JP2614449B2 - Insulation resistance measurement method compensated for ground resistance - Google Patents

Description

The present invention relates to a method for measuring the insulation resistance of an electric circuit or the like in a live state, and in particular, to an insulation which compensates for a ground resistance which cannot be ignored when a floating capacitance to the ground becomes large. It relates to a method for measuring resistance.

(Prior Art) Conventionally, a method of measuring the insulation resistance of a circuit as shown in FIG. 4 has been generally used for early detection of electric leakage and the like.

That is, the second-class ground wire of the receiving transformer T having a load of Z
Commercial frequency and different frequency from the oscillator OSC via the L _E
1 comprising measuring the low-frequency signal voltage V ₁ sin (2π ₁ t) input to the path L ₁ and L _2, the current transformer ZCT that allowed through the ground line L _E, exists between paths and ground insulation A leakage current that returns via the resistor R and the stray capacitance C is detected.

At this time, the component of frequency ₁ included in the output of the current transformer ZCT is detected by the filter 1, and the effective part in the leakage current is synchronously detected by the multiplier NULT using, for example, the output of the oscillator OSC. It was to measure the insulation resistance of the electric circuit.

 The above measurement theory will be described with reference to the equivalent circuit of FIG.

When a current is fed back to the oscillator OSC via a ground point _E of the ground line L _E and I (t) Therefore, if a component in phase with the input AC voltage, that is, a value proportional to the first term on the right side of the above equation (1) is detected by a method such as synchronous detection, a measured value inversely proportional to the insulation resistance R is obtained. be able to.

However, the (1) As is apparent from the equation, since the measurement method ignores even though not ground resistance to measure the current fed back through the earth to the ground line L _E, against the stray capacitance C Becomes larger, the effect of the grounding resistance appears, and the measured value is significantly different from the actual insulation resistance of the electric circuit.
As a means for compensating for this disadvantage, the same applicant has filed Japanese Patent Application No. 58-145464.
In "Insulation resistance measurement method", an insulation resistance measurement method that compensates for ground resistance is presented. The method is briefly described with reference to FIG. Husband to the ground line L _E s ₁ and _{2 (1} ≠
₂₎ made to enter the same voltage V consists signal from the oscillator OSC ₁ and OSC ₂ of the low-frequency signal generator. On the other hand, the output of the current transformer ZCT is branched into ₁ and ₂ band pulse filters BF, respectively.
P ₁ and the two oscillators OSC ₁ and other input terminal of the output respectively said synchronous detection circuit MULT ₁ and MULT ₂ of OSC ₂ and inputs to one input terminal of the synchronization via the BFP ₂ detection circuit MULT ₁ and MULT ₂ To enter. As a result, the in-phase component, ie, ig ₂ is obtained from the outputs of the effective components ig ₁ and MULT ₂ from the output of the synchronous detection circuit MULT ₁ , and the value is substituted into the following equation.

This makes it possible to measure V / R inversely proportional to the insulation resistance R of the electric circuit while compensating for the influence of the ground resistance r.

However, in the above method, the low-frequency measurement signals of frequencies ₁ and ₂ input to the electric circuit via the ground wire of the transformer are both signals in similar frequency bands, and therefore, two sources of these signals are used. Since the oscillators OSC ₁ and OSC ₂ are always required, the circuit configuration is complicated and expensive.

(Object of the Invention) The present invention has been made in order to eliminate the drawbacks of the conventional insulation resistance measuring method as described above, and to accurately measure the insulation resistance of a circuit by compensating for the influence of ground resistance. An object of the present invention is to provide a method for measuring insulation resistance of an electric circuit, which can simplify a circuit configuration by sharing one of two signals input to a ground line with a non-modulated carrier signal voltage used for data transmission or the like. I do.

(Summary of the Invention) In order to achieve this object, the invention according to claim 1 of the insulation resistance measuring method for compensating for the ground resistance according to the present invention comprises the steps of: frequency
A low-frequency signal voltage of a frequency f2 higher than f1 was applied, and the leakage currents of the frequencies f1 and f2 that returned to the ground line of the electric circuit were individually detected, and among the leakage currents, the leakage current of the frequency f1 was applied to the electric circuit. Synchronous detection is performed by an in-phase signal of the low-frequency signal of frequency f1 to extract an effective component ig1, and a phase shift of 90 ° from the low-frequency signal of frequency f1 in which the leakage current of frequency f1 is applied to the electric circuit among the leakage currents. The synchronous circuit detects the ineffective component ic1 by extracting the ineffective component ic1, the ineffective component ic1 and the signal io obtained by rectifying the ineffective component ic1 and the leakage current of the frequency f2, thereby performing a predetermined operation. Characterized in that the insulation resistance was measured.

The invention described in claim 2 is characterized in that, in the invention described in claim 1, the low frequency signal voltage of the frequency f2 is used as an unmodulated carrier signal voltage for data transmission. And

Further, the invention described in claim 3 is the invention according to claims 1 and 2, wherein the predetermined operation is: V ₁ was input signal voltage of the frequency f1, V ₂ is the input signal voltage of the frequency f2, omega ₁ is 2πf1, ω ₂ is 2πf2, k is V ₁ ^2/2, R
Is an insulation resistance.

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

First, before describing the insulation resistance measuring method according to the present invention, the defects of the conventional method will be described in some detail in order to help the understanding.

FIG. 3 is an equivalent circuit diagram when the ground resistance r is considered.

If a current is fed back in this case the oscillator via a ground point E OSC and I (t) of the oscillator voltage _{V 1 sinω 1 t I 1 (} t) = A 1 V 1 sinω 1 t + B 1 V 1 cosω 1 t ... (3) At this time, It goes without saying that if the ground resistance r is neglected in the equation (3), it becomes the same as the equation (1).

In equation (4), when stray capacitance to ground C = 0, A is However, since R≫r generally holds, A can be considered to be 1 / R, and the in-phase component in the above equation (3) becomes V / R, and the insulation resistance can be measured by detecting the in-phase component. However, when the stray capacitance C is large, even if the in-phase component is detected, the correct insulation resistance is not measured as shown by the equation (4).

The following shows how much such an error actually occurs.

In general, since R≫r, in the equation (4), R + r → R
Then It can be expressed as

Here, for example, if ₁ = 20 Hz, C = 5 μF, r = 100Ω, (ω ₁ Cr) ² = (2π × 20 × 5 × 10 ⁻⁶ × 100) ² 3.95 × 10 ^{−3 and} (ω ₁ Cr) ² ≪ It is one.

Therefore, equation (5) is May be considered.

Therefore, for example, ₁ = 20 Hz, C = 5 μF, R = 100 KΩ, r = 1
In the case of 00Ω, the value in {} of the above equation (6) is 1+ (ω ₁ C) ² Rr = 4.95, and the insulation resistance value 100KΩ to be detected from the in-phase component is
It will be measured as 100KΩ / 4.95 = 20.2KΩ.

As described above, as described above, the conventional insulation resistance measurement method ignoring the ground resistance has a defect that the measurement error becomes extremely large when the ground stray capacitance is large. Furthermore, since the stray capacitance to the ground includes the capacitance of the noise filter added to the power supply circuit of general electronic equipment, the stray capacitance to the ground tends to increase in the future, so that more accurate measurement results can be obtained with the conventional method. Will not be.

It is also known that the value of the ground resistance generally varies depending on the season and the like, and a method of accurately measuring the insulation resistance without being affected by these is desired.

Next, a method (Japanese Patent Application No. 58-154564) already proposed by the same applicant to eliminate this disadvantage will be briefly described.

That is, FIG. 5 shows a block diagram of an insulation resistance measuring apparatus according to this method, which relates to an insulation resistance measuring method in which the influence of ground resistance is compensated in the same manner as in the present invention.

This method, two ground lines L _E low frequency oscillator OSC, OSC ₂
After applying different frequencies ₁ and ₂ to the electric circuit L ₁ L ₂ and deriving the leakage current of both signals by the current transformer ZCT supplied to the grounding line, only the respective frequency signals are band-pass filtered BPF _1, BPF ₂ extracts through and obtain the active component ig ₁ and ig ₂ by synchronous detection by the phase signal from the signal source each, performs calculation of the two equations in further subtraction circuit SUB .

According to this method, the detailed description is omitted, but as a result, the measurement error caused by the current flowing through the capacitance component between the electric circuit and the ground flowing through the ground resistance is caused by two low-frequency signals ₁ ,
_{The two components} cancel each other out and are removed from the measurement result, so that an accurate insulation resistance value without the influence of the ground resistance can be obtained.

However, the arithmetic expression (2) used in this method
Since this is true when both low-frequency signals ₁ and ₂ belong to the same frequency band, there is a disadvantage in that the device requires both signal sources and the circuit configuration is slightly complicated. It is.

On the other hand, an insulation monitoring device for an electric circuit is generally installed on the roof of a building or a remote place, and the result is transmitted to a control room or the like where a person is resident through an electric circuit.

At this time, the signal superimposed on the electric line is generally modulated by monitoring a carrier signal of several KHz or more based on the monitoring result.

Therefore, if the carrier can be used for one of the two signals in the above-described insulation measuring means, the circuit configuration can be simplified and the cost can be reduced.

The present invention has been made in view of such circumstances, and an arithmetic expression for obtaining an insulation resistance is as follows in order to make this possible.

The in-phase component of the leakage current obtained by the input signal voltage V ₁ sinω ₁ t of the frequency ₁ , that is, the effective component ig ₁ is I _{1 of the} equation (2).
(T) is obtained by synchronously detecting the input signal voltage and obtaining its DC component.

here From equation (6) Becomes

Further, reactive component ics ₁ leakage current having a frequency ₁ to synchronous detection in (3) of I ₁ (t) and the input signal voltage V ₁ sin .omega ₁ t 90 ° phase-modulated allowed voltage V ₁ cos .omega ₁ t Ask by

By the way, B _{1 in the} equation (4) is generally R {r, (ω ₁ Cr) ² ≪
Since ω ₁ C is obtained by the condition of 1, ic ₁ = kω ₁ C (10).

On the other hand, the input signal voltage V ₂ sinω _{2 of} frequency ₂ (≫ ₁ )
t (ω ₂ = 2π ₂₎ due to the leakage current I ₂ (t) becomes _{I 2 (t) = A 2} V 2 sinω 2 t + B 2 V 2 cosω 2 t ...... (11). A _2, B ₂ is obtained by replacing the omega ₁ in the formula (4) A _1, B ₁ in omega _{2. 2 ω 2 »ω 1} next for _»1 of the input signal voltage, even R»r (ω ₂ ^Dr) ₂ «1
Not necessarily Is approximated.

Therefore, if the leakage current I ₂ (t) at frequency ₂ is rectified and its DC voltage io is obtained, ^{_{io 2 = V 2 2 (A}} 2 2 + B 2 2) become.

Substituting A ₂ and B ₂ in equation (12) into this gives Generally the low pressure path (hereinafter eg 500 KVA) plant such as the Te at approximately r = 0~100Ω, C = 0~10μF, if ₂ to about 1~3kHz Since R is determined to be equal to or larger than 2 k.OMEGA ( 14) In the equation Because the relationship holds And from equation (16) Becomes

By the way, the value k / R, which is inversely proportional to the insulation resistance R of the electric circuit, is obtained from the equation (8). Substituting equation (17) into this From equation (10) Substituting into equation (18) Becomes Accordingly, since V ₁ , V ₂ , ω ₁ , ω ₂ , and k are known, the insulation resistance of the electric circuit can be measured by the calculation of Expression (19) using the detected values ig ₁ , ic ₁ , and io.

FIG. 1 is a block diagram showing an embodiment of an insulation resistance measuring device according to the present invention.

4, the same symbols in FIGS. 4 and 5 have the same meaning.

That is, the output of the power amplifier PAMP the output of the oscillator OSC ₂ of the output of the oscillator OSC ₁ of the oscillation frequency ₁ oscillation frequency ₂ via the injection transformer OT is coupled to the ground line L _E. Thus, the voltages V ₁ sinω ₁ t and V ₂ sinω ₂ t exist between the electric circuits L ₁ and L ₂ and the ground.
Is input. Current transformer ZC earth wire L _E is allowed through
Frequency and the leakage current I ₁ of the frequency ₁ (t) during the output of the T
_{2 includes} a leakage current I ₂ (t), and a leakage current corresponding to the equation (3) is detected from the output of the filter BP ₁ that passes the component of frequency ₁ .

Enter the detected output to the synchronous detector MULT _1, MULT ₂ each one input terminal, inputting the output of the oscillator OSC ₁ to (V ₁ sinω ₁ t) to other one of the input terminals of MULT1 MU
The output of the LT1 is output DC component ig ₁ corresponding to (8). The other input of MULT2 connects the oscillator OSC1 output to 90
゜ Input to the translator PS and obtained 90 得 translator PS output V ₁ cosω ₁ t
The output of the entering MULT2 DC component ics ₁ corresponding to (10) below is output.

On the other hand, a filter BP2 that passes the current transformer ZCT output through frequency ₂
To enter. A leakage current component corresponding to equation (12) is obtained at the output of BP2. Further, when the BP2 output is input to the rectifier circuit DET, the output can obtain a DC component io corresponding to the equation (13). By inputting ig ₁ , ic ₁ , io obtained as described above to the arithmetic circuit AU, and performing the arithmetic processing of the equation (19) using the known V ₁ , V ₂ , ω ₁ , ω ₂ , An output proportional to 1 / R is obtained as the output of the arithmetic circuit AU, and the insulation resistance can be measured.

In the above description to obtain a frequency _{2 ( »1)} comprising a signal voltage from the oscillator OSC _2. However, there is a device that demodulates and outputs an alarm signal to a ground line, such as the "monitoring signal transmission device" of Japanese Patent Application No. 59-231874 by the same applicant and the same inventor. Therefore it is also possible to use a carrier signal voltage of the frequency ₂ of the unmodulated when not modulated by the alarm signal data as a substitute for the output voltage of the oscillator OSC _2.

In the description of the embodiment, the voltages input to the other input terminals of the synchronous detectors MULT ₁ and MULT ₂ are V ₁ sinω ₁ t and V ₁ cos, respectively.
ω ₁ t, and a single-phase two-wire case is shown for simplicity of explanation.
The magnitude of the voltage V ₁ does not have to be limited, and may be any voltage. The connection is not necessarily limited to a single-phase two-wire, but is a single-phase three-wire or three-phase three-wire. Obviously, it can be implemented based on the same principle.

(Effects of the Invention) Since the present invention measures the insulation resistance of the electric circuit by the method described above, it is possible not only to completely cancel the influence of the ground resistance but also to reduce the influence of the output resistance of the oscillator and the like. Also, since the compensation is performed in consideration of the ground resistance, it has a remarkable effect in accurately measuring the insulation resistance of an electric circuit including an electronic circuit which tends to increase the stray capacitance to the ground.

Further, as is clear from the embodiment, the measuring circuit for realizing the measuring method of the present invention is extremely simple and can be supplied at a low cost, so that it is particularly effective when applied to an insulation monitoring system for electric lines in factories and the like. is there.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment for realizing the insulation resistance measuring method of the present invention, FIG. 2 is an equivalent circuit diagram of an insulation resistance of an electric circuit, and FIG. 3 is an equivalent circuit in consideration of a ground resistance. FIG. 4 is a view for explaining a conventional insulation resistance measuring method, and FIG. 5 is a block diagram showing an embodiment for realizing Japanese Patent Application No. 58-154564 "insulation resistance measurement direction". T …… Transformer, L ₁ , L ₂ …… Electric circuit, L _E …… Grounding wire, OSC ₁ , OSC ₂
…… Oscillator, MULT ₁ , MULT ₂ … Synchronous detector, FIL …… Filter, BP ₁ , BP ₂ …… Filter, PS …… 90 ° phase shifter, PAMP …… Power amplifier OT …… Injection transformer, ZCT …… Current transformer, DET …… Rectifier, AU …… Operation circuit, SUB… Subtraction circuit.

Claims (3)

(57) [Claims] 1. A low-frequency signal voltage having a frequency f1 and a low-frequency signal voltage having a frequency f2 higher than the frequency f1 are applied to an electric circuit, and leakage currents at the frequencies f1 and f2 which are fed back to the ground line of the electric circuit are individually detected. At the same time, the leak current of frequency f1 among the leak currents is synchronously detected by an in-phase signal of a low frequency signal of frequency f1 applied to the electric circuit to extract an effective component ig1, and the leak current of frequency f1 among the leak currents The low frequency signal of frequency f1 applied to the electric circuit is synchronously detected by a signal shifted by 90 ° to extract an invalid component ic1, and rectifies the effective component ig1, the invalid component ic1 and the leakage current of the frequency f2. A method of measuring insulation resistance in which ground resistance is compensated, wherein insulation resistance of an electric circuit is measured by performing a predetermined operation using the obtained signal io. 2. The method according to claim 1, wherein the low-frequency signal voltage of the frequency f2 is an unmodulated carrier signal voltage for data transmission. . 3. The method according to claim 2, wherein the predetermined operation is V ₁ was Patent input signal voltage of the frequency f1, V ₂ is the input signal voltage of the frequency f2, omega ₁ is 2πf1, ω ₂ is 2πf2, k is V ₁ ^2/2, R is characterized in that an insulation resistance 3. The method for measuring insulation resistance according to claim 1, wherein said insulation resistance is compensated for ground resistance.