JPS6334986B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for measuring insulation resistance of a grounding system. Although a measuring device generally called a megger is used to measure the insulation resistance of electrical equipment, etc., it cannot be applied to measurements in a live line state. A known method for measuring insulation resistance in a live wire condition is to apply an AC voltage with a frequency lower than the commercial frequency to the circuit under test and detect the feedback current, but the feedback current includes a component that passes through stray capacitance. cannot be applied to live-line circuits with large stray capacitance. A method of extracting only the effective component from the feedback current is also used, but in a live circuit with a large stray capacitance, the reactive component becomes significantly large, resulting in a large measurement error. Furthermore, in online monitoring of live circuits, etc., it is inherently impossible to adjust the measurement system, so the measurement system needs to be non-adjustable, and it is necessary to accurately measure insulation resistance over a wide range due to insulation deterioration.

The method of the present invention is a useful method in that it is possible to measure insulation resistance over a wide range in a live line state, regardless of ground stray capacitance.

The present invention will be described in detail below with reference to the drawings.

FIG. 1 shows a measuring device according to the prior art. Examples of prior art methods include those detailed in Japanese Patent Publication No. 53-68290. In the following explanation, the example of the above-mentioned publication will be explained as a prior art.

In FIG. 1, the oscillator OSC is a low frequency oscillator, and oscillates at a frequency sufficiently lower than the commercial frequency. The output of the oscillator OSC passes through the filter F1, then the transformer TR5, and is sent to the ground line EL of the ground system. The resistance RO is the current i flowing through the grounding wire EL.
It is inserted in series with the grounding wire EL to detect. By the way, the "grounding line of the grounding system" EL means a line used for grounding in the grounding system. For example, in the case of a star-shaped three-phase circuit with a grounded neutral point, this is the line that connects the neutral point to the ground.

The current i flowing through the grounding wire EL is not only the current i _R circulating through the insulation resistance R to be measured, but also the stray capacitance C.
It also contains the _current component circulating at the system frequency due to the live line condition. If the angular frequency of the oscillator OSC is W ₁ and the oscillator voltage V on the secondary side of the transformer TR5 is V = Vosinw ₁ t, then the current i flowing through the ground wire due to the voltage at this frequency is: i = i _R + i _C =V _O /Rsinw ₁ t+w ₁ cVo cosw ₁ t.

When voltage V and current i are multiplied, V×i=Vo ² /R−Vo ² /2R cos2w ₁ t+w ₁ cVo ² /2 sin2w ₁ t. In other words, the DC component of the multiplier output is Vo ² /R, and if Vo is kept constant, a value that is inversely proportional to the insulation resistance is obtained, and this value is not related to the stray capacitance C.

By the way, in general, when comparing the sizes of i _R and i _C , i _R
is much smaller than i _C , and when trying to increase the sensitivity of the measuring device, saturation of the amplifier due to i _C occurs, causing problems with the circuit configuration. Therefore, the component of the second term in the equation must be made sufficiently small before multiplication. The traditional method in this regard is
The second term in the equation is reduced by adding to i in the equation a correction signal obtained by shifting the phase of the output using a 90° phase shifter PH and then inverting its polarity.

In this case, an important issue in this method is whether to use a current with an amplitude as large as this correction signal. In the above-mentioned publication, a method is adopted in which the amplitude of this correction signal is made adjustable by tap switching or the like. That is, in order to measure insulation resistance with high precision, manual adjustment of the correction signal was required. However, the value of stray capacitance fluctuates greatly in rainy weather, etc., and accuracy cannot be maintained unless the tap adjustment is made each time.

The method of the invention makes it possible to automatically determine the amount of this correction signal, eliminating the need for adjustment.

In other words, Vo′sinw ₁ t obtained by branching the oscillator output Vosinw ₁ t
(Vo′ does not have to be equal to Vo) is passed through a 90° phase shifter Ps to obtain V′=Vo′cosw ₁ t. Taking the product of V′ thus obtained and the current i in the equation, V′×i=w ₁ c/2VoVo′+w ₁ cVoVo′/2cos2w ₁ t+Vo
Vo′/2Rsin2w ₁ t In other words, we know that the DC component of the equation is w ₁ c/2VoVo′. This is proportional to stray capacitance. That is, the value obtained by multiplying the DC component of V'wi by 2/Vo'(Vo' is constant) becomes w ₁ CVo. Therefore, as a correction signal, the amplitude component of the signal V″=−Vo′cosw ₁ t, which is the polarity-inverted signal of V′
When the amplitude of V'' is automatically adjusted so that Vo' is equal to the DC component of V'xi multiplied by 2/Vo', the resulting signal is −w ₁ CVocosw ₁ t. If added to i in the equation, it becomes i-w ₁ CVocosw ₁ t=Vo/Rsinw ₁ t.

This means that the amplitude of the signal in the equation is inversely proportional to the insulation resistance R. It is clear that in order to obtain the DC component inversely proportional to R from the equation, the signal in the equation may be rectified, or it can be obtained by multiplying it by V as in the case of the equation.

FIG. 2 shows an embodiment of the present invention that implements the above. Oscillator with angular frequency w ₁ lower than the commercial frequency
The output of OSC passes through filter _F1 and then transformer
It passes through TR5 and is sent to the ground line EL of the ground system. The secondary side voltage of TR5 is assumed to be V. By passing the current flowing through the resistor Ro through a buffer amplifier BA, for example, a voltage proportional to the current i can be obtained at its output. (It is clear that this may be processed as a current without converting it to a voltage). On the other hand, the filter
The output of _F1 is applied to a 90° phase shifter ps.
a multiplier for the product of the output V' of the phase shifter ps and the output voltage of the buffer amplifier BA which is proportional to the current i;
When taken by MUP1, a signal obtained by multiplying both sides of the equation by Ro is obtained at the output of the multiplier MUP1. If you add it to low pass filter LPF1 to take the DC component of that signal, the output of low pass filter LPF1 will be
w ₁ CVoVo′/2Ro is obtained. The low-pass filter output is multiplied by 2/Vo' by the coefficient circuit 2/Vo' to obtain w ₁ CVoRo, which is added to one input of the subtraction circuit SUB1. On the other hand, the output of the 90° phase shifter PS is inverted by a polarity inversion circuit INV and then applied to a variable attenuator VR composed of transistors, FFTs (field effect transistors), etc. variable attenuator
The output of VR is rectified by a rectifier LD, and its DC component is added to the other input terminal of the subtraction circuit SUB1. The output of this subtraction circuit SUB1 is the amplifier AP3
The output is added to the control input of the variable attenuator VR and automatically adjusted so that the output of SUB1 becomes zero. Thus the variable attenuator
A signal −w ₁ CVoRo cosw ₁ t is obtained at the output of VR. The output of the variable attenuator VR is added to the output of the buffer amplifier BA by the adder ADD, and the output is Roi−w ₁ CVo on the left side of the equation.
Rocosw _1t is obtained. In other words, the output of the adder ADD is VoRo/Rsinw ₁ t, the amplitude of which is proportional to the insulation resistance. The voltage proportional to Vo/R can be obtained as the DC component even if the output of the adder ADD is rectified, but as shown in Figure 2, the DC component is created by multiplying the adder output and the oscillator output by the multiplier MUP2. By extracting it with the low pass filter LPF2, a value inversely proportional to the insulation resistance can be obtained at the output OUT.

In the above embodiment, the filter that passes only the frequency component of _w1 after the output of the buffer amplifier BA is omitted, but good results can be obtained by adding this filter. Further, in the above description, the current i was converted to the voltage iRo via the buffer amplifier BA for processing, but as already mentioned, the same processing can be performed using the current as it is.

Further, in order to eliminate the second term of the equation, the following method may be adopted in addition to the above method.

That is, by inverting the polarity of the voltage obtained by passing the oscillator output voltage to the current i through a 90 DEG phase shifter, a correction current with an amplitude a of current i'=-acosw ₁ t is obtained.

Adding this to i, we get i+i'=Vo/Rsinw ₁ t+(w ₁ CVo-a)cosw ₁ t from the formula, and taking the product of i+i' and V' in the same way as the formula, we get V'×(i+i ′)=(w ₁ CVo−a)/2Vo′+(w ₁ CVo−
a)/2Vo′cos2w ₁ t+VoVo′/2Rsin2w ₁ t. The DC component (w ₁ CVo-a)/2Vo' on the right side of the equation becomes zero when w ₁ CVo=a. That is, V′×
This is obtained when the DC component of (i+i') is automatically adjusted to zero, and i+i' includes only the effective component Vo/Rsinw ₁ t.

FIG. 3 shows a second embodiment of the invention that achieves the above.

The embodiment shown in FIG. 3 is basically the same as that shown in FIG. 2, and differs only in the method of erasing invalid components. Therefore, descriptions of parts having the same functions as those in FIG. 2 will be omitted. At the output of the buffer amplifier BA, iRo=VoRo/RSinw ₁ t+w ₁ cVoRo・cosw ₁ t is obtained, and at the output of the variable resistance attenuator VR, a signal of −a′cosw ₁ t (a′ is the amplitude of the voltage) is obtained. is obtained. Therefore, the output of adder ADD is
iRo−a′cosw ₁ t is obtained, and this iRo−a′cosw ₁ t and
Multiplier MUP1 takes the product of V' = V'ocosw ₁ t of the 90° phase shifter ps output, and adds the output of multiplier MUP1 to low-pass filter LPF1, so that the output of low-pass filter LPF1 has a DC component (w ₁ CVoRo−a′)/2Vo′ is obtained. Amplifier AP3 amplifies this DC component and controls variable attenuator VR.
It operates so that the DC component of the LPF 1 output becomes zero.

This type of technique is commonly used in automatic gain adjustment circuits and the like. As the DC component becomes zero, the reactive component included in the output of the adder ADD, ie, (w ₁ CVoRo−a') cosw ₁ t approaches zero. Therefore, the product of the output of the adder and the output Vo′sinw ₁ t of the filter F ₁ is the product of the multiplier MUP
2 and its DC component is passed through a low pass filter LPF2.
The output of low pass filter LPF2 is found by
VoVo'Ro/R is obtained at OUT. This amount is inversely proportional to the insulation resistance R. It is also clear that even if the output of the ADD is rectified, a value inversely proportional to the insulation resistance can be obtained.

According to the method of the present invention, insulation resistance can be measured with high accuracy without adjustment (automatic adjustment).

In the above description, the principle was explained by inserting a current detection resistor in series with the transformer TR5, but it is clear that the current detection method is not necessarily limited to this. That is, it is clear that the present method can also be used by inserting a current detection resistor in series with the primary winding of TR5. In this case, there is an effect that the measurement target can be completely insulated.

[Brief explanation of drawings]

R: Insulation resistance, C: Stray capacitance, EL: Grounding wire for grounding system, TR5: Transformer, F ₁ , F ₂ : Filter,
ADJ: Manual adjuster, PH: 90 degree phase shifter, Ro: Current detection resistor, AP ₃ , AP ₁ , AP ₂ : Amplifier, OSC:
Oscillator, MUP, MUP ₁ , MUP ₂ : Multiplier, ps:
90° phase shifter, INV: polarity inversion circuit, VR: variable attenuator, LD: rectifier, SUB1: subtraction circuit, LPF ₁ ,
LPF ₂ : Low pass filter, ADD: Adder, 2/
Vo′: Coefficient circuit. Figure 1 is a block diagram of a conventional insulation measuring device. 2 and 3 are system diagrams of the insulation resistance measuring device of the present invention, respectively.

Claims (1)

[Claims] 1. A low-frequency signal source inserted and connected to a grounding line, a current detection unit that detects a current flowing through the grounding line based on the low-frequency signal, and a phase difference between the output of the low-frequency signal source.
means for shifting the phase by 90 degrees; means for multiplying the 90 degrees phase-shifted signal by the current of the current detection section; means for multiplying the DC component of the product by a constant; a variable attenuator that controls the amplitude value of the signal to be equal to the DC component multiplied by the constant; and arithmetic means for adding or subtracting the output signal of the variable attenuator and the DC signal multiplied by the constant. An insulation resistance measuring device characterized by: 2. In claim 1, means for obtaining a signal of a difference between the current of the current detection unit and a first current having a phase different from the low frequency signal source by 90 degrees, the signal of the "difference", and the means for obtaining a DC component of the product of a low frequency signal source and a second signal having a phase difference of 90°; and means for automatically adjusting the magnitude of the first current so that the DC component becomes zero. An insulation resistance measuring device characterized by: