JPH0243410B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for measuring insulation resistance, etc. of an electric circuit.

(Prior art) Conventionally, in order to detect troubles in electrical circuits such as power transmission lines at an early stage, it has been possible to constantly monitor the insulation resistance of electrical circuits and detect abnormalities as soon as possible to prevent damage such as burnout due to insulation resistance deterioration. It is being done.

Various methods have been devised for measuring the insulation resistance of electrical circuits, one example of which is shown in FIG.

In the figure, T is a power receiving transformer, and its low-voltage secondary circuits 2 and 3 are connected to a load Z, and one of these circuits, for example, circuit 3, is generally grounded by a grounding wire 4. The ungrounded electric path 2 is coupled to the ground through an insulation resistance R and a ground capacitance C. It should be noted that insulation resistance and stray capacitance to the ground also exist in the grounding line, but their explanation will be omitted below.

In order to measure the insulation resistance R of such an electric line, for example, a low frequency signal generator for measurement OSC ₁ and a zero-phase current transformer ZCT are inserted and connected to the grounding wire 4 as shown in the figure. A low frequency signal f ₁ for measurement is applied to the low frequency signal f 1 for measurement, and the low frequency signal for measurement is returned to the ground wire via the insulation resistance R and the ground capacitance C.
The leakage current component of f ₁ is detected by the zero-phase current transformer ZCT, and the synchronous detection circuit MULT ₁ derives the component in phase with the applied low frequency voltage, that is, the effective component, which is inversely proportional to the insulation resistance R. It is used to obtain signals.

However, as mentioned above, in the conventional measurement method, when the ground capacitance is large, the current due to the ground capacitance in the leakage current of frequency f ₁ is larger than the current due to the insulation resistance, so the error of the synchronous detector There was a problem that the active ingredient could not be detected correctly.

In addition, as a method for measuring insulation resistance without being affected by ground capacitance, as disclosed in JP-A-56-63270 and JP-A-57-45469, two types of frequencies different from the commercial frequency are applied to the electrical circuit. There is a method of measuring insulation resistance by applying a voltage and detecting the two types of frequency signals that are returned to the electric circuit, weighting the detected voltage, converting it into a predetermined signal, and then multiplying the converted signal. .

However, the above two methods require squaring or multiplication, and the multiplier used to perform multiplication generally performs logarithmic transformation of each input signal using the log characteristics (nonlinearity) of semiconductor elements. At the same time, the converted values are added, and the result of the addition is exponentially transformed (inverse logarithm transformation) to obtain the multiplication result. Even if a large amount of variation occurs, the result will have an extremely large error,
There is a problem that it is difficult to perform accurate insulation resistance measurements, and multipliers with small multiplication errors are expensive, so using such a high-precision multiplier makes the measurement device itself expensive. It was hot.

(Objective of the Invention) The present invention was made to solve the problems of the conventional method of measuring insulation resistance of electrical circuits, etc., and is simple and inexpensive in structure, and also reduces ground capacitance. It is an object of the present invention to provide an insulation resistance measurement method that can measure insulation resistance with high accuracy without using multiplication processing that causes influence and measurement errors.

(Summary of the Invention) In order to achieve this object, the present invention provides a measurement rectangular wave signal containing a large number of harmonic components or a measurement low frequency signal of two frequencies different in frequency but synchronized in phase to an electric line. detect the plurality of high frequency components in the measurement rectangular wave signal or the two-frequency measurement low frequency signal flowing back to the ground line via the insulation resistance or capacitance of the electric path, and A signal obtained by frequency converting the reflux component of one of f ₁ and f ₂ (f ₁ < f ₂ ), for example, the reflux component of f ₂ so that it becomes equal to the other signal f ₁ , and the reflux component of f ₁ Measures are taken to obtain an output inversely proportional to the insulation resistance R or an output proportional to the ground capacitance C by detecting the combined voltage of both.

Note that any method may be used to frequency convert either the detected measurement signal f ₁ or f ₂ to the other frequency; for example, a means for frequency shifting using an amplitude modulator may be used. The purpose can be achieved with a simple configuration.

(Example) Hereinafter, the present invention will be described in detail based on an example shown in the drawings.

FIG. 1 is a block diagram showing one embodiment of the present invention.

In the figure, T is a power receiving transformer, 2 and 3 are secondary circuits, 4 is a grounding wire, ZCT and 5 are a zero-phase current transformer inserted into the grounding wire, and a transformer coupled to the oscillating OSC ₁ . This is the same as in FIG. 2 above.

In this embodiment, a rectangular wave oscillator containing a high frequency component is used as the measurement signal oscillator, and the circuit for detecting the leakage current flowing back into the ground wire 4 is constructed as follows.

In other words, the zero-phase current transformer ZCT output that passes through the grounding wire 4 is transmitted through the amplifier AMP to two different frequencies.
branched into two band bus filters BPF ₂ and BPF ₃ , and one of them, for example, modulates the output of BPF ₂ .
After the output is applied to the low-pass filter LPF for frequency conversion, the output is input to the subtraction circuit SUB ₂ via the phase shifter PS.

On the other hand, the output obtained from a part of the circuit of the square wave oscillator OSC ₂ applied to the electric circuit is input to the modulation signal input terminal of the modulator MOD, and the subtraction circuit
The other input terminal of SUB ₂ is connected to the bandpass filter.
The output of the BPF ₂ is input, and the output of the subtraction circuit SUB ₂ is input to the contact circuit DET, so that a signal inversely proportional to the insulation resistance R of the electric circuit is obtained at its output terminal.

The operation of the measuring circuit configured as described above will be explained below using mathematical expressions.

That is, if the voltage applied to the oscillator is V (volt), its duty ratio is 50%, the commercial power supply voltage of the secondary circuit is V ₀ (volt), and its frequency is f ₀ , then
The leakage current i flowing back to the grounding wire 4 is: i=[V ₀ /R] sinω ₀ t + ω ₀ CV ₀ cosω ₀ t + [V ₁ /R] ^sin ₊ ω ₁ t [V ₃ /R] cos3ω ₁ t+... It is expressed as +Cω ₁ V ₁ cosω ₁ t + 3Cω ₁ V ₁ cos3ω ₁ t +…… ……. However, V ₁ , V _{3 .} . . are the voltages of each harmonic component of the rectangular wave signal flowing through the electric circuit.

The output corresponding to this current i is the zero-phase current transformer ZCT
is obtained at the output end of the amplifier AMP through
The passing frequency of each filter BPF ₂ and BPF ₃ in the subsequent stage is
If f ₁ and 3f ₁ , the respective filter outputs x ₁ and x ₃
x ₁ =V ₁ /Rsinω ₁ t+Cω ₁ V ₁ cosω ₁ t ... x ₃ =V ₂ /Rsin3ω ₁ t + 3Cω ₁ V ₁ cos3ω ₁ t ...

Apply the output x ₃ of the filter BPF ₃ to one input of the amplitude modulator MOD, and apply the frequency to the other input of the MOD.
If a voltage of frequency 4f ₁ obtained from the output of the frequency divider before the oscillator that outputs f ₁ is applied and modulated, a frequency-shifted signal y is obtained at the output of the low-pass filter LPF provided at the output of the modulator.

y=V ₃ /Rsin (3-4) ω ₁ t + 3C ω ₁ V ₃ cos (3-4) ω ₁ t = -V ₃ /Rsin ω ₁ t + 3Cω ₁ V ₃ cos _{ω 1} t... phaser
PS is filter BPF ₃ as well as low pass filter
This is provided to compensate for the phase shift in the LPF and obtain the signal y expressed by the equation.

By the way, voltage V, frequency f ₁ , duty ratio 50
% rectangular wave is expanded into a Fourier series, it becomes 4V/π (sinω ₁ t + 1/3 sin3ω ₁ t + 1/5 sin5ω ₁ t +...). Therefore, since V ₁ = 4V/π and V ₃ = 4V/3π, 3V ₃ = V ₁ , so the second term of the equation is equal to the second term of the equation, so subtract the difference between the equations. calculator
If taken by SUB, the output of SUB will be x ₁ -y=V ₁ +V ₃ /Rsinω ₁ t... Therefore, the output of the subtractor can be converted into a rectifier
If the DET is used for rectification, the output OUT will be V ₁ +V ₃ /R, that is, an output that is inversely proportional to the insulation resistance.

Also, in the calculation of the difference between the expressions x ₁ −y,
When the components of the second term of the equation and the second term of the equation are significantly large and are not equal to each other due to the precision of the circuit, a cancellation residual occurs, and x ₁ - y provides a voltage that is correctly inversely proportional to the insulation resistance. However, in this case, by synchronously detecting x ₁ -y with the applied low-frequency voltage, it is possible to eliminate the influence of the above-mentioned residual component whose phase differs by 90 degrees from the applied voltage.

Also, since the sum of x ₁ and y is 3V ₃ = V ₁ , x ₁ + y = 2Cω ₁ V ₁ cosω ₁ t ...(6), which includes the capacitance C. Since this is the equation, the capacitance of the electric circuit can be calculated by obtaining the voltage proportional to the ground capacitance.

In the above embodiment, components of frequencies f ₁ and 3f ₁ were used as measurement low frequency signals, but
The present invention is not necessarily limited to this in any way, and it is clear that similar results can be obtained even if, for example, two-frequency signals of frequencies fo _and f _n are used as the applied low-frequency voltage.

Also, there is no need to limit the applied voltage to a rectangular wave;
For example, two measurement signal oscillators may be provided to perform similar measurements.

That is, if measurement signals with frequencies f ₁ and f ₂ are applied to the electric circuit, and the applied voltage components with angular frequencies ω ₁ and ω ₂ are a ₁ and a ₂ , respectively, the formula is i=V ₀ /Rsinω ₀ t+ω ₀ CV ₀ cosω ₀ t +a ₁ /Rsinω ₁ t+ω ₁ Ca ₁ cosω ₁ t +a ₂ /Rsinω ₂ t+ω ₂ Ca ₂ cosω ₂ t... It can be transformed as follows, and the filter BPF ₂ ,
If the passing frequencies of BPF ₃ are ω ₁ /2π, ω ₂ /2π, then the filter BPF ₂ ,
The outputs x ₁ and x ₂ of the BPF ₃ can be expressed as x ₁ =a ₁ /Rsinω ₁ t+ω ₁ Ca ₁ cosω ₁ t ... x ₂ =a ₂ /Rsinω ₂ t+ω ₂ Ca ₂ cosω ₂ t ....

Furthermore, in order to shift the output of the filter BPF ₃ to the frequency ω ₁ /2π, the angular frequency ω _p (however, ω _p
−ω ₂ =ω ₁ ), the low-pass filter
The output y of the LPF is y=a ₂ /Rsin(ω ₂ −ω _p )t +ω ₂ Ca ₂ cos(ω ₂ −ω _p )t =−a ₂ /Rsinω ₁ t+ω ₂ Ca ₂ cosω ₁ t...' Therefore, when the low-pass filter output y is multiplied by ω ₁ a ₁ /ω ₂ a ₂ and the difference from x ₁ is found using the subtractor SUB, x ₁ −a ₁ ω ₁ /a ₂ ω ₂・y=a ₁ / R[1+ω ₁ /ω ₂ ] sin
ω ₁ t... Since a ₁ , ω ₁ , and ω ₂ are known, it is possible to measure a value inversely proportional to the insulation resistance R by rectifying the subtracter output with a rectifier DET.

Furthermore, in this example, as the angular frequency ω _p of the signal used for frequency modulation, a signal with the relationship ω _p −3ω ₁ = ω ₁ ω _p −ω ₂ = ω ₁ was used, so when calculating the insulation resistance, the signal x It was found by subtracting ₁ and y, but frequency modulation is performed using a signal with the following relationship: ω _p = 3ω ₁ = ω ₁ ω _p = ω ₂ − ω ₁ (when ω ₂ > ω ₁ ) If this is done, the signs of the expressions and '' expressions will be inverted, so it is clear that the configuration should be such that x ₁ and y are added.

(Effects of the Invention) Since the present invention is configured and functions as described above, it exhibits a remarkable effect in measuring the insulation resistance and capacitance of an electrical circuit in a live state using an extremely simple method.

In addition, in the above-mentioned embodiment, only the case of a two-phase two-wire electric circuit was explained for the sake of simplicity, but it should be noted that the present invention is not limited to this, and can also be applied to, for example, a three-phase wire electric circuit. It probably won't be necessary.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing one embodiment of the present invention;
FIG. 2 is a block diagram showing a conventional method for measuring insulation resistance of an electric circuit. T...Power receiving transformer, 2 and 3...Electric circuit, 4...
...Ground wire, AMP...Amplifier, ZCT...Zero phase current transformer, BPF ₁ , BPF ₂ , BPF ₃ ...Band pass filter, MOD...Modulator, LPF...Low pass filter, PS...Phase shift container, SUB...subtractor, DET...
...detection circuit, OSC ₁ and OSC ₂ ...oscillator.

Claims (1)

[Claims] 1. A low frequency signal voltage for measurement including at least two frequencies f ₁ and f ₂ different from the commercial frequency is applied to the electric line via the grounding wire of the transformer, etc. by electromagnetic induction or series coupling, etc., and the grounding wire The leakage current flowing _back to the grounding wire is detected by a zero-phase current transformer connected to
After separating and extracting the leakage current component of f ₂ through a filter, the leakage current component of frequency f ₂ is frequency-converted to the same frequency as f ₁ by amplitude modulating,
The sum of two signals obtained by converting the leakage current component due to capacitance included in the frequency-converted leakage current and the leakage current component due to capacitance included in the leakage current at frequency f ₁ to the same level. An insulation resistance measuring method characterized in that the difference is used to obtain an output that is inversely proportional to the insulation resistance of an electrical circuit.