JPH1078461A - Measuring method for insulation resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a measuring method which surely removes the influence of an always changing capacitance to groud when the insulation resistance of an electric circuit is measured in a live-wire state. SOLUTION: The leakage component of a low-frequency signal f1, for measurement, which is detected by a residual current transformer ZCT is input also to a synchronous detector MULT2, a signal in which the pase of the low-frequency signal f1, for measurement, from an oscillator OSC is advanced by 90 deg. is used as a reference signal in the synchronous detector MULT2, and a reactive component, i.e., a capacitance to ground, in the leakage component is obtained. The capacitance to groud is converted into a DC signal by using a rectifier RC. Then, when an output from the synchronous detector MUTL2 is on the positive side, a voltage level is adjusted so as to be lowered from a present level by using an automatic voltage controller AV. When the output is on the negative side, the level is adjusted so as to be raised from the present level. The DC signal is detected synchronously with the reference signal by using a synchronous detector MULT3. When the DC signal is applied to the current transformer ZCT so as to be in an opposite phase by using a capacitor load C4 which is installed at a connecting line 4, the residual current transformer ZCT can obtain a signal in which the capacitance to ground is always canceled.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for measuring an insulation resistance of an electric circuit or the like in a live state, and more particularly to a method capable of always measuring an insulation resistance component by canceling a ground capacitance.

Conventionally, in order to measure the insulation resistance of an electric circuit in a live state, an insulation resistance measurement method using a circuit as shown in FIG. 2, for example, has been used. In the figure, a primary side electric circuit 1 and a secondary side electric circuit 2 are connected via a power receiving transformer T, and a ground wire 3 extends from the secondary side electric circuit 2. The ground line 3 is provided with a zero-phase current transformer ZCT and an injection transformer OT, and an amplifier AMP, a low-pass filter LPF, and a synchronous detector MULT1 are connected in series to the output terminal of the ZCT. The low frequency signal oscillator OSC is connected.

In the circuit configured as described above, a low frequency signal for measurement having a frequency f1 different from the commercial frequency is applied to the electric circuit via the injection transformer OT, so that the circuit generally exists between the electric circuit 2 and the ground. After detecting a leakage current component due to the measurement low-frequency signal f1 returning to the ground line 3 via the ground capacitance C0 and the insulation resistance R0 by the zero-phase current transformer ZCT coupled to the ground line 3, the component is detected. The insulation resistance of the electric circuit is calculated by performing synchronous detection using the low frequency signal for measurement. In this case, the leakage component detected by the zero-phase current transformer ZCT is input to the synchronous detector MULT through the amplifier AMP and the low-pass filter LPF that extracts only the measurement low-frequency signal component, and the reference signal of the synchronous detector MULT is output. Uses a part of the measurement low-frequency signal OSC.

Further, the circuit described above causes the zero-phase current transformer ZCT and the core of the injection transformer OT for detecting the leakage current of the low-frequency signal component for measurement returning to the ground line 3 to be in opposite phases to each other. A new loop connection line 5 penetrating therethrough, a variable capacitor CV is inserted into the loop connection line 5 and the variable capacitor CV is adjusted to make the invalid current during the leakage current of the low frequency component included in the output of the zero-phase current transformer. Cancel the minute. Therefore, the insulation resistance value can be obtained while reducing the influence of the ground capacitance C0.

[0005]

However, since the ground capacitance C0 constantly changes, the canceling effect may not be obtained if the value of the variable capacitor CV is kept constant. It is also difficult to adjust frequently according to the capacity C0, and especially C0 and C
When the difference from v is large, there is a problem that the canceling effect is hardly obtained. The present invention has been made in view of the above, and it is an object of the present invention to provide an insulation resistance measuring method capable of reliably removing the constantly changing influence of the ground capacitance C0.

[0006]

In order to achieve the above object, a method for measuring insulation resistance according to the present invention comprises applying a low-frequency measurement signal having a frequency different from a commercial frequency to an electric circuit and returning the signal to a ground line of the electric circuit. Canceling the ground capacitance component included in the leakage current by applying an ineffective part of the leakage current of the low frequency measurement signal to the zero-phase current transformer coupled to the ground line in a reverse phase. It is characterized by.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail based on embodiments shown in the drawings. FIG. 1 is a diagram for explaining one embodiment of the insulation resistance measuring method according to the present invention, in which a primary side electric circuit 1 and a secondary side electric circuit 2 are connected via a power receiving transformer T. A ground wire 3 extends from the secondary side electric circuit 2. The ground line 3 is provided with a zero-phase current transformer ZCT and an injection transformer OT, and an output terminal of the ZCT has an amplifier AM.
P1, low-pass filter LPF, synchronous detector MULT1
Are connected in series, and a low-frequency signal oscillator for measurement OSC is connected to the injection transformer OT. The above configuration is the same as the conventional one.

In this embodiment, a synchronous detector MULT2, an automatic voltage controller AV, a synchronous detector MULT3 and an amplifier AMP2 are connected to the output terminal of the low-pass filter LPF.
Are connected in series, and the output terminal of the amplifier AMP2 is connected to a new connection line 4 passing through a zero-phase current transformer ZCT for detecting a leakage current of a low-frequency signal component for measurement returning to the ground line 3. I have. The synchronous detectors MULT1 and MULT1
3, the output signal of the oscillator OSC is applied as a reference signal, and the synchronous detector MULT2 is applied to the oscillator OSC.
A signal obtained by advancing the phase of the output signal of C by 90 ° is obtained by the phase shifter PS, and is applied as a reference signal.

In the above-described configuration, a low-frequency signal for measurement having a frequency f1 different from the commercial frequency is applied to the electric circuit via the injection transformer OT to thereby provide a ground capacitance C0 between the electric circuit 2 and the earth and a ground capacitance C0. After detecting a leakage current component of the low-frequency signal f1 for measurement returning to the ground line 3 via the insulation resistance R0 by the zero-phase current transformer ZCT coupled to the ground line 3, the component is detected. By performing synchronous detection using the signal, the insulation resistance of the electric circuit is calculated. In this case, the leakage component detected by the zero-phase current transformer ZCT passes through the amplifier AMP1 and the low-pass filter LPF that extracts only the low-frequency signal component f1 for measurement, and passes through the synchronous detector MUL.
A part of the measuring low-frequency signal oscillator OSC is used as the reference signal of the synchronous detector MULT1, which is input to T1.

Further, the leakage current component detected by the zero-phase current transformer ZCT is also input to the synchronous detector MULT2 through the amplifier AMP1 and the low-pass filter LPF.
The oscillator OULT is used as a reference signal for the synchronous detector MULT2.
Since a signal obtained by advancing the phase of the measurement low-frequency signal f1 from the SC by 90 ° is used, an ineffective portion of the leakage component, that is, a capacitance component to the ground can be obtained. After this ground capacitance component is converted into a DC signal by the rectifier RC, the automatic voltage controller AV lowers the voltage level from the current level when the output from the synchronous detector MULT2 is positive, and when the output from the synchronous detector MULT2 is negative, Is adjusted to raise the level from the current level. Then, it is synchronously detected with the reference signal by the synchronous detector MULT3, and the amplifier A
Capacitor load C provided on connection line 4 via MP2
By being added to the current transformer ZCT in the opposite phase at 4, the zero-phase current transformer ZCT can always obtain a signal in which the ground capacitance component is canceled.

[0011]

According to the present invention, the above-described method cancels the change in accordance with the change in the electrostatic capacitance to the ground, so that the correct insulation resistance value can always be measured.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating an embodiment of an insulation resistance measuring method according to the present invention.

FIG. 2 is a view for explaining a conventional insulation resistance measuring method.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Primary side electric circuit 2 ... Secondary side electric circuit 3 ... Grounding line 4 ... Connection line 5 ... Loop connection line CV ... Variable capacitor C4 ... Capacitor load ZCT ...・ Zero-phase current transformer OT ・ ・ ・ Injection transformer AMP ・ ・ ・ Amplifier LPF ・ ・ ・ Low pass filter MULT ・ ・ ・ Synchronous detector AV ・ ・ ・ Automatic voltage controller RC ・ ・ ・ Rectifier

Claims (1)

[Claims] A low-frequency measurement signal different from a commercial frequency is applied to an electric circuit, and an ineffective part of the low-frequency measurement signal that is fed back to a ground line of the electric circuit is supplied to a zero-phase current transformer coupled to the ground line. A capacitance component to ground included in the leakage current is canceled by applying the electric current to a reverse phase.