JPH0369073B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a ground fault detection device for a variable voltage variable frequency power system.

A conventional example of this type of ground fault detection device is shown in FIG. In the figure, 1 is an AC power supply, 2 is a variable voltage variable frequency power supply device (hereinafter abbreviated as a VVVF device), 3 is an output transformer, and 4 is an AC motor that is a load of the system. The output transformer 3 has an open tertiary winding 31, and its output is input to a protective relay (overvoltage relay) 5 as a ground fault detector. 6 is a neutral point resistor of the grounding circuit of the output transformer 3.

In this configuration, the AC motor 4 includes:
Electric power from an AC power source 1 whose voltage and frequency are controlled by a VVVF device 2 is supplied via an output transformer 3, and the AC motor 4 rotates at a speed corresponding to the frequency f of the input system. In order to prevent overheating due to overexcitation of the AC motor 4 that occurs during low frequency operation, the output of the VVVF device 2 is controlled so that the system voltage E and frequency f have the following relationship.

E=Ko・f (Ko: constant) ...(1) Now, if a single-phase ground fault occurs at point F in the figure, the zero-sequence voltage will be applied to the open end of the tertiary winding 31 of the output transformer 3. Vo appears and this zero-sequence voltage Vo is guided to the protective relay 5, and if the value is larger than the setting value of the protective relay 5, this outputs a detection signal and the occurrence of a one-phase ground fault is detected. Ru.

However, this zero-sequence voltage Vo is affected by the grid frequency f in the same way as the grid voltage E, so if a one-phase ground fault occurs when the AC motor 4 is driven at a low frequency, a situation may occur where it cannot be detected. .

That is, the protective relay 5 has a frequency f equal to the commercial frequency.
Based on the value of the zero-sequence voltage Vo at the time of one-phase complete ground fault when fs and voltage E are the constant operating voltage Es, the detection sensitivity is selected to be about 20 to 50% in consideration of incomplete ground fault.

Therefore, for example, if the sensitivity is set to detect a 40% incomplete ground fault under the commercial frequency fs,
If the frequency at the time of a ground fault is less than fs x 40%, the zero-sequence voltage will be Vs x 40% even if the ground fault is a complete ground fault.
This has the disadvantage that it is difficult to detect this and one-phase ground faults can be overlooked.

This invention was made in order to eliminate the above-mentioned drawbacks of the conventional system, and includes a physical quantity detector that detects a physical quantity proportional to the frequency of the grid, and the setting value of the protective relay corresponds to the output of the above-mentioned physical quantity detector. This is a variable voltage variable frequency power system that can reliably detect single-phase ground faults even when the system is operating at low frequencies, thereby increasing the reliability of system protection compared to conventional systems. The purpose of this invention is to provide a ground fault detection device.

An embodiment of the present invention will be described below with reference to the drawings.

In Fig. 2, 7 is a pilot generator, which is connected to the AC motor 4, and whose output is es.
is input to the protective relay 8 as a set value input.
This protective relay 8 receives the zero-sequence voltage Vo appearing at the open end of the open tertiary winding 31 of the output transformer 3 as a detection input, and its setting value increases or decreases in proportion to the voltage es, which is the setting value input. It is structured as follows. Since the other configurations are the same as those in FIG. 1, the same reference numerals are given.

In this configuration, the voltage es output by the pilot generator 7 increases or decreases in proportion to the frequency f of the grid, so the setting value of the protective relay 8 that constantly receives this voltage es is not fixed, but corresponds to increases and decreases in frequency. It becomes a value that increases or decreases. Therefore, when a one-phase ground fault occurs, the protective relay 8
Vo is compared with a set value proportional to the grid frequency f at the time of a one-phase ground fault. For example, if the grid frequency f is the commercial frequency fs, up to 40% incomplete ground fault can be detected. If the sensitivity is set to obtain
%, by lowering the set value to 40%, a detection signal is output not only in the case of a complete ground fault but also in the case of an incomplete ground fault of 40%.

In this embodiment, the output of the pilot generator 7 and the output of the open tertiary winding 31 of the output transformer 3 are directly applied to the protective relay 8, but the levels and signals are converted through input devices (not shown). You may also give it as a gift.

Note that when the AC motor 4 is an induction motor, the output es of the pilot generator 7 is not exactly proportional to the frequency f of the system, but since the slip is usually about 5%, it is not practical. No problem.

In the above embodiment, the zero-phase voltage Vo is output from the output transformer 3.
However, other methods such as using the voltage across the neutral point resistor 6 may be used.
The same effect can be obtained by detecting a voltage corresponding to the zero-phase voltage and applying it to the protective relay 8.

In this embodiment, the output of the pilot generator 7 is used as the setting value input for the protective relay 8.
A physical quantity detector is provided to detect other physical quantities such as voltage, current, and rotational speed that are proportional to the frequency f of the system, and the detected value is directly or converted into an appropriate signal/level and used as the above-mentioned setting value input. It's okay,
An example is shown in FIG.

In FIG. 3, 10 is a voltage detector,
The phase-to-phase voltage of the grid is guided through the voltage transformer 9,
This detection force is applied to the protection relay 8 directly or converted into a signal/level suitable for inputting a set value by an input device (not shown). This phase-to-phase voltage will hardly fluctuate even when one phase is grounded when the output transformer 3 is ungrounded or resistance grounded, and this voltage is proportional to the frequency f of the system. The same effect as in the example can be obtained.

Note that even if the system load is not an AC motor, the present invention can be implemented and the same effects as described above can be obtained as long as the voltage/frequency ratio is controlled to be constant.

As described above, according to the present invention, the output of a physical quantity detector that detects a physical quantity proportional to the frequency of the system is given as a setting value input to a protective relay whose detection input is a zero-sequence voltage or a voltage corresponding to this. As a result, when the frequency of the grid decreases, the setting value of the protective relay also decreases, making it possible to reliably detect single-phase ground faults during low-frequency operation of the grid, compared to conventional methods. Therefore, the reliability of ground fault protection can be improved.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram of a conventional ground fault detection device for a variable voltage variable frequency power system, Fig. 2 is a circuit diagram of an embodiment of the ground fault detection device for a variable voltage variable frequency power system according to the present invention, and Fig. 3 is a circuit diagram of a conventional ground fault detection device for a variable voltage variable frequency power system. FIG. 3 is a circuit diagram of another embodiment of the invention. In the figure, 2... variable voltage variable frequency power supply device, 3... output transformer, 7... pilot generator, 8
...Protective relay, 9...Voltage transformer, 10...Voltage detector. In the drawings, the same reference numerals indicate the same or equivalent parts.

Claims (1)

[Claims] 1. A protective relay to which the zero-sequence voltage of the system or a voltage corresponding thereto is introduced as a detection input, and a physical quantity detector that detects a physical quantity proportional to the frequency of the system, wherein the output of the physical quantity detector is A ground fault detection device for a variable voltage variable frequency power system, characterized in that a set value input to the protective relay is applied to the protective relay. 2. The ground fault detection device for a variable voltage variable frequency power system according to claim 1, wherein the physical quantity detector is a pilot generator that is connected to an AC motor connected to the system. 3. The ground fault detection device for a variable voltage variable frequency wave power system according to claim 1, wherein the physical quantity detector is a voltage detector that detects interphase voltage of the system.