JPS64891B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a ground fault detection device for ungrounded power distribution lines. To detect ground faults in non-grounded distribution lines, conventional methods include a ground fault overvoltage relay that responds to the zero-sequence voltage of the bus bar, and a ground-fault direction relay that responds to the phase difference between the zero-sequence voltage and the zero-sequence current of each feeder. Detection was done through cooperative action with In this case, in order to obtain the specified ground fault detection sensitivity (for example, the ground fault resistance is 6000 ohms for a 6.6KV bus), the zero-sequence voltage detection level of the ground fault overvoltage relay must be adjusted by performing an artificial grounding test. There is a need. Therefore, when there is a change in the system configuration, such as an increase or decrease in the number of feeders, the ground capacity of the system changes, creating the inconvenience of having to conduct an artificial grounding test and readjust each time. . When detecting a ground fault in an ungrounded distribution line, this invention
The purpose is to keep the ground fault detection sensitivity constant regardless of changes in the ground capacity of the system. This invention calculates the relationship between the zero-sequence voltage and the earth-fault current at each earth-to-earth capacity value when the earth-to-earth capacity is sequentially changed when the earth-fault resistance is the same, and calculates the zero-sequence voltage and earth fault current that satisfy this relationship. This method attempts to detect ground faults based on the short circuit current. Figure 1 shows an example of an ungrounded distribution line system, where 1 is a bus bar, 2A and 2B are feeders, 3 is a grounding transformer that detects zero-sequence voltage, and 4A and 4B are zero-sequence current transformers. . If feeder 2A causes a ground fault, the earth-to-ground capacity of the healthy line will be at the ground fault point.
Currents I g ′ and I _{g ″} flow through C _s (earth capacitance per arm) and the earth capacitance C (earth capacitance per arm) of the faulty line, respectively. If the ground fault resistance is R _g and the phase voltage is E _a , this equivalent circuit can be shown as shown in FIG.
If we find the ground fault current I _g and zero-sequence voltage V _p from the same figure, we get I _g = E _a /R _g +1/jw ₃ (C + Cs) V _p = I _g /jω ₃ (C + C _s ). . Here, if the ground fault detection sensitivity is set to 6000 ohm in a 6.6K distribution line (in this case, E _a =
6600/√3=3800V), and when we actually calculated each value in the previous equation for each ground capacity, we obtained the following results. In this table, the zero-sequence voltage assumes the value at the time of a complete ground fault as 100%.

[Table] The relationship between zero-sequence voltage V _p and ground fault current I _g in this table is shown in a graph as shown in Figure 3. What this curve means is that at any point on this curve, the ground fault detection sensitivity is independent of the ground capacity.
It represents the relationship between zero-sequence voltage V _p and ground fault current I _g at 6000 ohms. Therefore, when the zero-sequence voltage and ground fault current are determined from the actual system, and the relationship between the two is within the area above the graph shown in Figure 3, it is assumed that a ground fault with a ground fault resistance of 6000 ohms or less will occur. It becomes possible to detect this as having occurred. FIG. 4 shows an embodiment of the invention. 1 is 6.6KV
The bus bar, 2 is a feeder, 3 is a grounding transformer, and 4 is a zero-phase current transformer, which are not much different from those in FIG. The zero-sequence voltage obtained from the grounding transformer 3 is transformed into an appropriate value by the auxiliary transformer 5. This transformed voltage (this is referred to as V _p ') is phase-shifted by, for example, 45 degrees in advance by the phase shift circuit 6 (this output is set as A).
shall be. ) is in phase with the ground fault current, and then shaped into a square wave output B by the square wave shaping circuit 7,
This square wave output B becomes one input of the AND gate 8. The output of the AND gate 8 is input to an integrating circuit 9. When the level determination circuit 10 detects that the integrated value exceeds a certain value, the relay drive circuit 11 operates based on the output from this point, and the relay 12 operates. The operation of the relay 12 notifies the ground fault detection or selects and disconnects the feeder. The voltage corresponding to the zero-sequence voltage obtained from the auxiliary transformer 5 is also connected to the diode 13, the resistor 14 and the capacitor 1.
The rectifying and smoothing circuit 16 consisting of 5 converts the current into direct current and smooths the current. The upcoming DC voltage is applied to a nonlinear circuit 19 consisting of, for example, a Zener diode 17 and a resistor 18, where it is converted into a nonlinear output such that the output is −E _p for a zero-sequence voltage V _p as shown in FIG. be done. This nonlinear output is provided as one input of a differential amplifier 21 via a bias resistor 20. The zero-phase voltage from the grounding transformer 3 is supplied to the primary side of the zero-phase current transformer 4 together with a capacitor 22 whose capacity is set to be the same as the ground capacity C of the feeder 2. The current flowing through the capacitor 22 is I=jω ₃ CV _p
However, as can be understood from FIG. 1, this is because the ground fault current of the faulty line does not flow to the zero-phase current transformer 4, so it is necessary to correct this.
Note that if the ground capacitance C is sufficiently smaller than the ground capacitance C _s of the system, the capacitor 22 may be omitted. The output of the zero-phase current transformer 4 is converted into an appropriate value by an auxiliary current transformer 23 (this is designated as I _g '), and further converted into a voltage by a resistor 24. This voltage is given as another input of the differential amplifier 21. The differential amplifier 21 constitutes a waveform shaping circuit 25 that converts the voltage of the resistor 24 into a square wave, and at this time, the square wave level is controlled by the nonlinear output from the nonlinear circuit 19. In other words, the smaller the zero-phase voltage V _p is, the larger the slice level of the square wave is. . The output from the waveform shaping circuit 25 becomes an input to the AND gate 8. In the above configuration, it is assumed that a ground fault occurs, and at this time, the zero-sequence voltage V _p is small and the ground fault current I _g is large. Since the zero-phase voltage V _p is small, the resistance 2 in the waveform shaping circuit 25
Assume that voltage No. 4 has a high slice level, so that the square wave output from the circuit 25 has the same width as the square wave output B, as shown in C (see FIG. 6). Therefore, the output D of the AND gate 8 at this time has the same width as the square wave outputs B and C. This output D is integrated by an integrating circuit 9. The integrated output becomes as shown in E, and when it reaches a predetermined level, an output F is output from the level determination circuit 10 and the relay 12 is activated. If the above-mentioned zero-sequence voltage V _p is small and the ground fault current is even larger, the slice level in the waveform shaping circuit 25 is the same, so
The width of the square wave output C from circuit 25 becomes even wider. However, even with this, the output D of the AND gate 8 does not change. Therefore, even in this case, relay 1
2 works. Also assume that the zero-sequence voltage V _p is small and the ground fault current is smaller than in the above case. In this case, since the slice level in the waveform shaping circuit 25 is the same, the width of the square wave output C becomes narrower due to the smaller ground fault current. Therefore, and gate 8
Since the width of the output D becomes narrower and the discharging time becomes longer than the charging time in the integrating circuit 9, the output E does not reach the specified level and therefore the relay 12 does not operate. Next, assume that the zero-sequence voltage V _p is large and the ground fault current I _g is small. In this case, the slice level in the waveform shaping circuit 25 becomes low, but the ground fault current I _g
Because of the small width of the square wave output C from the circuit 25, the width of the square wave output C from the circuit 25 is the same, so that the relay 12 operates in the same manner as described above. If the ground fault current is larger, the slice level remains the same, so the width of the square wave output of the circuit 25 becomes wider;
The output D of the AND gate 8 does not change, and the relay 12 operates in this case as well. Conversely, when the ground fault current is smaller than in the above case, the width of the square wave output from the circuit 25 becomes narrower, and in this case the relay 12 does not operate. As understood from the above explanation, the zero-sequence voltage
When V _p is small, the relay 12 operates when the ground fault current I _g is greater than a large value, and the zero-sequence voltage V _p
If is large, the relay 12 operates even if the ground fault current I _g is a sufficiently small value or more. This relationship satisfies the relationship shown in FIG. 3. As detailed above, according to the present invention, it is now possible to always detect ground faults with the same ground fault detection sensitivity regardless of changes in the system configuration. This has the effect of eliminating the inconvenience caused by adjusting the sensitivity by performing a grounding test.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of a power distribution system, FIG. 2 is an equivalent circuit diagram of FIG. 1, FIG. 3 is a V _p -I _g characteristic curve diagram, and FIG. 4 is a circuit diagram showing an embodiment of the present invention. FIG. 5 is a characteristic diagram of the nonlinear circuit, and FIG. 6 is a waveform diagram for explaining the operation. 1...System bus, 2...Feeder, 3...Grounding transformer, 4...Zero-phase current transformer, 6...Phase shift circuit, 7...Square wave shaping circuit, 8...And gate, 9...Integrator circuit, 1
0... Level determination circuit, 12... Relay, 16... Rectifying and smoothing circuit, 19... Nonlinear circuit, 25... Waveform shaping circuit.

Claims (1)

[Claims] 1 A grounding transformer that detects the zero-sequence voltage of each feeder, a zero-sequence current transformer that detects the ground fault current of each feeder, and the zero-sequence voltage detected by the grounding transformer as input, When the detection sensitivity is the same,
a nonlinear circuit that produces a nonlinear output with respect to a characteristic curve showing the relationship between zero-sequence voltage and ground fault current for each ground capacity of the feeder; and a ground fault detected by the zero-sequence current transformer with the nonlinear output as a slice level. A waveform shaping circuit that shapes the current into a square wave and creates a square wave with the same width as the zero-sequence voltage and ground fault current that satisfy the characteristic curve, and the square wave output and the zero-sequence voltage that satisfy the characteristic curve. an AND gate that receives as input an output that is phase-shifted to be in phase with the current and made into a square wave; an integrating circuit that integrates the output of the AND gate; and when the integrated output of the integrating circuit exceeds a predetermined level. A ground fault detection device for non-grounded distribution lines consisting of an operating relay.