JPH0515046A - Ground fault detector - Google Patents

Abstract

PURPOSE:To block high frequency components generated from an inverter inserted for load control and eliminate the influence of the high frequency components upon a portion after a level discriminator by connecting a low pass filter to a zero-phase current transformer for detecting any ground fault current in AC cable way. CONSTITUTION:A low-pass filter 9 connected between a zero-phase current transformer 3 and a level discriminator 6 cuts off high frequency components and transmit only low-frequency components to the level discriminator. The low pass filter 9 blocks a carrier frequency and its harmonics as high frequency components, and should effectively pass the power source frequency components in delta grounding and the third harmonic components of an operating frequency and the power supply frequency in star grounding. When an inverter 10 is inserted at the load side, the waveform detected by the zero-phase current transformer 3 contains the high-frequency components; but the low pass filter 9 cuts off the high frequency components of the inverter, so that the waveform input to the level discriminator 6 has the commercial frequency of an AC cable way 1, and the ground fault detector operates without being affected by the high frequency components.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ground fault detection device for shutting off an AC electric line when a ground fault occurs in the AC electric line, which is an unnecessary operation due to a high frequency component from an inverter inserted on the load side of the AC electric line. , Related to prevention of non-operation.

[0002]

2. Description of the Related Art FIG. 6 is a block diagram showing a conventional ground fault detecting device disclosed in, for example, Japanese Patent Laid-Open No. 61-15422, and FIG. 7 is an operation waveform diagram thereof. In the figure, 1 is an AC circuit, 2
Is a circuit breaker installed in this AC circuit 1 and 3 is an AC circuit 1
Is a primary fault winding, for example, a zero-phase current transformer, 4 is an electromagnetic device provided to trip the circuit breaker 2, and 5 is a switching element connected in series with the electromagnetic device 4, such as a thyristor. is there. The circuit breaker 2, the electromagnetic device 4, and the thyristor 5 constitute a breaking means. 6 is a level discriminator connected to the zero-phase current transformer 3 for discriminating the level of its output, 7 is connected to this level discriminator 6, and a signal width discriminator for discriminating its output time width, 8 is a signal width It is a trigger circuit that is connected to the discriminator 7 and that generates a trigger signal in accordance with its output. The signal discriminator 7 and the trigger circuit 8 constitute a signal discriminating means. The output side of the trigger circuit 8 is connected to the gate electrode of the thyristor 5, and the trigger signal turns on the thyristor 5 to trip the circuit breaker 2 via the electromagnetic device 4 to cut off the AC circuit 1.

The operation of the conventional ground fault detecting apparatus as described above will be described with reference to the waveform chart of FIG. When a ground fault occurs in the AC power line 1, a ground fault current shown in FIG. This ground fault current is detected by the zero-phase current transformer 3, and the detection signal shown in FIG. 6 (b) is obtained at the output side thereof. This detection signal is input to the level discriminator 6, and when the level exceeds the threshold value TH1, as shown in FIG. 7 (c),
A pulse signal having a corresponding time width during the period when the detection signal level of the zero-phase current transformer 3 exceeds the threshold value TH1 is generated. This pulse signal is input to the signal width discriminator 7, and if the pulse signal width is a predetermined width t1 or more than 2 milliseconds, the signal width discriminator 7 generates an output signal as shown in FIG. 7 (d). ,
In response to this output signal, the trigger circuit 8 generates the trigger signal shown in FIG. 7 (e). The thyristor 5 is turned on by this trigger signal, the breaker 2 is pulled out via the electromagnetic device 4, and the AC electric circuit 1 is cut off.

By the way, in recent years, the use of the ground fault detection device in a circuit for performing load control by inserting an inverter on the load side has increased. The circuit configuration in this case is shown in FIG. 8, and its operation waveform charts are shown in FIGS. 9, 10, and 11. In the figure, 1 to 8 are the same as the above-mentioned conventional device. Reference numeral 10 is an inverter, which is used for speed control of the induction motor and often converts the frequency to supply power to the load. The electric power from the inverter 10 includes a high frequency around 1 kHz or around 10 kHz used for converting the frequency. Reference numeral 11 is a distribution transformer. Generally, in a 200 V distribution circuit, one phase of the delta connection is grounded. 12 is the ground fault resistance when a ground fault occurs around the load, and 13 is the stray capacitance between the load circuit wire and ground.

The operation of the circuit in which the inverter is inserted on the load side of the ground fault detecting device as described above when no ground fault occurs will be described with reference to FIG. A current with a steeple waveform as shown in Fig. 11 (a) flows to the floating capacitance, and the zero-phase current transformer 3
The output as shown in (b) is output to the level discriminator 6. The output corresponding to the portion exceeding the threshold TH1 of the level discriminator 6 is
The pulse train becomes narrow as shown in 11 (c). Since this pulse width is less than the predetermined time t1, it does not exceed the threshold TH2 of the signal width discriminator 7 as shown in FIG. 11 (d). That is, the signal width discriminator 7 blocks the signal due to the current flowing through the floating electrostatic capacitance to prevent unnecessary operation of the ground fault detection device.

Next, the operation of the circuit in which the inverter is inserted on the load side of the ground fault detection device during a ground fault will be described with reference to FIG.
A ground fault current as shown in FIG. 9 (a) flows through the ground fault resistor 12 due to the occurrence of the ground fault, and the zero-phase current transformer 3 outputs the output as shown in FIG. 9 (b) to the level discriminator 6. Threshold value TH of level discriminator 6
The output corresponding to the portion exceeding 1 becomes a discontinuous minute pulse due to the high frequency used for frequency conversion as shown in FIG. 9 (c). This minute pulse is input to the signal width discriminator 7, and although the duration of the discontinuous minute pulse is equal to or longer than the predetermined time t1, the accumulation of the minute pulse during this period is detected by the signal width discriminator 7. Since the threshold value TH2 is not exceeded, the trigger circuit 8 does not generate a trigger signal and the load is not cut off even if a ground fault occurs.

Further, the combined current of the current flowing through the floating electrostatic capacitance 13 and the ground fault current according to the length of the phase electric wire between the inverter 10 and the load is as shown in FIG. 10 (a). Since this combined current flows through the electrostatic capacity (capacitor), it has a steeple part in the high frequency part. Therefore, the output of the zero-phase current transformer 3 does not exceed the threshold value TH1 of the level discriminator 6 at the basic output, but the peak of the waveform exceeds the threshold value TH1. As a result, the output from the level discriminator 6 becomes a continuous minute pulse and is input to the signal width discriminator 7. Since the signal width discriminator 7 integrates continuous minute pulses, it may exceed the threshold TH2 of the signal width discriminator 7,
The trigger circuit 8 generates a trigger signal, and the load is cut off even if a ground fault exceeding a predetermined level has not occurred.

[0008]

In the conventional ground fault detection device as described above, when the inverter is inserted into the load side as described above and applied to the circuit for load control, the frequency conversion of the inverter is performed. Even if a ground fault occurs due to the influence of the used high frequency, the load is not cut off, or if the phase wire between the inverter and the load is long, a ground fault occurs due to the unbalanced current including the high frequency flowing in the stray capacitance. There was a problem that the load was cut off even if it was not done.

The present invention has been made to solve the above problems. Even if the present invention is applied to a circuit for performing load control by inserting an inverter on the load side, unnecessary operation due to high frequency components from the inverter, high frequency components It is an object of the present invention to provide a ground fault detection device capable of preventing non-operation due to the influence of.

[0010]

[Means for Solving the Problems] Claim 1 according to the present invention
The ground fault detection device, a zero-phase current transformer that detects the ground fault current of the AC circuit, a low-pass filter that is connected to the zero-phase current transformer and blocks high-frequency components contained in the signal of the ground fault current, A level discriminator that generates a pulse signal having a time width corresponding to a period in which the level of the ground fault current signal that has passed through this low-pass filter exceeds a predetermined value, and an output signal when the pulse signal has a predetermined width or more. Is provided with a signal width discriminating means and a shutoff means which is actuated by the output signal from the signal width discriminating means to shut off the AC electric circuit.

Further, the ground fault detecting device according to the second aspect is provided with a low-pass filter having a configuration including an operational amplifier capable of adjusting the gain in accordance with the output characteristic of the zero-phase current transformer.

A ground fault detecting device according to a third aspect of the present invention comprises a ground fault detecting device according to the second aspect, which includes a counter and a timer for outputting an output signal when the output signal from the signal width determining means is present twice or more within a predetermined time. It is a thing.

[0013]

In the ground fault detecting apparatus according to the present invention, the low-pass filter connected to the zero-phase current transformer blocks high-frequency components generated from the inverter inserted for load control, so that the level discriminator and its subsequent components can be controlled. Eliminate the influence of high frequency components.

In the ground fault detecting device according to the third aspect of the present invention, the counter and the timer prevent the transient output due to the time constant of the low pass filter.

[0015]

EXAMPLES Example 1. First Embodiment FIG. 1 is a block diagram showing a ground fault detecting device showing a first embodiment of the present invention, and FIGS. 2 to 3 are operation waveform diagrams thereof. In the figure, 1 to 8 are the same as those described in the conventional device. 9 is a low-pass filter connected between the zero-phase current transformer 3 and the level discriminator 6,
The high frequency component is cut and only the low frequency component is transmitted to the level discriminator 6.

Here, the main frequency component of the ground fault current is 200
In general delta grounding system in V circuit, operating frequency (0 to 120 Hz), power supply frequency (50 or 60 Hz),
The carrier frequency (800 to 20 KHz) and its harmonic components. In a general star ground system with a 400 V circuit, the operating frequency (0 to 120 Hz), the third component of the power supply frequency (150 or 180 Hz), the carrier frequency (800 to 20 kHz) and its harmonics It is an ingredient.

The low-pass filter 9 blocks the carrier frequency, which is a high-frequency component, and its harmonics, and effectively activates the power frequency component in the delta ground and the third-order component of the operating frequency and the power frequency in the star ground. Need to pass. Therefore, it is desirable that the cutoff frequency is 100 to 360 Hz, which is about twice the frequency to be passed, and the attenuation characteristic is 12 dB / 0 ct or more so that the carrier frequency of 800 Hz is attenuated.

In the ground fault detecting device constructed as above, when the inverter is inserted on the load side, the zero-phase current transformer 3
The waveform detected by contains a high frequency component as shown in Fig. 2 (a) and Fig. 3 (a), but since the low pass filter 9 cuts off the high frequency component of the inverter, the waveform input to the level discriminator 6 2 (b) and 3 (b) are commercial frequencies of the AC circuit 1, and the ground fault detection device operates only with a predetermined ground fault detection component without being affected by high frequency components.

Example 2. FIG. 4 is a diagram showing an embodiment of a low pass filter according to claim 2 of the present invention. In the figure, 1
3 to 3 are the same as those described in the first embodiment. 14
Is an adjustment resistor, 15 is an operational amplifier, and has a feedback path of a resistor 16 and a capacitor 17.

Since such a low-pass filter has a variable gain and an appropriate gain setting can be made according to the output characteristics of the zero-phase current transformer 3, the zero-phase current transformer 3 is used for ground fault detection. It is suitable for ground fault detectors. Also, by using the operational amplifier 15, compared to a low-pass filter that consists of a combination of resistors and capacitors. The circuit configuration can be miniaturized.

Example 3. Third Embodiment FIG. 5 is a block diagram showing a ground fault detection device showing a third embodiment of the present invention. In the figure,
2 to 10 are the same as those described in the first embodiment. The low-pass filter 9 is the operational amplifier 15 described in the second embodiment.
It is equipped with a low-pass filter using. Reference numeral 18 denotes a counter, which outputs a trigger signal to the trigger circuit 8 when the number of pulses input from the signal width discriminator 7 is two or more. Reference numeral 19 is a timer that enables the counter 18 for a predetermined time from the last pulse from the signal width discriminator 7. Counter 18
Is reset after a lapse of a predetermined time from the last pulse.

The low-pass filter 9 using the operational amplifier 15 has a time constant, and a transient output is generated by the time constant when the power supply is turned on. When the transient output waveform is equal to or larger than the predetermined time width t1 which is the discriminant value of the signal width discriminator 7, the ground fault detection device of the second embodiment described above malfunctions although the ground fault does not occur. Generally, the transient output waveform is large at the beginning, but decays sharply after that. Therefore, the output from the signal width discriminator 7 is a single-shot pulse. Further, the output of the signal width discriminator 7 by the ground fault detection becomes a continuous pulse. Therefore, by providing the counter 18 and the timer 19 as described above, the single-shot pulse (transient output due to the time constant at the time of power-up) regulated by the counter 18 does not operate within the predetermined time. The ground fault detection device has few unnecessary operations and operates when the pulse is a continuous pulse (output by ground fault detection) that is two or more times.

Incidentally, the counter 18 and the timer in the above
Even if a time constant circuit composed of a capacitor and a resistor is used instead of 19, the same effect can be obtained.

[0024]

As described above, according to the present invention, the load is not cut off even if a ground fault occurs due to the influence of the high frequency used for the frequency conversion of the inverter, and the phase between the inverter and the load is reduced. It is possible to obtain a ground fault detection device that does not perform unnecessary operation in which load is cut off even when a ground fault does not occur due to an unbalanced current including a high frequency wave that flows through a stray capacitance when the electric wire is long.

[Brief description of drawings]

FIG. 1 is a block diagram showing a ground fault detection apparatus showing a first embodiment of the present invention.

FIG. 2 is a non-operating waveform diagram of the ground fault detection apparatus according to the first embodiment of the present invention.

FIG. 3 is an unnecessary operation waveform diagram of the ground fault detection apparatus according to the first embodiment of the present invention.

FIG. 4 is a circuit diagram of a low pass filter showing a second embodiment of the present invention.

FIG. 5 is a block diagram showing a ground fault detection device showing a third embodiment of the present invention.

FIG. 6 is a block diagram showing a conventional ground fault detection device.

FIG. 7 is an operation waveform diagram of a conventional ground fault detection device.

FIG. 8 is a circuit configuration diagram when an inverter is inserted in a conventional ground fault detection device.

9 is an operation waveform diagram for explaining the non-operation of FIG.

FIG. 10 is an operation waveform diagram illustrating an unnecessary operation of FIG.

FIG. 11 is an operation waveform diagram illustrating an unnecessary operation of the floating capacitance.

[Explanation of symbols]

1 AC line 3 Zero-phase current transformer 6 level discriminator 7 Signal width discriminator 8 Trigger circuit 9 low-pass filter 10 inverter 12 Ground resistance 13 Stray capacitance 15 operational amplifier 18 counter 19 timer

Claims (3)

[Claims] 1. A zero-phase current transformer that is electromagnetically coupled to an alternating-current circuit and detects a ground fault current in the alternating-current circuit, and a high-frequency component included in the detected ground-fault current that is connected to the zero-phase current transformer. A low-pass filter that blocks the signal, a level discriminator that generates a pulse signal of a time width corresponding to a period in which the level of the ground fault current that has passed through the low-pass filter exceeds a predetermined value, Ground fault detection characterized by including a signal width determination means for generating an output signal when the pulse signal has a width equal to or greater than a predetermined width, and a shutoff means which operates in response to the output signal from the signal width determination means to shut off the AC electric circuit. apparatus. 2. The ground fault detection device according to claim 1, wherein the low-pass filter includes an operational amplifier. 3. The ground fault detecting device according to claim 2, further comprising a counter and a timer which output an output signal when the output signal from the signal width determining means is present twice or more within a predetermined time. .