JPH0743403B2 - Insulation resistance measurement method - Google Patents

Description

The present invention relates to a method for measuring the insulation resistance of an electric circuit in a hot line state, and in particular, a transient response and filtering of a current transformer that cannot be ignored when measuring a large insulation resistance value. The present invention relates to an insulation resistance measuring method that compensates for the influence of noise frequency components in the vicinity of the measurement low-frequency signal frequency that cannot be removed by a measuring instrument.

[Prior art]

 A conventional insulation resistance measuring method is shown in FIG.

The signal generated by the low-frequency signal generator for measurement is injected into the ground wire through the injection transformer, and this signal voltage causes the insulation resistance R _g and the capacitance C _g to ground to flow back to the ground wire. A method of measuring the insulation resistance of the circuit by detecting only the leakage current component of the low-frequency signal for measurement with a current transformer and then detecting only the insulation resistance component with a synchronous rectification unit is used. . Also, the synchronizing signal for performing synchronous rectification must be in phase with the measuring low-frequency signal voltage injected into the ground line through the injection transformer, and the leakage current of the measuring low-frequency signal is Since the measurement low-frequency signal voltage injected by the filter is out of phase with the measurement low-frequency signal voltage, it is common to provide a phase correction unit that corrects the phase difference.

[Problems to be solved by the invention]

The measurable insulation resistance value when the conventional insulation resistance measuring method is used is determined by the noise equivalent voltage included in the pass band width and the input equivalent noise voltage of the filter following the current transformer.

First, the input converted noise voltage will be described with reference to FIG. 2 (b) showing an equivalent circuit of the electric circuit including the insulation resistance R _g , the injection transformer and the current transformer.

The voltage injected by the injection transformer into the circuit is generally 0.5V.
Therefore, the leakage current I ₁ when the insulation resistance R _g is 100 kΩ is 5 μA (0.5 V / 100 kΩ), and the number of turns of the secondary winding of the current transformer is generally about 2000 turns. The next current I ₂ is about 2.5 nA (5 μA / 2000 turns). Thus the voltage across the current transformer termination resistor R _L about as R _L = 100Ω 250nV (= 10
It becomes 0Ω × 2.5nA). Although it is possible to increase the R _L voltage across the larger the terminating resistor R _L, typically for primary current / secondary current phase characteristic of the current transformer for temperature deteriorates R _L
Is used below 100Ω.

Moreover, even if the input conversion noise of the commercially available ultra-low noise operational amplifier is good, it is 3 to 5 nV.
There will be 50 noise components. When performing high accuracy, high stability, and high insulation resistance detection, the noise component is 1 /
It needs to be 1000 or less, and the conventional method cannot detect the insulation resistance R _g > 100 kΩ with high accuracy and high stability.
Next, the noise voltage included in the pass band width of the filter will be described.

Generally, a band pass filter is used as the filter in order to pass the low frequency signal for measurement, and the pass band has a constant frequency width centered on the low frequency signal frequency for measurement. Since a noise component due to the input-converted noise voltage is present in the pass band of this filter, the pass band width of the filter may be narrowed in order to reduce the noise component of the output of the filter.

However, if the pass band width is narrowed, the cutoff frequency of the filter becomes close to the measurement low-frequency signal frequency, so that the input voltage / output voltage phase characteristic of the filter with respect to the temperature of the measurement low-frequency signal frequency deteriorates.

For this reason, the passband width cannot be narrowed unnecessarily, and a noise component due to the input-referred noise voltage is generated in the output of the filter, and when measuring high insulation resistance, the low-frequency signal for measurement and the noise component are separated. There was a drawback that made it difficult.

[Means for solving problems]

Means for solving the problem will be described with reference to FIG.

The leakage current component of the electric circuit detected by the current transformer is input to the analog / digital converter (hereinafter referred to as A / D converter) via the filter.

The A / D conversion unit is controlled by the control unit, and quantizes the analog input using the low frequency signal for measurement as a trigger signal, and then stores the quantized waveform data in the waveform storage unit.

This relationship is shown in FIG.

The control unit generates an A / D conversion control signal at time T ₁ intervals at time T _{2 using} the zero voltage point A of the measurement low frequency signal as a trigger point, and applies it to the A / D conversion unit. The A / D conversion unit converts the analog input signal into a digital value each time an A / D conversion control signal is applied and sends the digital value to the waveform storage unit. The waveform storage unit stores the digital value of the A / D conversion unit output in the address determined by the storage address setting signal preset for each A / D conversion control signal generated by the control unit. That is, during time T ₂ , (T ₂ / T ₁ +
1) The analog / digital conversion is performed once, and the waveform digital data is stored in the prepared storage location for this number of times.
In addition, the number of times is related to the high-speed Fourier transform performed by the calculation unit later.
It is preferably 2 ⁿ (n is a natural number) times, but it is not particularly limited. Further, the trigger point is set to the zero voltage point A for the sake of explanation, but in order to correct the phase delay of the analog signal due to the current transformer and the filter, the trigger point is moved by the phase delay and set as a point other than the zero voltage point. I don't care. The waveform digital data stored in the waveform storage unit is subjected to high-speed Fourier transform by the calculation unit. The fast Fourier transform is a general method for converting time domain data in which a series of data is real numbers into frequency domain data. That is, waveform digital
By performing the fast Fourier transform on the data, the signal component of the specific frequency can be divided into the real number part and the imaginary number part to obtain the calculation result.

Furthermore, the low frequency signal frequency is selected as this specific frequency, and in the calculation result, the real part is the same phase component as the measurement low frequency signal used as the trigger signal, and the imaginary part is the same low frequency signal and π / 2 phase. Insulation resistance from
It is possible to distinguish between the leakage current component due to R _g and the leakage current component due to the ground capacitance C _g . That is, when the trigger voltage point is set to zero voltage, the real part after the fast Fourier transform becomes the leakage current component due to the insulation resistance R _g , and the imaginary part becomes the leakage current component due to the ground capacitance C _g .

However, only when there is no phase delay due to the current transformer and filter.
If there is the same phase delay, a circuit for correcting the phase delay may be provided after the filter or the trigger voltage point may be changed. Generally, the frequency resolution when high-speed Fourier transform is performed is A / D conversion control signal T ₂ time and A / D conversion number N (= 1 +
It is given by (2 / (T ₂ × N)) depending on T ₂ / T ₁ ). The Fast Fourier Transform is a technique for detecting a specific frequency component, but if it is realized differently, it is a band pass filter that passes only the specific frequency component, and the pass bandwidth is the frequency resolution, that is,
It is determined by the A / D conversion control signal T ₂ time and the A / D conversion number N.

As described above, the filtering technique using the high-speed Fourier transform is a digital filter, and a filter with a narrow pass band can be easily realized with high accuracy and high stability without depending on environmental changes such as temperature and temperature. Furthermore, the phase characteristics with respect to temperature changes can be ignored, and the analog filter in the preceding stage of the A / D conversion unit can be made simpler to provide a device with high stability due to environmental changes.

〔Example〕

In Fig. 1 (a), the insulation resistance R _g and the capacitance to ground C _{g are} measured by the low-frequency signal for measurement applied to the ground wire through the transformer for injection.
A leakage current passing through is generated. This leakage current is detected by the current transformer and input to the A / D converter through the filter.

Also, the control unit triggers the low frequency signal as described above.
Sends an A / D conversion control signal to the A / D converter. The waveform storage section
The A / D converted digital data is stored in a predetermined address. This address is determined by the control unit. When a series of A / D conversion is completed and digital data storage in the waveform storage section is completed, the control section sends a calculation execution signal to the calculation section, and the calculation section outputs the digital data in the waveform storage section. Fast Fourier transform. In the result of the faster Fourier transform,
Only the real part is extracted and output from the same frequency components as the low-frequency signal frequency for measurement. This output is the insulation resistance of the circuit R _g
Inversely proportional to the value of. Further, it is easy to carry out a numerical process such as calculating an average value of the number of outputs for higher precision.

[Advantages of the Invention] Compared with the conventional method using synchronous rectification, the present invention, which performs digital calculation after A / D conversion, significantly reduces the number of adjustment points and improves the assembly workability.

By using the method of high-speed Fourier transform, it becomes possible to separate and remove the noise component in the vicinity of the measurement signal frequency that could not be removed by the filter.

By using high-speed Fourier transform and digital processing, highly stable and highly accurate measurement of high insulation resistance is possible without being affected by changes in phase characteristics due to environmental changes.

[Brief description of drawings]

1 (a) is a diagram showing a concept and an embodiment of the present invention, FIG. 1 (b) is a diagram showing a timing for explaining the concept, and FIG. 2 (a) is an insulation resistance measuring device by a conventional method. FIG. 2B is an equivalent circuit diagram for explaining the problems of the conventional measurement method.

Claims (1)

[Claims] 1. A low-frequency signal voltage for measurement different from a commercial frequency is applied to a circuit through an injection transformer to a ground wire of the transformer, and a leakage current component of the low-frequency signal for measurement is detected.
After removing the unnecessary current component detected at the same time with a filter, the output of the filter is converted into a digital value at a fixed period and at a fixed time interval with a synchronization signal obtained by waveform shaping from the low frequency signal for measurement as a trigger. An insulation resistance measuring method characterized in that an insulation resistance component of a measurement signal frequency component is separately detected by storing in a waveform data storage unit and performing a high-speed Fourier transform in a calculation unit using the waveform data.