JPH02234071A - Ground fault detector - Google Patents

Abstract

PURPOSE:To detect a ground fault even in an electric system with large variation of the voltage and frequency by providing means for the detection of voltages against the ground for each phase and the suppression of its tortion, for calculations of two phase difference and total addition of output signals for each phase, and for the detection of the ground fault from those mentioned above. CONSTITUTION:The subject device consists of voltage dropping devices 21a-c dividing the voltages against the ground for each phase by resistors, converters 22a-c for absolute values of the aforementioned output voltages, filter circuits 23a-c smoothing the above, a subtracting device 24a calculating the different voltage for the circuits 23a and 23b, similar subtracting devices 24b, 24c, an adding device 25 calculating the total output voltage for the circuits 23a-c, a gain setting device 26 for this output voltage, a gain multiplier 27, and comparators 28a-c detecting the ground fault from the above output signal and the subtracting devices 24a-c. The voltages against the ground for each phase are converted into the DC signals through the circuits 23a-c, then the output voltages for each phase are equal and the output signals of the subtracting devices 24a-c are zero but the positive or negative voltage is outputted when the ground fault is generated, and finally when the two phases voltage difference exceeds a certain rate of an overall detected voltage, the ground fault is detected by the comparators 28a-c. The detection at the same level can be thereby performed without reducing the detection accuracy.

Description

Detailed Description of the Invention [Object of the Invention] (Industrial Application Field) The present invention relates to a ground fault detection device for a variable frequency power supply system. (Prior Art) Figures 4 and 5 show conventional ground fault detection devices for variable frequency power supply systems. In Figure 4, from a three-phase AC power supply 1,
Steps down to low voltage. A variable frequency power supply device 4 is connected via an input transformer 2 and an electromagnetic contactor 3 that opens and closes the circuit. The variable frequency power supply device 4 consists of a forward converter 5 that converts AC power to DC power, a DC reactor 6 that smoothes this DC power, and an inverse converter II 7 that converts DC power to AC power of an arbitrary frequency. The output of the converter 7 is connected to an AC motor 9 through an output transformer 8, and the rotation speed of the AC motor 9 is controlled. The ground fault detection device includes three terminals on the primary side of the input transformer 2, the input side of the variable frequency power supply device 4, and the input side of the AC motor 9.
This is because the circuit is isolated by the input/output transformer, so it may not be possible to detect a ground fault depending on the location. Figure 5 shows a detailed diagram of the ground fault detection device. The ground fault detection device includes a zero-phase transformer 10 that detects the zero-phase voltage of the power supply system, a protective fuse 10a of the zero-phase transformer, and a ground fault overvoltage relay that detects a ground fault from the output voltage of the zero-phase transformer IO. 11. Consists of a suppression resistor 12 that suppresses ground fault current flowing due to a ground fault. The primary winding of the zero-phase transformer 10 is star-connected, and its neutral point is connected to ground. 2
The secondary winding has an open delta connection, and when the sine wave three-phase AC voltage of the system is input to the primary side, zero voltage is always output to the secondary side. When a power grid has a ground fault, one phase with three-phase voltage is fixed at ground potential and becomes unbalanced, causing voltage to occur on the secondary side. When the voltage exceeds a certain predetermined value, the ground fault overvoltage relay 11 operates, outputs a detection signal, and opens the electromagnetic contactor 3 to protect the system. Here, arbitrary AC power is supplied to the AC motor 9 by the variable frequency power supply device 4 via the output transformer 8, and the AC motor 9 rotates at a speed corresponding to the frequency of the AC power. Therefore, when controlling the speed of the AC motor 9, the frequency and voltage of the AC power are controlled. Variable frequency power supply device 4
The output voltage contains many high frequencies and has large waveform distortion.
When the ground fault overvoltage relay 1l is used in such a power supply system, the detection setting value is fixed, so the detection sensitivity of the ground fault overvoltage relay 11 changes with changes in the power supply voltage, and at low voltage, the ground fault detection Sometimes it was impossible. In addition, when trying to solve this problem by lowering the detection setting value, there were problems such as false detection of ground faults due to high voltage. Furthermore, due to large waveform distortion, zero-sequence voltage was generated even when no ground fault occurred, resulting in frequent false ground fault detections. Since this waveform distortion varies greatly depending on the output capacity and output frequency of the variable frequency power supply device 4, it has been impossible to set it to cover the entire operating range.

(Problems to be Solved by the Invention) As described above, the conventional technology has a limited detection range and has problems such as false detection due to waveform distortion of the output voltage and inability to detect due to low output voltage. In addition, because the method detects zero-phase transformation, there were problems such as difficulty in determining the type of ground fault and ground fault phase, and the time required for restoration work. In view of the problems of the prior art, the present invention solves problems in power supply systems with large waveform distortion, such as variable frequency power supplies, and in low-voltage,
The purpose of this paper is to provide a ground fault detection device that can detect ground faults even in power supply systems where the frequency changes significantly. [Structure of the Invention] (Means for Solving the Problem) Therefore, in order to achieve the above object, the present invention detects the ground voltage of the power supply system for each phase, and converts the detection signal into an absolute value for each phase. A first means for converting, a second means for suppressing distortion of the output signal of the first means for each phase, and a third means for calculating a difference between two phases in each phase output signal of the second means. ,
a fourth means for calculating the sum of each phase output signal of the second means; a fifth means for adjusting the output of the fourth means; an output signal of the fifth means and an output signal of the third means; A ground fault detection device is provided, which is equipped with a sixth means for detecting cadastral land. (Function) In the ground fault detection device configured in this way, waveform distortion can be suppressed even in power systems with large waveform distortion, or power systems where output voltage and frequency vary over a wide range, and detection suitable for the output voltage and frequency can be performed. The level can be set, and ground faults can be detected without false detection or failure to detect. (Example) An example of the present invention is shown in Figure 1. A step-down converter 2 that divides the ground voltage of each phase using a resistor in a variable frequency power supply device.
1 a = c, a converter 22a = c that converts the output voltage into an absolute value, a filter circuit 23a = c that smoothes the output voltage, an output voltage of the filter circuit 23a,
A subtracter 24a that calculates the difference in output voltage of the filter circuit 23b, a subtracter 24b that similarly calculates the difference voltage between the filter circuit 23b and the filter circuit 23c, and a subtracter 24c that calculates the difference voltage between the filter circuit 23c and the filter circuit 23a. , filter circuits 23a, 23b, 23c
an adder that calculates the sum of the output voltages of the adder 25, and a setting device 2 that sets a gain for determining a ground fault for the output voltage of the adder 25;
6. A multiplier 27 that multiplies the output voltage of the adder 25 by the gain set by the setter 26, and subtraction that calculates the output signal of the multiplier 27 to which the ground fault detection level is set and the voltage difference between each two phases. The comparator 28a=c detects a ground fault by comparing the output signal of the comparator 24a=c. The method for detecting ground faults in power supply systems using the above configuration will be explained. The ground voltage of each phase of the power supply system is
The voltage is stepped down by the absolute value converters 22a to 22C, respectively.
Each phase is converted into a DC signal through the filter circuit 23a=c. In a power supply system where the output voltage includes a large amount of waveform distortion, such as the variable frequency power supply device 4, malfunctions due to surge voltage can be prevented by applying a filter.
In addition, by converting the signal to a DC signal, it is possible to obtain almost the same filter effect over a wide range of frequencies, and the filter effect itself becomes higher. The output voltage of the filter circuit 23a=c usually has the same magnitude for each phase unless there is a ground fault. Therefore, the subtractor "2"
The output signal of 4a"c is normally "zero". However, if a certain phase has a ground fault, a voltage with positive or negative polarity is output. The ground fault detection level is determined by the adder. The value is obtained by multiplying the sum of the detected voltages of each phase by the multiplier 27 by the gain set by the setter 26.In other words, the difference voltage between the detected voltages of each two phases is the sum of the detected voltages of each phase by the multiplier 27. When a certain percentage is exceeded, a ground fault is detected by the comparators 28a, b, and c.Another embodiment of the present invention is shown in FIG. 2, and its structure, operation, and effect will be explained.In FIG. is constructed by adding a sequence circuit 29 for determining the ground faulted phase to that shown in Fig. 1. Note that the same symbols as in Fig. 1 indicate the same actions, so the explanation will be omitted. Regarding the actions, Let's take a U-phase ground fault as an example. When the U-phase has a ground fault with a certain impedance, the voltage of the U phase becomes extremely close to the ground potential, and the step-down converter 21 a
= The detection voltage of c is lower in the U phase than in the other v and w phases. Therefore, the outputs of the subtractors 24a to 24C are such that 24a is negative,
24b is zero and 24c is positive. In case of 1 wire ground fault, other 2
Since the detection voltages of the phases are equal, the outputs of the subtractors 24a and 24c have the same detection level, although their polarities are different.
Both comparators 28a and 28c detect a ground fault. Therefore, the sequence circuit 29 performs subtraction [124
The polarity of the output of a"c is monitored, and if it is negative, it is determined to be a ground fault phase, and the ground fault phase is detected under the AND condition with comparator 28a=c.The effect is to detect the ground fault phase. This makes it easier to limit the cadastral location in the event of a ground fault, and the restoration work can be greatly shortened. Still another embodiment of the present invention is shown in Fig. 3, and its configuration, operation, and effects will be explained. In Fig. 3, a sequence circuit 30 for determining the type of ground fault is added to the structure shown in Fig. 1. The same symbols as in Fig. 1 indicate the same functions, so the explanation will be omitted. ．The action will be explained by taking U phase, ■ phase, and ground fault as an example. If U phase and ■ phase each have a ground fault with a certain impedance, the detected voltage of step-down converter 21a=c will be equal to the respective ground fault impedance. Assuming that the ground fault impedance of the U phase is smaller than the ground fault impedance of the ■ phase, the outputs of the subtracters 24a''c are negative for 24a, negative for 24b, and positive for 24c. In the case of two-wire grounding, two of the three phases are fixed at the grounded potential, so there is no phase that detects the same voltage, and the subtracter 2
All three outputs of 4a=c always have positive or negative polarity. Therefore, the sequence circuit 30 subtracters 24a to
Monitor the polarity of the output of C, and if there is a zero, connect one wire to ground. If there is no zero, it is determined that there is a two-wire grounding, and the OR conditions of comparison @28a to C are ANDed to detect the type of cadastral property. As an effect, it is possible to detect the type of ground fault, whether it is a 1-wire ground fault or a 2-wire ground fault, which makes it easier to determine the location of the ground fault in the event of a ground fault, and the recovery work can be greatly shortened. Also, this time the step-down device 21a=c is
Although we have explained voltage division using a resistor, detection using a step-down transformer is also possible, and the same effect can be obtained. [Effects of the Invention] According to the present invention, even in a power supply system such as a variable voltage variable frequency power supply device where the waveform distortion is large and the voltage and frequency vary over a wide range, the detection level can be set based on the output voltage. Therefore, it is possible to perform detection at the same level over a wide range without reducing detection accuracy. Furthermore, since ground faults are detected independently for each phase, it is possible to limit the location of ground faults.

[Brief explanation of the drawing]

Figures 1, 2, and 3 are configuration diagrams showing different embodiments of the present invention, Figure 4 is a configuration diagram of a conventional example, and Figure 5 is a detailed diagram of a conventional ground fault detection circuit. be. 1...3-phase AC power supply 2...Input transformer 3.
...Electromagnetic contactor 4...Variable voltage variable frequency power supply device 5...Forward converter 6...DC reactor 7
...Inverse converter 8...Output transformer 9...
・AC motor 10...Zero-phase instrument transformer 11
...Ground fault overvoltage relay 12...Suppression resistor 21...
・Step-down converter 22... Absolute value converter 23...
・Filter circuit 24...Subtractor 25...Adder 26...Detection level setter 27...
Multiplier 28...Comparator 29...Ground fault phase sequence circuit 30...Ground fault type sequence circuit agent Patent attorney Noriyuki Chika Yudo Ken Deshimaru

Claims (1)

[Claims] In a configuration that includes at least a step-down device that steps down the ground voltage of a variable frequency power supply system, and a detector that detects a ground fault in the power system using the ground voltage stepped down by the step-down device, a first means for converting the absolute value to a fourth means for calculating the sum of each phase output signal of the second means; a fifth means for adjusting the output of the fourth means by ground voltage; and an output signal of the fifth means and the fourth means. A sixth means for detecting a ground fault based on the output signal of the third means.
A ground fault detection device characterized by comprising the following means.