18 is switched and differential element output 2
Outputs 3. Further, the ratio element signal 24 is the suppression voltage |E _R | generated in the resistor 12 and the differential voltage |E _O |
Calculation of |E _R |−|E _O |K is detected, and by keeping a certain ratio in the derivation conditions for voltage |E _R | and |E _O |, in the event of an internal accident, |E _O | ＞
|E _R | becomes. Therefore, the level detector 19 does not switch, and the output signal 24 of the NOT element 21 becomes high level, so the AND element 22
The input signal condition is satisfied and the output relay operates. The above operation is illustrated in a characteristic diagram as shown in FIG. In other words, Fig. 4 shows 1 expressed in the same way as the conventional characteristic diagram Fig.
This is a terminal ratio characteristic diagram, and the straight line 25 is the locus in the event of an internal fault on the bus bar in a one-terminal power supply, and it is necessary to consider the slope up to I _D / I _T = 0.5 in the event of an internal fault with outflow current. is as described above. The operating characteristic 27 is a characteristic of the differential element, and indicates that the level detector 18 in FIG. 3 detects the differential current _ID above a specified value. In addition, the operating characteristic 28 is a characteristic _of a ratio element _, and _the level detector 19 in _FIG . ｜E _R ｜−｜E ₀ ｜=I _T −ηI _D
≧K. Note that the operating characteristic 28 is such that the slope η≒I _T /I _D starts from the origin, but the minimum detection value condition of the level detector 19 is such that I _T operates at a specified value I _T1 or more. However, since it is not within the scope of the present invention, a detailed description will be omitted. At the time of an internal fault in the bus bar, the relationship between the suppression current I _T and the differential current is on the straight line 25 or on a straight line in a direction with a larger slope. The characteristic 28 of the ratio element should be set below the slope of the straight line 25. In addition, if the current detector CT is saturated during an external fault and an erroneous differential current is generated, depending on the saturation situation, it may exceed the limit of operating characteristics 27 of the differential element and the third
There is a possibility that the level detector 8 shown in the figure may switch. The measure to prevent the output relay 14 from malfunctioning in this case is the operating characteristic 28 of the ratio element.As long as the relationship between the suppression current I _D is maintained within the hatched area of the operating characteristic 28 in FIG.
The output is locked. Next, the principle of operation will be explained with reference to FIG. 5 in the case of an external accident in which the current detector CT saturation is significant, which is the object of the present invention. The waveform I _F is an example of a current outflow waveform flowing to a fault point, and the actual fault current waveform is under the most severe conditions for CT saturation. In other words, this is a case where the transient DC current is 100% superimposed on the peak value of the AC current when an accident occurs, and the AC current is
If I _FP and the decay time constant of the DC component current are T, then the fault current is

It is represented by [Formula]. Waveform I ₁ is an example of the waveform of the CT secondary current at the external fault end. The primary current of this external fault end (outflow end) CT is waveform I _F , and its magnitude is the same as that of the fault current, so it is the most Conditions are such that it is easy to become saturated. The shaded part of the waveform is the CT secondary current waveform, and the dotted line is the CT
It represents the time when is not saturated. The waveform _I2 is the error differential current, and the severe condition is that the inflow end
CT is unsaturated and only the outflow end (fault end) CT has a waveform
If it is saturated like I ₁ , a difference between the waveform _IF and the waveform I ₁ will occur as the error differential current I ₂ , resulting in a waveform as shown by diagonal lines. At this time, the input of the ratio differential relay 15 in Fig. 3 is the operation input I _D
The waveform I ₂ shown in Fig. 5 is applied, and the suppression input |E _R |
Since the waveform |I ₁ | is applied, the voltages generated across the resistors 11 and 12 in FIG. 3 are |E _O | and |E _R
| eventually becomes |E _O |∝|I ₂ |, |E _R |∝|I ₁ |, so the waveform of the voltage |E _R |−|E _O | is K ₁ |I ₁ |−
The waveform is the same as that of K ₂ |I ₂ |. The level detector 19 is designed to switch only when |E _R |−|E ₀ |≧K, and when |E _O |>|E _R |, the output mode is set to a steady state. Therefore, the input to the level detector 19 is |E _R |−|E _O |
Only input waveforms and magnitudes in the zero range matter.
As shown in Figure 5, the waveform of |E _R |−|E _O |=K ₁ |I ₁ |−K ₂ |I ₂ |
Once I ₂ starts to generate, it disappears, but until CT saturation is reached, an output is generated in proportion to the current I ₁ . That is, in the first wave after an accident occurs, the CT is unsaturated during the time t ₁ until the sum of the residual magnetic flux of the CT and the magnetic flux generated by the fault current reaches the saturated magnetic flux of the CT, and the CT secondary current I ₁ is Since it occurs in proportion to the primary current I _F and no error fluctuation current I ₂ occurs, K ₁ | I ₁ | −K ₂
|I ₂ | is proportional to the current I ₁ . Next, the second wave is largely influenced by the decay time constant T of the DC component of the fault current I _F , but if it occurs on the negative wave side for a time t ₂ minutes, the magnitude of the negative wave component (proportional) value is generated during time t ₃ as the sine component of the second wave, and CT is the non-saturation period of waveform I ₁ during t ₂ + t ₃ .
Something between t ₂ + t ₃ will occur at K ₁ |I ₁ |−K ₂ |I ₂ |. The input of the level detector 19 is K ₁ | I ₁ | −
Since K ₂ |I ₂ |, the output signal 25 of the level detector 19 is K 1 |I 1 | at time t ₁ of the first wave and time t ₂ +t ₃ of the second wave, as shown in _{FIG .} −K ₂ ｜I ₂ ｜
generates an output pulse by switching in the range where the magnitude of
To delay the signal for T ₁ hour with delay circuit 20
The signal 24 of the NOT circuit 21 becomes a continuous signal as shown in FIG. This extension time T ₁ is a long decay time T of the DC component current, and in the second wave, the current I ₁ is t ₂ + t ₃
is short and the magnitude of K ₁ |I ₁ |−K ₂ |I ₂ | is small, so if the _second pulse of signal 25 in FIG. It is sufficient to extend the pulse to the fourth wave. As described above, even if the CT secondary current is significantly saturated in the first wave due to the residual magnetic flux of the CT, and even if saturation continues to be large in the second wave and subsequent waves due to the DC component current, the detection value of the level detector 19 of the ratio element and A sufficient countermeasure can be taken by appropriately setting the signal extension time _T1 of the OFF delay circuit 20. Figure 5 shows the waveform at the time of an external fault, but even in the case of an internal fault, if the CT is significantly saturated, a differential current will be generated at the same time as the fault occurs, and the differential current will be the secondary current of the CT at each power supply terminal. Because it is the sum, it is always larger than the CT secondary current at each end | E _O |
>|E _R |, so it is impossible for the level detector 19 to detect it. As described above, the present invention focuses on the difference between the secondary current waveform and error differential current waveform due to CT saturation at the time of internal fault and external fault, and reliably distinguishes between internal and external faults even in the case of extreme CT saturation. By providing a ratio element that can be used to prevent malfunctions, reliability is greatly improved by improving the performance of the relay, and this also has an extremely significant effect on reducing the size and cost of CTs. It is.

[Brief explanation of drawings]

Fig. 1 is a block configuration diagram showing the principle of a conventional ratio differential relay, Fig. 2 is a ratio characteristic diagram of the ratio differential relay shown in Fig. 1, and Fig. 3 is a ratio differential relay showing an embodiment of the present invention. 4 is a block diagram showing the principle of the relay, FIG. 4 is a ratio characteristic diagram of the ratio differential relay of FIG. 3, and FIG. 5 is a waveform diagram illustrating the principle of the ratio differential relay of the present invention. 1... Bus line, 2-1 to 2-n... CT, 3-1
~3-n...Input device, 4,5,9,16...Transformer, 6,11,12...Resistor, 7,10,1
7... Full wave rectifier circuit, 8, 15... Ratio differential relay, 13, 18, 19... Level detector, 14...
...Output relay, 20...OFF delay circuit, 2
1...NOT circuit, 22...AND circuit, 23...
Output signal of differential element, 25... Signal of ratio element.
Note that the same reference numerals in the figures indicate the same or equivalent parts.

Claims (1)

[Claims] 1. A current differential amount deriving circuit that detects a plurality of branch currents in a power system using a current transformer and derives a differential amount obtained by vector-synthesizing the detected secondary currents of the current transformers; a current suppression amount derivation circuit that derives a suppression amount proportional to the maximum absolute value or the sum of the absolute values of the plurality of current transformer secondary currents; and a first level that detects an output signal of the current differential amount derivation circuit. a detection element; a level detection circuit that compares the instantaneous value of the output of the current suppression amount deriving circuit with the instantaneous value of the absolute value of the output of the current differential amount deriving circuit, and generates an output signal when the suppression amount is larger; , a second level detection element including an OFF delay circuit that extends the output signal of the level detection circuit for a predetermined time, and the output signal of the first level detection element is negated by the output of the second level detection element. A ratio differential relay characterized in that it is configured as follows.