JPH0552466B2 - - Google Patents

Description

[Detailed Description of the Invention] (Field of Industrial Application) The present invention relates to a method of compensating for temperature changes or secular changes in circuit constants of a device that measures the insulation resistance and ground stray capacitance of electric circuits etc. in a live line state. .

(Prior Art) Conventionally, it has been common to use a method of measuring the insulation resistance of an electrical circuit as shown in FIG. 1 for early detection of troubles in electrical circuits such as leakage of electricity.

This is the second of the receiving transformer T with a load Z.
Connect the grounding wire L _E to a frequency different from the commercial power frequency1 _.
A low frequency signal for measurement is supplied to the circuit L ₁ and the circuit L ₂ by passing it through the transformer OT connected to the low frequency signal oscillator for measurement OSC, or by cutting the ground wire and connecting the oscillator in series with it. Apply voltage,
A current transformer ZCT passing through the grounding line L _E detects the leakage current that returns to the grounding line via the insulation resistance Ro and ground stray capacitance Co existing between the electric line and the ground, and the leakage current is detected by the amplifier AMP. After amplifying,
In addition to the filter FIL, only the frequency ₁ component is selected, and the effective component in the leakage current (i.e., the component in phase with the applied low-frequency voltage for measurement) is detected, and this is detected using, for example, the output signal of the oscillation canceling OSC. It was configured to perform synchronous detection using a multiplier MULT to measure the insulation resistance of the electrical circuit.

To further explain the measurement theory, if the measurement signal voltage applied to the ground line L _E is a sine wave, for example, Esinω ₁ t (ω ₁ = 2π ₁ ), the signal is returned via the ground point E. The leakage current I at frequency ₁ is I=V/Rosinω ₁ t+ω ₁ CoVcosω ₁ t (1), so the component in phase with the applied AC voltage, that is, the first term on the right side of equation (1) above, If the proportional value was detected by means such as synchronous detection, a measured value inversely proportional to the insulation resistance Ro could be obtained, and from this, the insulation resistance value of the electrical circuit could be determined. However, in this way, the leakage current that returns to the ground wire can be absorbed by the current transformer.
If the leakage current component of frequency ₁ included in the output of the current transformer is detected by ZCT and then selectively outputted by filter FIL, the phase of the leakage current of frequency ₁ will always shift in the current transformer → amplifier → filter system. , in order to synchronize these, it is necessary to compensate for this phase shift, and for this purpose, a phase shifter PS is inserted into the first input terminal or second input terminal of the synchronous detector. They compensated for phase shifts and synchronized with each other.

In other words, by providing this phase shifter PS, the phase difference between the voltages applied to the first and second input terminals of the synchronous detector becomes zero when there is no stray capacitance Co to the ground (Co = 0). It was set in advance and fixed.

However, in the conventional method as described above, if the phase characteristics at frequency ₁ of the current transformer ZCT, filter FIL, etc. change due to temperature changes or secular changes in the characteristics of the parts used, a phase error with the initial adjustment value will occur. However, there was a drawback that it could not provide accurate measurement results. In order to deal with these problems, extremely high-quality zero-phase current transformers or filters with little variation in characteristics have traditionally been required, but even with these, it has been difficult to eliminate the effects of phase errors. .

(Object of the Invention) The present invention has been made to solve these drawbacks, and is a method for compensating the phase difference of components used in an insulation resistance measuring device without using a high-quality current transformer or filter. An object of the present invention is to provide a compensation method for an insulation resistance measuring device that can perform accurate insulation resistance measurements.

(Summary of the Invention) In order to achieve the above object, the present invention applies a measurement low frequency signal voltage having a frequency ₁ different from the commercial frequency to an electric line, and reduces the leakage current of the frequency ₁ returning to the grounding line by a first change. A second current transformer and a first filter having the same characteristics as the first current transformer and the first filter are detected from the current flowing to the load connected to the low frequency signal source for measurement. 2
the frequency through a current transformer and a second filter.
By detecting the component ₁ and measuring the insulation resistance using the second filter output and the first filter output, it is possible to detect phase characteristic fluctuations due to fluctuations in the circuit constants of current transformers and filters. Take steps to limit the impact.

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

First, before explaining the surveying method according to the present invention, the shortcomings of the conventional method will be explained in some detail to aid understanding.

Leakage current component I at frequency ₁ shown in equation (1)
If θ is the phase shift generated in the system of current transformer ZCT, amplifier AMP, and filter FIL, then filter FIL output I ₁ is I ₁ = V/Rosin (ω ₁ t + θ) + ω ₁ CoVcos (ω ₁ t + θ) ... (2), which is applied to the first input terminal of the synchronous detector MULT.

Furthermore, if the voltage applied to the second input terminal of the synchronous detector is, for example, _ap sin (ω ₁ t + θ ₁ ) with a constant amplitude, then
The DC component D obtained from the output of the synchronous detector is D = ₁ × _p ( ₁ + ₁ ) ... (3) (- means DC component) = Va _p / 2 Rocos (θ - θ ₁ ) − ω ₁ C _p Va _p sin (θ−θ ₁ ) ...(4) Therefore, when θ = θ ₁ , the DC output Do is Do = Va _p /2Ro ... (5) and V and a _p are constant. It is possible to measure a value that is inversely proportional to the insulation resistance Ro. Therefore, when the phase shift θ-θ ₁ is not zero, the error E of D with respect to the above Do is E=Do-D/Do=1-cos(θ-θ ₁ ) +ω ₁ CoRosin(θ-θ ₁ )... (6) becomes.

Now, for example, when θ−θ ₁ = 1 (degrees), f ₁ in equation (6)
= 25Hz, Ro = 20KΩ, and Co = 5μF, ω ₁ CoRo = 15.7, so the error ε is 27.4%, which means that the measurement error becomes significantly large.

Furthermore, the phase variation characteristics of the current transformer ZCT with respect to temperature variations, for example -10 to 60 degrees Celsius, range as much as ±1 degree at f ₁ =25 Hz, further increasing the measurement error.

The present invention proposes a method for suppressing the occurrence of errors due to the above-mentioned phase shift as much as possible.

FIG. 2 is a circuit diagram showing an embodiment of the insulation resistance measuring method according to the present invention, and the same symbols as in FIG. 1 have the same meanings.

In the figure, an oscillator OSC for generating a low frequency of frequency ₁ is connected in series to the ground line L _E via a transformer OT, and a voltage V is applied to the ground line L E. At this time, the impedance of the transformer inserted in series with the ground wire is selected to be sufficiently low. Furthermore, the current transformer ZCT output is amplified.
By inputting the amplifier output to the AMP and applying it to the filter FIL that passes the frequency ₁ component and removes the commercial frequency component, an output corresponding to equation (2) can be obtained,
This is applied to the first input terminal 1 of the synchronous detector MULT.

On the other hand, if, for example, a resistor R is connected to the secondary side of the transformer OT for applying a low-frequency signal, and a constant current i is caused to flow, i=V/Rsinω ₁ t (7). This current i is detected by a second current transformer ZCT 1 having the same characteristics as the current transformer ZCT, and is transferred to a second current transformer ZCT ₁ having the same characteristics as the amplifier AMP.
AMP ₁ and its output is applied to a second filter FIL ₁ having the same characteristics as the filter FIL, the output i ₁ of the filter FIL ₁ is i ₁ = aV/Rsin (ω ₁ t + θ')... …(8) becomes. where θ′ is from current transformer ZCT ₁ to filter
This is the phase shift at frequency ₁ up to FIL ₁ output, and generally θ≒θ'. This is current transformer ZCT ₁ and ZCT,
This is because the characteristics of the amplifiers AMP ₁ and AMP and the filters FIL ₁ and FIL are almost the same. Also, a is the gain of these systems.

Therefore, the output of filter FIL ₁ is transferred to phase shifter PS
In addition, by setting θ=θ and applying the output of the phase shifter PS to the second input terminal of the synchronous detector MULT, the output OUT ₂ of the synchronous detector MULT
From a calculation similar to the relationship in equation (3), D ₁ = aV ² /2R (9) can be obtained. Here, since V and a are constant, the insulation resistance can be measured by knowing the value of the output OUT. To explain these relationships in more detail, due to temperature fluctuations, the phase θ included in equation (2) changes to θ + ε ₁ , and the phase θ′ included in equation (8) changes to θ′ + ε′. Then, the phase difference between them is θ+ε ₁ − (θ′+ε′)=θ−θ′+ε ₁ −ε′, but the phase shifter PS is set so that θ−θ′=0.
Since the phase shift ε ₁ −ε′ _is set at If selected, ε ₁ −ε′ becomes extremely small, and the occurrence of an error can be ignored.

In addition, if you can use a filter FIL whose phase characteristics vary little due to temperature, it is economical because you do not need to insert the filter FIL _1. In this case, the fixed phase shift of the filter FIL can be replaced by a phase shifter. It is clear that adjustments can be made in PS.

As explained above, according to the method of the present invention, a system with the same characteristics as the system of current transformer ZCT → filter FIL is provided in current transformer ZCT ₁ → filter FIL ₁ , so the temperature of the total phase shift of each system , the fluctuation due to aging can be relatively the same, and the phase shifter PS
By making only the fluctuations small, it becomes possible to offset each fluctuation. Note that a highly stable phase shifter can be easily realized by connecting multiple stages of phase shifters with low positional shift, and even if there is a phase fluctuation due to constant fluctuations of the component parts of each phase shifter (not shown). Fluctuations in the overall phase characteristics can be made extremely small. Figure 3 is a circuit diagram showing another embodiment in which the measurement signal voltage is detected by a zero-phase current transformer. Terminate the separate winding with a resistor R,
The current flowing through this is detected by current transformer ZCT ₁ .

FIG. 4 is a circuit diagram showing another embodiment of the present invention, in which a separate winding provided in a transformer OT passing through a grounding wire L _E is terminated with a resistor R, and the current flowing through this is detected.

In this embodiment, a single-phase, two-wire case is shown for ease of explanation, but the present invention is not necessarily limited to this, and the same applies even in the case of a single-phase, three-wire system or a three-phase, three-wire system. It is clear that it can be implemented based on the principle, and although the grounding wire was cut and inserted when applying the low frequency signal, it may also be passed through the transformer connected to the oscillator, and the synchronous detector MULT If the phase of the voltage applied to the second input terminal of the synchronous detector is shifted by 90 degrees from the above example, a value proportional to the stray capacitance to ground can be obtained from the output of the synchronous detector. It is also possible to measure stray capacitance.

Although not shown in this embodiment, the absolute value of the output I ₁ of the filter FIL expressed by equation (2) is detected separately by a rectifier circuit, and the phase difference δ between the output of the filter FIL and the output of the filter FIL ₁ is detected, and by calculating _| I ₁ |cosδ, a value proportional to the reciprocal of the insulation resistance Ro can be obtained. Since this method can be obtained, it can also be used as a compensation method for circuit phase fluctuations in insulation resistance measurement, similar to the method described above.

Furthermore, in the above explanation, a zero-sequence current transformer is used to detect zero-sequence current.
Although ZCT is used, the present invention does not need to be limited to this in any way, and for example, without using a zero-phase current transformer ZCT,
Cut the grounding wire L _E and insert a low resistance in series with it,
The voltage value across this resistor may be detected. However, in this case, the electric phase current transformers ZCT and ZCT ₁ in the embodiment shown in Fig. 2 are removed, and the filters FIL and FIL _{1 are} replaced.
All you have to do is leave only the

Furthermore, although the measurement signal voltage has been described as a sine wave, it is not limited to this. For example, it may be a rectangular wave, and its fundamental wave component or high frequency component may be used.

(Effects of the Invention) Since the present invention is constructed and functions as described above, it is possible to detect the leakage component of the applied signal of the signal voltage for measurement in a circuit for measuring the insulation resistance, relative stray capacitance, or resistance of the electric circuit. The device is configured to perform the various measurements using the measurement signal obtained through a similar circuit having the same characteristics as the amplifier, filter, etc. used in the circuit, and is subject to changes in temperature or aging of the parts used. Since the changes in the characteristics of the measurement circuit due to the above cancel each other out, it is very effective in accurately measuring various characteristics of the electric circuit at a very low cost.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing a conventional method for measuring insulation resistance. FIG. 2 is a block diagram showing one embodiment of the invention, and FIGS. 3 and 4 are block diagrams showing other embodiments of the invention.

Claims (1)

[Claims] 1. A low frequency measurement signal having a frequency _f1 is applied to an electrical line whose one end is grounded via a grounding line, and the grounding line is connected via a current transformer, an amplifier, and a filter coupled to the grounding line. By deriving the low frequency signal that flows back to the ground line and synchronously detecting this low frequency signal using the in-phase component of the applied signal, a signal that is inversely proportional to the ground insulation resistance of the electric line can be generated. A method of measuring the insulation resistance or grounding capacity of the electrical circuit by synchronously detecting the leakage current flowing back into the circuit using a signal whose phase has shifted by 90 degrees from the applied signal to detect a signal proportional to the grounding capacity of the electrical circuit. , the reference signal supplied to the synchronous detection means is inputted through a second current transformer, an amplifier, and a filter having substantially the same characteristics as those used for deriving the reflux signal, thereby performing a synchronous detection. A compensation method for a measuring device for insulation resistance, etc., characterized in that a phase shift between two signals is prevented. 2. A phase shifter is inserted into a system consisting of a second current transformer, an amplifier, and a filter that supplies a reference signal to the synchronous detector, and the phase between the feedback low frequency signal input to the synchronous detector and the reference signal is adjusted. 2. A method for compensating an apparatus for measuring insulation resistance, etc. according to claim 1, characterized in that the relationship can be adjusted to a desired value. 3. Apply a low frequency signal for measurement with a frequency of f ₁ to the electrical line whose one end is grounded via the grounding wire, and return the signal to the grounding wire via the current transformer, amplifier, and first filter coupled to the grounding wire. Deriving the low frequency signal and determining the absolute value |I ₁ | of this low frequency signal,
A part of the low frequency signal applied to the ground wire is supplied to a series circuit of a second current transformer, an amplifier and a filter having the same characteristics as those used for deriving the freewheeling signal, and the output of the second filter is Detect the phase difference δ between and the first filter output signal, |I ₁ |・cosδ
1. A method for compensating an insulation resistance, etc., measuring device, characterized in that a value inversely proportional to the ground insulation resistance of the electrical circuit is determined by the following calculation. 4. A resistor is inserted in place of the current transformer connected to the grounding wire, and the reflux component of the low frequency signal is derived from both ends of the resistor, and the second current transformer is removed. A method of compensating for an apparatus for measuring insulation resistance, etc., according to claim 3.