JPS6243572A - Insulation resistance measurement - Google Patents

Abstract

PURPOSE:To always enable accurate measurement of insulation resistance regardless of increase in the floating capacitance, by shifting the frequency so that one of leakage components of two low frequency signals to be applied to an electric circuit is equal in the frequency to the other thereof. CONSTITUTION:Ground leakage currents of two low frequency signals which differ from the commercial power source frequency applied to an electric circuit through the earth wire of a transformer T are separated and extracted with a zero-phase current transformer ZCT connected to the earth wire and a band pass filter BPF2-3 connected thereto to shift the signal frequency of one of the two leakage components to that of the other thereof. In other words, the frequencies of two low frequency signals shall be expressed by f1 and f2 (f1<f2) and the signal of f2 of the two low frequency signals to be fed back to the ground wire is mixed with a signal of the sum (f1+f2) of two low frequency signals applied to the electric circuit. Then, component f1 lone is extracted from the mixed outputs and is subtracted from the leakage component of the f1 signal to cancel the invalid component. Thus, the valid component alone adversely proportional to the insulation resistance value of the electric circuit is derived thereby enabling the detection of the insulation resistance irrelevant to the invalid component.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method of measuring insulation resistance of an electric circuit or the like in a live line state.

(Prior Art) Conventionally, the insulation resistance of electrical circuits has been constantly monitored for early detection of electrical leakage, and the method shown in No. 21 WK has been common as this type of measurement method.

In other words, in order to monitor the insulation resistance of the secondary line LILz of the power receiving transformer T, for example, which is provided with the second type grounding line Lv on the secondary side wire ring, as shown in the same figure, the above-mentioned An oscillator 0801 inserted into the grounding line Lg applies a low frequency signal f+ different from the commercial frequency to the electrical line LIL2, and connects the grounding line LEK to the zero-phase current transformer ZCT1 to reduce the insulation resistance R existing between the electrical line LIL2 and the ground. The leakage current of the low frequency signal f flowing back to the grounding line LE via
Therefore, the insulation resistance of the electric circuit was obtained by extracting the signal and synchronously detecting it using the multiplier M[JLT+ and the output of the oscillator 0801.

In order to facilitate understanding of the present invention, this conventional insulation resistance value calculation method will be explained in detail.

In other words, if the measurement system shown in FIG. 2 is expressed as an equivalent circuit, the third
As shown in the figure, insulation resistance R is connected in parallel to the low frequency oscillator O8C+.
・ and stray capacitance C are connected, and if the current that returns to the ground line LE through these is ↑1, then I 1 = (v/R・) + jωtCV...
...(1) However, ■ is the applied voltage of the generator 08C1,
ωl is the angular velocity of the applied low frequency signal f1, and ω1=2πf
! It is.

Now, the current 11i expressed by this equation is the oscillator 08C
If the in-phase component (effective component), that is, the first term of the in-phase component, is extracted by means such as synchronous detection using the output of I, this becomes a value that is inversely proportional to the insulation resistance R, and the insulation resistance can be derived from this. .

However, according to this method, as the stray capacitance increases, the level of the reactive component of the leakage current increases, and accurate extraction of the effective component becomes difficult due to the dynamic range constraints of the synchronous detection circuit, etc. mentioned above. There was a flaw that made it difficult.

One way to reduce this reactive component is to lower the frequency of the applied low-frequency signal f1, but this is uneconomical since the zero-phase current transformer used has to be made larger in order to increase its actance. .

Generally, a frequency of about 10 to 30 Hz is used as the low frequency signal f1, so it is extremely difficult to measure insulation resistance due to an increase in stray capacitance.

(Objective of the Invention) The present invention has been made to eliminate the deficiencies of such conventional methods for measuring insulation resistance of electrical circuits, and provides a method that enables accurate insulation resistance measurement at all times regardless of increases in stray capacitance. The purpose is to provide

(Summary of the Invention) For this purpose, in the present invention, the low frequency signals applied to the electric circuit are two waves, one of the MM acid components of the two low frequency signals is frequency shifted to be the same as the other frequency, and By subtracting the frequency-shifted signal from the other signal and canceling the reactive component, only the effective component that is inversely proportional to the insulation resistance is fully derived, and the insulation resistance of the electric circuit is detected regardless of the reactive component.

(Example) The present invention will be described in detail below based on an illustrated example.

Before that, the principle of the present invention will be explained in detail using mathematical formulas.

That is, if one more low frequency signal is added to be applied to the electric path and it is designated as f2, the resulting leakage current I2 is similarly expressed as ω2=2πf2.

Therefore, if we subtract the equation (1) from the equation (1) above...
...(3) is obtained, and since there is no reactive component Kanakura in this equation, the insulation resistance can be measured regardless of the increase in its level.

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

In the figure, T, L+, L2 ZCT and Lx are the power receiving transformer, secondary line, zero-phase current transformer and 2m grounding wire, which are inserted in the grounding line LE, as in the above figure 2. The oscillator 08C2 simultaneously outputs two low frequency signals f'+f2, each having a different frequency.

Also, the zero-phase current transformer ZCT output is branched into two.

One of them is a bandpass filter BP that extracts only fl.
Fz and one input of the subtracter 5UBT via the amplifier AMP, and the other branch output of the zero-phase current transformer ZCT is fl
Bandpass filter BPFs that extracts only
5MOD, low pass filter LPF and phase shifter P8f-
The output of the oscillator 08Cz, that is, the signal fl+fz is input to the modulation signal input terminal of the modulator MOD.

The operation of the insulation resistance measuring circuit configured as described above will be explained below using a mathematical formula.

Now, if the voltages of the two signals fs and fz applied to the electric circuit from the oscillator 08C2 are both the same 9 (volts), then
The bandpass BPFz output (signal fl output) χ1 is χ1=CV/R1) si*ωlt+ωICTcosωI
t−・・−・−・(4) Also, the output of bandpass BPFa (signal f2 output) χ2 is zt=(V/R)simωzt+ωzCVcosa+z
(-...-(51) Next, the output of the modulator MOD contains the two mixed components of (f1 + f2) ± f2.
If only the difference y between the two is extracted using the next-stage low-pass filter LPF, only the negative component remains, which is expressed by the following equation:
The frequency is shifted to t-fl.

y = (V/R1) sin (ω2-(ω1+ω2)
)t+ω2CV(9)(ω2-(ω1+ω2))t=-(V/R1)siaω+t+ωzcVcosa+t
t ・・−・−・(6) Then, if this signal and the signal multiplied by 08701 by the amplifier AMP of Sfilter BPFzb are subtracted by the subtractor 5tJBT, (ω2/ω1)χ1−y=( V/R) ((ω2/ωt)
+1) sinω1t (7) This is a signal proportional to the effective component of the leakage component of the low frequency signal, so by rectifying it or synchronously detecting it, the insulation resistance of the electric circuit can be adjusted. can be found.

Note that the phase shifter PS is for correcting the phases of the two signals to be subtracted in the subtracter 5UBT.

With this configuration, as is clear from the explanation above, the insulation resistance can be determined regardless of the reactive component of the leakage current.

2. In carrying out the present invention, it is not necessary to limit it to the nine embodiments described above, and various other embodiments may be considered. for example,
The fl and fl signal voltages applied to tvlII are vl and v
It is easy to understand that the gain of the amplifier AMP may be set to (Vz/Vt).(ω2/ω1) in this case.

(Effects of the Invention) Since the present invention is configured as described above,
It is extremely effective in accurately determining the insulation resistance of an electric circuit without being affected by reactive components.

[Brief explanation of the drawing]

The first factor is a block diagram showing an embodiment of the A8 edge resistance measuring method of the present invention, FIG. 2 is a block diagram showing a conventional insulation resistance measuring circuit, and FIG. 3 is an equivalent circuit diagram of FIG. 2. T......Power receiving transformer Ls, Lt・
・・・・・・Electric circuit R・・・・・・Insulation resistance,
C...... Ground capacitance, Z...
・・・－・・・Load on electric circuit. ZCT・・・・・・Zero phase current transformer, O8C+, 0
8C2...... Oscillator, BPF+ to BrF3...... (nd pass filter,
M[LTt......Synchronous detection circuit,
LPF・・・・・・Ronox filter PS・
...... Phase shifter, 8UBT...
...Subtractor.

Claims (2)

[Claims] (1) A zero-phase current transformer that couples the ground leakage currents of two low-frequency signals different from the commercial power frequency applied to the power line through the transformer's ground wire to the ground wire, and a bandpass filter connected to the zero-phase current transformer. The insulation resistance of the electrical circuit is detected by separating and extracting the leakage components of the two low-frequency signals, shifting the frequency of one of the leakage components of the two low-frequency signals to the signal frequency of the other, and subtracting these two to derive only the effective component. A method for measuring insulation resistance, characterized in that the method is configured to: (2) The two low frequency signal frequencies are f_1 and f_2, where (f_1<f_2), and f_2 of these two low frequency signals returned to the ground line and the two low frequency signals applied to the electric line are By mixing the sum of the signals f_1+f_2, extracting only the f_1 component from this mixed output, and subtracting this component and the leakage component of the f_1 signal, only the effective component that is inversely proportional to the insulation resistance value of the electric circuit is derived. The insulation resistance measurement method according to claim 1, characterized in that the insulation resistance measurement method is configured to perform the following: