JP2772426B2 - Phase compensation method for zero-phase current transformer - Google Patents

Description

Description: BACKGROUND OF THE INVENTION The present invention relates to a method for compensating for variations in phase characteristics when detecting a leakage current in an electric circuit or the like using a zero-phase current transformer.

(Prior Art) Conventionally, an insulation measuring device as shown in FIG. 5, for example, has been used to measure the insulation resistance of an electric circuit in a live state.

In the figure, T is a power receiving transformer, and 1 and 2 are the power receiving transformers T.
Is a ground line connected to the secondary circuit. A current transformer ZCT and an injection transformer OT are provided on the ground line 3, and an amplifier AMP, a filter FIL, and a synchronous detector MULT are respectively connected in series to the output of the current transformer ZCT, while a low-voltage measuring device is connected to the injection transformer. Frequency signal oscillator OSC is connected.

In the measuring system thus configured, the injection transformer is used.
By applying a measurement low-frequency signal to the electric circuit via the OT, the ground line 3 is generally passed through the electric circuits 1, 2 and the grounding capacitance Co and the insulation resistance Ro existing between the load device Z and the earth. After detecting a leakage current component due to the measurement low-frequency signal that is fed back to the ground line 3 with a zero-phase current transformer (hereinafter referred to as ZCT) coupled to the ground line 3, the component is synchronously detected using the measurement low-frequency signal. Is performed to calculate the insulation resistance of the electric circuit.

In this case, the leakage current detected by the ZCT
The signal is input to a synchronous detector 9 via a filter 8 for extracting only the low-frequency signal component for measurement, and a part of the output of the low-frequency signal oscillator OSC is used as a reference signal of the synchronous detector.

However, when a relatively large current flows as the temporary current of the ZCT, there is a disadvantage that the phase of the secondary winding current of the ZCT fluctuates due to this current, and the insulation resistance cannot be measured accurately.

That is, the leakage current that returns to the ground line contains both the current component of the commercial frequency and the current component due to the applied low-frequency signal for measurement. Generally, the voltage of the low-frequency signal for measurement should be set to a level sufficiently lower than the commercial frequency voltage of the electric circuit (for example, 100 V or 200 V) in consideration of the influence on the load equipment, and should be about 1-2 V (frequency is about 10-20 Hz). It is common to choose.

Therefore, the leakage current of the commercial frequency is
The phase of the low-frequency component in the output of the ZCT is about 500 to 1000 times, so the ZCT
Is observed to fluctuate greatly under the influence of the magnetic core. 4th
The figure shows that the resistance R = 1, which corresponds to the insulation resistance Ro, as shown in FIG.
3.33KΩ and the capacitor C equivalent to the ground capacitance Co = 8.5
This figure shows that the insulation resistance measurement results fluctuate greatly due to changes in the leakage current at the commercial frequency when the above-described measuring means is used in a circuit in which μFs are connected in parallel. It can be seen that the measured output increases non-linearly due to changes in the leakage current of the commercial frequency. This increase is
This does not occur unless the phase characteristics of the ZCT fluctuate. When detecting a component (effective component) in phase with the applied low-frequency voltage in low-frequency leakage current, such phase fluctuation results in insulation resistance There is a drawback that the measured value directly has an adverse effect as an error.

(Object of the Invention) The present invention has been made in view of the above-mentioned drawbacks of the conventional insulation resistance measuring device, and corrects the phase compensation of a current transformer that fluctuates due to a leakage current of a commercial frequency by a simple method. It is an object of the present invention to provide a phase compensation method for a zero-phase current transformer.

(Summary of the Invention) In order to achieve this object, the phase compensation method of the current transformer according to the present invention is arranged such that, as the primary current of the current transformer ZCT increases, the phase of the secondary output current of the output of the ZCT becomes non-linear. , The magnitude of the leakage current of the commercial frequency contained in the output of the ZCT is detected, multiplied by a factor, and subtracted from the output of the conventional insulation resistance measurement device to approximately reduce the measurement error. We will take steps to reduce it.

(Examples) Hereinafter, the present invention will be described in detail based on examples shown in the drawings.

FIG. 1 is a view showing an embodiment of the present invention, in which T is a power receiving transformer, and 11 and 12 are 2 of the power receiving transformer T.
A secondary line 13 is a ground line provided on the line 12, ZCT is a zero-phase current transformer coupled to the ground line 13, and OT is an injection transformer connected to the ground line 13.

The output of the zero-phase current transformer ZCT is applied to one input of a synchronous detector MULT via an amplifier AMP and a filter FIL.
The output of the oscillator OSC is applied to the other input of the MULT.

On the other hand, the output of the amplifier AMP is added to a filter FIL2 for detecting a commercial frequency component, rectified by a rectifier DET, and multiplied by a coefficient by a coefficient circuit COF. The output of the coefficient circuit COF and the synchronous detection circuit MULT
The difference with the output of is calculated by the subtraction circuit.

Thus, the output OUT is approximately compensated for by the phase lead of the zero-phase current transformer due to the increase in the leakage current of the commercial frequency, and is output.

This is to determine the advance of the phase in the current transformer by knowing the magnitude of the leakage current of the commercial frequency from the relationship shown in FIG. 4 and to determine the output of the coefficient circuit COF, that is, the leakage of the commercial frequency. An output that is proportional to the magnitude of the current and that is inversely proportional to the insulation resistance value is obtained. With this output, the output of the synchronous detector MULT is corrected, and an accurate insulation resistance value can be obtained.

In the embodiment of the present invention, the case of the single-phase two-wire system has been described, but the present invention can also be applied to a single-phase three-wire or three-phase three-wire circuit. In addition, a low-frequency voltage for measurement is applied to the ground wire or the electrical circuit via the injection transformer OT to search for a defective insulation resistance, and the zero-phase current transformer ZCT is connected to the terminal at the far end from the low-frequency voltage injection point for measurement. When measuring the insulation resistance of the circuit section, as shown in FIG. 2, it is difficult to obtain a reference signal for synchronous detection directly from a part of the output of the oscillator OSC. In FIG. 1, the low-frequency signal voltage for measurement existing between 12) and the ground may be detected by the high input impedance amplifier AM1 and used as a reference signal.

Although the output of the zero-phase current transformer ZCT is used to detect the leakage current of the commercial frequency in the embodiment of FIG. 1, a dedicated zero-phase current transformer is separately provided or a ground-fault relay is used. The output of a zero-phase current transformer (not shown) may be used. Further, it is apparent that the same effect can be obtained even if the current transformer and the injection transformer are configured to penetrate the electric lines 11 and 12 together.

(Effects of the Invention) Since the present invention is configured and functions as described above, the phase error of the zero-phase current transformer can be compensated by a simple method, and the measurement error is smaller than the conventional insulation resistance measuring device. It is extremely effective in doing

[Brief description of the drawings]

FIG. 1 is a diagram showing an embodiment of the present invention, FIG. 2 is a diagram showing a modified embodiment of the present invention, FIG. 3 is a diagram showing a circuit in which a resistor and a capacitor are connected in parallel, and FIG. FIG. 5 is a diagram showing a relationship between a commercial frequency current and a measurement output when a conventional insulation resistance measuring method is used in the circuit shown in FIG. 5, and FIG. 5 is a block diagram showing a conventional insulation resistance measuring method. T: Power receiving transformer, ZCT: Zero-phase current transformer, OT: Injection transformer, AMP, AM1 ... Amplifier, FIL, FIL2 ... Filter, MULT
…… Synchronous detector, OSC …… Oscillator, DET …… Rectifier, SUB ……
Subtractor.

Continuation of the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G01R 27/18

Claims (2)

(57) [Claims] A low-frequency voltage different from a commercial frequency is applied to an electric circuit, and a leakage current due to the low-frequency voltage contained in a ground wire of the electric circuit or in an output of a zero-phase current transformer coupled to the electric circuit is reduced. The difference between the output value obtained by synchronous detection using the low-frequency voltage as a reference signal and the value obtained by multiplying the rectified value obtained by rectifying the leakage current of the commercial frequency contained in the output of the zero-phase current transformer by a factor. A phase compensation method for the zero-phase current transformer in insulation resistance measurement or the like, wherein the phase variation due to the magnitude of the measured current of the zero-phase current transformer is compensated to measure the insulation resistance of the electric circuit. . 2. A zero-phase current transformer in an insulation resistance measurement or the like according to claim 1, wherein a low-frequency voltage existing between an electric circuit and ground is detected and used as said reference signal. Phase compensation method of the detector.