JPS64892B2 - - 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 detects ground faults below a certain ground fault detection sensitivity from a vector diagram determined by the zero-sequence voltage at the time of a ground fault, the fault phase voltage, and the voltage drop at the ground fault point. be.

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 of the fault phase is V _a , this equivalent circuit can be shown in Figure 2. Figure 3 is a vector diagram drawn based on this equivalent circuit. Note that θ indicates the angle between the phase voltage V _a of the fault phase and the ground fault current Ig. From the relationship shown in Figure 3, V _p = R _g I _g tanθ Therefore, I _g tanθ = V _p /R _g Here, when R _g ≦6000 ohm, I _g tanθ≧V _p /6000. Therefore, by finding the ground fault current I _g and tanθ and comparing them with V _p /6000, Therefore, it is possible to detect a ground fault with a ground fault detection sensitivity of 6000 ohms.In this case, the ground fault current I _g cannot be actually measured at the relay point, but if the ground capacity C of the fault line is set, the fault can be detected. Jω3CV _p
Therefore, the ground fault current I _g can be obtained as I _g = I _p + Jω3CV _p .

FIG. 4 shows an embodiment of the invention based on the operating principle described above. 1 is 6.6KV bus, 2 is feeder, 3
is the zero-sequence voltage V _p of the grounding transformer, and the phase voltage V _a of each phase,
Detect V _b and V _c . 4 is a zero-phase current transformer. The phase voltage detected by the grounding transformer 3 is applied to a ground fault phase determination element 5, where a ground fault phase is detected. When the phase voltage of the ground fault phase, for example, the a phase, is ground faulted, the phase voltage V _a is outputted. This phase voltage V _a is applied together with the zero-phase voltage V _p to the phase difference detection circuit 6, where the phase difference θ is detected. In other words, as understood from Fig. 3, there is a relationship between the phase voltage V _a and the zero-sequence voltage V _p as follows: V _p /V _a = sinθ, and therefore the phase voltage V _a and the zero-sequence voltage
If V _p is given, the phase difference θ can be found from the previous equation as θ=sin ⁻¹ V _p /V _a . Phase difference detection circuit 6
The output corresponding to the phase difference θ is given to the calculation circuit 7, where tan θ is calculated.

Zero-sequence voltage obtained by the grounding transformer 3
V _p is supplied to the primary side of the zero-phase current transformer 4 via a capacitor 8 having a capacity of Jω3C. Therefore, the output of zero-phase current transformer 4 is I _p + Jω3CV _p , that is, the ground fault current
The output corresponds to I _g . Note that if the ground capacitance C is sufficiently smaller than the ground capacitance C _s of the system, the capacitor 8 may be omitted. The output of the zero-phase current transformer 4 is given to the multiplication circuit 9 together with the output of the calculation circuit 7, where both output values are multiplied. This multiplication result corresponds to the above-mentioned I _g tan θ.

The zero-phase voltage V _p is also given to the divider circuit 10,
If the ground fault detection sensitivity is set to 6000 ohms,
Calculate V _p /6000. This calculation result corresponds to V _p /R _g in the previous equation. This calculation result and the output of the division circuit 10 are given to a comparison circuit 11, where the comparison result of both values. Outputs when the relationship I _g tanθ≧V _p /6000 is established. Relay 12 is activated by this output. The fact that the relay 12 operated in this way means that a ground fault occurred when the ground fault resistance was less than 6000 ohms. The operation of the relay 12 notifies the ground fault detection or selects and disconnects the feeder.

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 vector diagram, and FIG. 4 is a circuit diagram showing an embodiment of the present invention. DESCRIPTION OF SYMBOLS 1...System bus, 2...Feeder, 3...Grounding transformer, 4...Zero phase current transformer, 5...Ground fault phase discrimination element, 6...Phase difference detection circuit, 7...Calculation circuit, 9
...multiplication circuit, 10 ... division circuit, 11 ... comparison circuit, 12 ... relay.

Claims (1)

[Claims] 1 A grounding transformer that detects the zero-sequence voltage and phase voltage of each feeder, a zero-sequence current transformer that detects the ground fault current of each feeder, and a zero-sequence voltage and fault detected by the grounding transformer. A circuit that inputs the phase voltage of the phase and outputs a value corresponding to tanθ when the phase difference between the phase voltage and the ground fault current is θ, and a circuit that outputs a value corresponding to tanθ, and an output corresponding to the tanθ of the zero-phase current transformer. a circuit that multiplies the output, and a comparator circuit that compares the multiplication output with an output corresponding to a value obtained by dividing the zero-sequence voltage by the ground fault detection sensitivity, and outputs an output when the multiplication output is greater than or equal to the output. A ground fault detection device for ungrounded distribution lines.