JPS59173775A - Detector for ground-fault of variable voltage and variable frequency power system - Google Patents

Abstract

PURPOSE:To detect surely a one-phase ground-fault accident even when a system is operated with a low frequency by controlling the set value of a protective relay through a physical quantity detector which detects a physical quantity proportional to the frequency of a power system. CONSTITUTION:The output from a variable voltage and variable frequency power supply device 2 is impressed to an AC motor 4 of a load of the power system through an output transformer 3. A pilot generator 7 is interlocked by this motor 4 to detect a physical quantity proportional to the frequency of the power system, and a sensitivity set value, which is proportional to the system frequency, of the protective relay 8 is controlled and changed in accordance with this detection output. Consequently, a zero-phase detection voltage from an open tertiary winding 31 of the transformer 3 whose output is changed in accordance with the frequency becomes the set value or higher even for the ground-fault at a point F of the power system when the motor 4 is driven with a low frequency, and the one-phase ground-fault accident is detected surely through the relay 8.

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 VVV l" device), 3 is an output transformer, and 4 is an AC motor that is the load of the system. Output transformer 3 has an open tertiary winding 31, the output of which 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 a VVVF device 2.
Electric power from an AC power supply 1 whose voltage and frequency are controlled by In order to prevent overheating due to overexcitation of the AC motor 4, 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 1-phase ground fault occurs at point F in the diagram,
A zero-sequence voltage Vo appears at the open end of the tertiary winding 31 of the output transformer 3, and this zero-sequence voltage Vo is guided to the protective relay 5, and if its value is larger than the set value of the protective relay 5, This outputs a detection signal and the occurrence of a one-phase ground fault is detected.

However, since this zero-sequence voltage Vo is affected by the grid frequency f in the same way as the grid voltage E, if a one-phase ground fault occurs during low-frequency drive of the AC electric TA4, a situation may occur where it cannot be detected. .

That is, the protective relay 5 considers incomplete ground faults based on the value of zero-sequence voltage ■0 at the time of one-phase full-metal ground fault when the frequency f is the commercial frequency fs and the voltage E is the constant operating voltage E's. and 20
The detection sensitivity is selected to be approximately 50%.

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 fsX40% or less, the ground fault is a complete ground fault. However, since the zero-phase voltage drops to 40% or less of VsX, it is difficult to detect this, and there is a drawback that a one-phase ground fault is overlooked.

This invention was made in order to eliminate the above-mentioned drawbacks of the conventional technology, and includes a physical quantity detector that detects a physical quantity proportional to the frequency of the system, and the setting value of the protective relay is set to the output of the physical quantity detector. By adopting a configuration that varies accordingly, it is possible to reliably detect single-phase ground faults even during low-frequency operation of the system, thereby increasing the reliability of system protection compared to conventional systems. The purpose 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, a pilot generator 7 is connected to the AC motor 4, and its output es is manually inputted to the protective relay 8 as a setting value input. This protective relay 8 receives the zero-sequence voltage V, o appearing at the open end of the open tertiary winding 31 of the output transformer H3 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 configured to do this. 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 system frequency f, so the setting value of the protective relay 8 that constantly receives this voltage e's is not fixed, but changes as the frequency increases or decreases. The value increases or decreases accordingly.

Therefore, when a phase-to-ground fault occurs, the protective relay 8 compares the input zero-sequence voltage Vo with a set value that is proportional to the frequency f of the system at the time of a one-phase ground fault. , 40 when the frequency f of the grid is the commercial frequency fs
If the sensitivity is set to detect incomplete ground faults up to %, the frequency f at the time of a phase ground fault is fs
Even in this case, 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 a 40% incomplete ground fault.

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 connected directly to the protective relay 8.
Alternatively, the signals may be provided after level/signal conversion through an input device (not shown).

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 system frequency f, but the slip is usually about 5%, so
There are no practical problems.

In the above embodiment, the zero-phase voltage ■0 is taken out from the third and next winding 31 of the output transformer 3, but the neutral point resistor 6
The same effect can be obtained by detecting the voltage corresponding to the zero-sequence voltage using other methods or means such as using the voltage across the voltage and applying it to the protective relay 8.

In this embodiment, the output of the pilot generator 7 is used as the setting input of the protective relay 8, but the system frequency f
It is also possible to provide a physical quantity detector that detects other physical quantities such as voltage, current, and rotation speed that are proportional to , and use the detected value directly or by converting it into an appropriate signal/level as the above-mentioned setting value input. An example of this is shown in FIG.

In FIG. 3, numeral 10 is a voltage detector, from which the phase-to-phase voltage of the system is guided through a voltage transformer 9, and the detected power is directly or by a human-powered device (not shown) set to a signal level suitable for manual input. is converted into and applied to the protective relay 8. 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.

In addition, this invention can be used even if the system meal prep is not an AC motor.
If the power system is controlled at a constant voltage/frequency ratio,
When carried out, the same effect as above can be obtained.

As described above, according to the present invention, a 4m electric appliance that uses a zero-sequence voltage or a voltage corresponding to this as a detection input,
By using a configuration in which the output of a physical quantity detector that detects a physical quantity proportional to the frequency of the grid is given as a manual setting value, when the frequency of the grid decreases, the above-mentioned setting value of the protective relay also decreases, so it It is possible to reliably detect even single-phase ground faults, and the reliability of ground fault protection can be improved compared to the conventional method.

[Brief explanation of the drawing]

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 figures, the same reference numerals indicate the same or equivalent parts. Agent Makoto Kazuno

Claims (3)

[Claims] (1) A protective relay to which the zero-sequence voltage of the system or a voltage corresponding to this is introduced as a detection input, and a physical quantity detector to detect a physical quantity proportional to the frequency of the system, and the output of the physical quantity detector is the output of the protective relay. A ground fault detection device for a 01 variable voltage variable frequency power system, characterized in that it is applied as a settling input to the protective relay. (2) Ground fault detection in a variable electric ratio variable frequency power system according to claim 1, characterized in that it is a physical quantity detector or a pilot generator connected to an AC motor connected to the system. Device. (3) A ground fault detection device for a J-variable voltage variable frequency power system according to claim 1, wherein the physical quantity detector is a voltage detector that detects the phase-to-phase voltage of the system.