JPS63175772A - Phase compensating method for insulation resistance measuring instrument - Google Patents

Abstract

PURPOSE:To correct the phase shift of a low-frequency signal for measurement at low cost all the times by adjusting the phase of the low-frequency signal for measurement which is applied to a synchronous detector provided on the secondary side of a current transformer coupled with an earth line, and approximating the output of the synchronous detector to zero. CONSTITUTION:The output terminal of a power amplifier PAMP is connected to the secondary side of a transformer OT through a changeover switch SW1 and a phase control circuit PC which generates the low-frequency signal for measurement is connected to the input side of the PAMP. When the switch SW1 is connected to a broken-line side, the primary side of the transformer OT is terminated by a resistance R1, so a current which flows through a conductor penetrating the current transformer ZCT through a capacitor C shifts in phase by 90 deg. through the PAMP. At this time, the value or polarity of the output D1 of the synchronous detector MULT is discriminated and the phase of the low-frequency signal for measurement applied to the MULT through a filter FIL is adjusted to approximate the output D1 to zero, thereby eliminating the phase shift of a measuring circuit.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method of compensating for temperature changes, secular changes in circuit constants, etc. in a device that measures the insulation resistance of electrical circuits or the like in a live line state.

(Prior Art) Conventionally, it has been common practice to monitor the condition of an electrical circuit using a method of measuring insulation resistance of the electrical circuit, as shown in FIG.

This can be done by inserting a transformer O↑ connected to the second type grounding wire II K of the power receiving transformer T with a load Z, a measuring low frequency signal oscillator O8C having a frequency fx different from the commercial power supply frequency, or A low frequency voltage for measurement is applied to the electrical circuits 1 and 2 by inserting and connecting the oscillator O8C in series with the grounding line Lx, or by passing the electrical circuit 1.2 through a toroidal core transformer to which the oscillator is connected. , the current transformer ZCT K penetrates the grounding wire Lm, so that the insulation resistance Ro and the stray capacitance to ground Co existing between the electric path and the ground are reduced.
After detecting the leakage current of the measurement low frequency signal that returns to the grounding line via the amplifier AMP.

Only the frequency f1 component is selected by the filter FIL, and this is synchronously detected by the multiplier MULT using, for example, the output signal of the oscillator O8C to calculate the effective component (O
It was constructed to detect the UT error (that is, a component in phase with the applied low frequency voltage) and to measure the insulation resistance of the smear path.

To help understand the present invention, the measurement theory thereof will be further explained.

If the measurement signal voltage applied to the grounding line LΣ is, for example, a sine wave and V sinωtt (ω1=2π, fl), then the frequency f that returns to the grounding line LE via the grounding point E is
The leakage current I in 1 is expressed as , and is a component that is in phase with the applied AC voltage.

In other words, if a value proportional to the first term on the right-hand side of equation (1) is detected using a means such as synchronous detection, this value will be inversely proportional to the insulation resistance Ro. You can ask for it.

However, in the conventional method of detecting the leakage current that returns to the grounding line with the current transformer ZCT and selectively outputting the leakage current component of the frequency fx included in the detected leakage current component with the filter FIL, two normal current transformers → amplifier → Since the phase of the leakage current of frequency fl is shifted in the filter system, R from these synchronous detection outputs.

In order to obtain a value that is inversely proportional to , it is necessary to compensate for this phase shift. For this purpose, conventionally, as shown in the figure, the first input terminal of the synchronous detector MtJLT or

By inserting a phase shifter PS into the second input terminal, the above phase shift is corrected and mutual synchronization is achieved. i.e. phase shifter PS′? : Provided in a state where there is no stray capacitance Co to the ground (Co=O).

The first of the synchronous detectors. The phase difference between the voltages applied to the second input terminal was set in advance to be zero.

However, in the conventional method as described above, the current transformer ZCT
, filter FIL, phase shifter PS, etc., fluctuate due to temperature changes or changes in the characteristics of the parts used over time. This results in a phase error with the initial adjustment value, making it impossible to provide correct measurement results. was there. In order to cope with these problems, the influence of the phase error has conventionally been minimized by employing extremely high-quality current transformers or filters with little variation in characteristics.

Even so, it was difficult to completely eliminate that influence.

(Objective of the Invention) The present invention has been made to eliminate the drawbacks of the conventional insulation resistance measurement method as described above, and to constantly correct the phase shift of the measurement signal at low cost without requiring expensive parts. However, it is an object of the present invention to provide a phase adjustment method for an insulation resistance measuring device that can always provide accurate measurement results.

(Summary of the invention) (
In principle, the present invention achieves this objective.

A low frequency signal voltage for measurement different from the commercial power supply frequency is applied to the electric line via the ground line, and the leakage component of the low frequency signal for measurement that returns to the ground line via the insulation resistance and stray capacitance between the electric line and the earth is detected. In an apparatus for measuring the insulation resistance of an electrical circuit by extracting the signal through a current transformer coupled to the grounding wire and synchronously detecting this signal, a new conductor is passed through the current transformer, and , A signal whose phase has shifted by 90 degrees from the low frequency voltage for measurement is energized through the conductive wire in a state where the low frequency voltage for measurement is not applied to the electric line, and the output of the synchronous detector obtained at this time is zero. Automatically adjusts the phase of the reference measurement low-frequency signal voltage applied to the synchronous detector.
Or configure it to be adjusted manually.

In this state, if the current to the conducting wire is stopped and normal measurement is performed, fluctuations in the phase characteristics of the measuring circuit are compensated for, and the measurement results obtained at this time will be accurate.

Examples) Before explaining the measuring method according to the present invention, a conventional method and its drawbacks will be explained in some detail to aid understanding.

The leakage current component I at frequency f1 shown in equation (1) is the current transformer ZCT, amplifier AMP, and filter FIL.
If the phase shift that occurs when passing through the O system is θ, then the filter FIL output 1 is -... (2) This is the first input terminal of the synchronous detector MOLT. applied.

In addition, the voltage applied to the second input terminal of the synchronous detector is, for example, a □ sin (ωs t+01) with a constant amplitude.
Then, the output of the synchronous detector, that is, the effective component, is D=I
xXa□5i11(ωtt-IJt) ・・・・・・
・・・・・・(3) (□ means to remove components with angular frequency of 01 or higher) ・・・・・・・・・(4) Therefore, when σ=σl, the output DO is ') *”+aO is constant, so insulation resistance R.

A value that is inversely proportional to K can be measured. Therefore, the above is true when the phase shift 0-01 is not zero.

The error E of D for
in (a-θl) (6).

Now 9 For example, when σ-01 = 1 (degree), fx in equation (6)
=25Hz, Ro=20Ω, Co=5. When c+F, ωtCoRo=15.7, so the error C is 27
．． It can be seen that the measurement error becomes significantly larger when the number is 4.

The present invention proposes a method for minimizing the occurrence of measurement errors due to the above-mentioned phase shift.

FIG. 1 is a circuit diagram showing an embodiment of an insulation resistance measuring device according to the present invention, which is used to measure the insulation resistance of electrical circuits 1 and 2, one of which is grounded, similar to that shown in the third case. This is the device.

That is, the second type grounding conductor Lx is connected to one side 2 of the secondary circuit 1.2 of the transformer T having the load 2.

A current transformer ZCT passes through the ground line Lg and connects each transformer OT by direct coupling.

Further, the output terminal of the power amplifier PAMP is connected to the secondary side of the transformer OT via a changeover switch SWI.

Furthermore, a phase control circuit pc that generates a measurement low frequency signal having a frequency fx is connected to the input end of the power amplifier PAMP.

Similarly, the primary impedance of the transformer OT is low;
It does not interfere with the grounding function of the electrical circuit. Said current transformer ZC
The output of the secondary coil of T is input to the synchronous detector MULT via the amplifier AMP and the filter FIL that removes the commercial frequency, and the output is divided into two routes by the second switch SWz, one route is connected to the resistor R2 and the capacitor CI. The other is fed back to the output terminal 0UT2 via a holding circuit connected in an inverted manner, and the other is fed back to the phase control circuit pc via a holding circuit consisting of a resistor R3 and a capacitor C2.

The reference signal for the periodic detector MULT is as follows.

The output of the phase control circuit PC is applied, and the two changeover switches SWl and SWz are controlled by the output of the phase control circuit PC.

Further, a signal line 3 including a capacitor C in series is connected to one side of the changeover switch SW1 in a loop such that a part thereof passes through the current transformer ZCT and reaches the negative electrode of the power amplifier PAMP.

The operation and operation of the insulation resistance measuring device configured as described above will be described in detail below.

First, when the first switch SWI is connected to the solid line side as shown in FIG. 1, the leakage current at frequency f1 flowing through the grounding line Lx is given by equation (1).

And the output of the synchronous detector is given by equation (2) as described above. On the other hand, when the switch SWt is connected to the dashed line side, the primary side of the transformer OT is terminated with the resistor R1, so if the output voltage of the power amplifier PAMP is e, then the output of the power amplifier PAMP is connected via the capacitor C. The current I2 flowing through the conductor passing through the current transformer is I2=ωI Cecosω1t...
...(71) Also, the output 3 of the filter FIL at this time is Is = ωtc6cos(ω1t+σ) −−−−
------- (8) Therefore, the output D1 of the synchronous detector MULT is Ih = l3xa□5in (ωIt + θl) (
The □ mark has the same meaning as formula 31.

That is, as is clear from equation (9), K, GJI, C,
If e and a□ are constant, when 1θ-θ11〈-, θ-θ
If l>0, Dt(0, and 0-θ1〈0, Dl〉0. Therefore, when the value or polarity of DI is determined, the side effect low frequency signal a.5in(ω1
By adjusting the phase θ1 of t+σ1) to bring Dl closer to zero, θ−θ1−+Q can be obtained.

FIG. 2 shows the output waveforms of each part of the measuring device shown in FIG. 1. In the same figure, (a) is the switch S.
The output voltage of the synchronous detector MTJLT when Wx and SWz are switched at a cycle T/2, (a) shows the case where the switch is a solid line K, and (b) shows the case where the switch is a broken line. Also, (b) shows the waveform of the output OUT z, and during the period when the switches SWl and SWz are in solid lines, the synchronous detectors Mt and ILT
The output appears as it is, but during the period when the switch is indicated by the broken line, the holding circuit K which is entangled with the resistor R2 and the capacitor C1 works.
Therefore, the voltage immediately before switching is maintained as it is. Further, in the same Figure 2, the symbol ``→'' indicates the input voltage of the phase control circuit PC, that is, the voltage across the capacitor Cz. When SWz is a broken line, synchronous detector MtJ
The LT output voltage appears as is, but when the switch is in the solid line, the value immediately before switching is held.

As is clear from the figure, according to the circuit described above, even if the switches SW 1 and SW t are switched at a period of T/2, it is inconvenient to calculate the insulation resistance value of the electric path from the signal appearing at the output terminal OUT*. does not occur, so while calculating the insulation resistance value using the output, the voltage across the capacitor C2, that is.

Using a voltage of (/-1) in FIG. 2, adjust the phase of the reference signal applied to the synchronous detector MULTK so that the voltage becomes zero.

Similarly, in this case, the switches SWI and SWz are switched in conjunction with each other, but the switching cycle does not need to be limited to T/2 as in the above embodiment, and the point is that the low frequency signal for measurement is not applied to the electric line. Any means may be used as long as it applies a low frequency measurement signal to the signal line passing through the current transformer and controls the voltage across the capacitor C2 to be zero at that time.

Furthermore, current transformer ZCT, amplifier AMP.

90 at the root of filter FIL and synchronous detector MULT
The means for energizing the phase-shifted measurement low-frequency signal is not limited to the above-described embodiment, and other methods may be used.

Furthermore, since such a phase control circuit can be realized using known technology, a detailed description thereof will be omitted. In addition, in order to avoid transient phenomena that occur in the output of the synchronous detector when the switch SWI is switched, only the output of the synchronous detector when the transient phenomenon becomes steady after switching is used. switch 8
A sampling circuit may be further added between Wx.

In the above embodiment, a single-phase two-wire electric circuit is shown, but it is clear that a single-phase three-wire or three-phase three-wire electric circuit may be used.

Furthermore, the method of inserting the capacitor C can be further expanded based on the same principle.

(Effects of the Invention) As explained above, the present invention compensates for the phase fluctuation characteristics of the measuring circuit of an insulation resistance measuring device, so it is possible to provide a measuring device with extremely high accuracy.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing one embodiment of the present invention. FIG. 2 is a diagram showing waveforms of various parts showing an embodiment of the present invention, in which (a) is the output waveform of the synchronous detector, and (b) is the output terminal 0.
The output waveform of the UT2, (c) is a diagram showing the voltage across the capacitor C2, and FIG. 3 is a block diagram showing a conventional insulation resistance measuring device. T......Trans, 1,2...
...Electric circuit. Lz・・・・・・− Ground wire, E・・・・・・・
...Grounding point. ZCT...Current transformer, AMP...
・・・・・・Amplifier, FIL・・・・・・
··filter. MTJLT・・・・・・・Synchronous detector, O
SC......Oscillator, OT...
...Impression transformer. PS・・・・・・・・・Phase shifter, SWI S
Wz......Switch, PC...
・・・・・・Phase control circuit. PAMP...--Power amplifier. Patent applicant: Toyo Tsushinki Co., Ltd.

Claims (1)

[Claims] 1. A low frequency signal voltage for measurement with a frequency f_1 different from the commercial frequency is intermittently applied to the electric line via the grounding wire of the transformer, and the signal voltage included in the output of the current transformer coupled to the grounding wire is applied. In an apparatus for measuring the insulation resistance of an electrical circuit by synchronously detecting the leakage current at the frequency f_1 with the low frequency signal voltage, a conductor passing through the current transformer is connected to a conductor that is 90° out of phase with the low frequency signal voltage. The phase of the low frequency signal voltage applied to the synchronous detector is automatically adjusted so that the synchronous detection output approaches zero when a current of a predetermined value that has changed is caused to flow in a state where the low frequency voltage is not applied to the grounding wire. 1. A phase compensation method for an insulation resistance measuring device, characterized in that phase characteristic fluctuations of a measuring circuit are compensated for by adjusting the phase characteristic of the measuring circuit.