JPH028529B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for removing phase errors when detecting leakage current in electrical circuits, etc. using a current transformer.

(Prior art) Conventionally, as a method of measuring the insulation resistance of a power line, etc. in a live line state, a low-frequency voltage for measurement is applied to the ground wire on the low-voltage secondary side of the power receiving transformer, and the insulation resistance of the low-voltage power line is applied to the ground wire. A method of measuring the insulation resistance of an electric circuit by detecting a leakage current that returns through a low resistance, ground capacitance, and detecting a component in phase with the measurement low frequency voltage component, that is, an effective component in the leakage current. is widely practiced.

In this case, in order to detect the leakage current, a current transformer (hereinafter abbreviated as ZCT) that passes through the grounding wire is used.
This is generally the case.

(Problem to be solved by the invention) However, when a relatively large current flows as the primary current of the ZCT, this current changes the magnetic core inductance characteristics of the ZCT, so the phase of the secondary winding current changes. This causes an error in phase detection. Therefore, there was a problem in that it was not possible to accurately measure the insulation resistance of the electric circuit.

In other words, the leakage current that returns to the grounding wire contains commercial frequency components and applied low frequency components, but generally the applied voltage as the measurement voltage is set to It is sufficiently low compared to the secondary voltage of the frequency (e.g. line voltage 100 or
The applied voltage for measurement is about 2 to 3 V compared to 200 V).
Therefore, the current situation is that the commercial frequency leakage current is significantly larger than that of the low frequency component mentioned above, and for this reason, the phase of the low frequency component in the ZCT secondary winding output changes as the commercial frequency component increases or decreases. Large fluctuations are observed. On the other hand, as mentioned above, when detecting the effective component in the same phase as the applied voltage among the low frequency components, such phase fluctuations will directly affect the measured value of insulation resistance as an error, so ZCT Accurate measurement was impossible due to large phase error fluctuations in the output.

The influence of magnetic core inductance when a large current flows through a current transformer will be explained below.

Figure 3 shows the secondary winding (n-turn) output of the ZCT that passes through the line I through which the current I is flowing, and connects it to the resistor _R0.
This shows the case where the terminal is terminated at . If this is shown as an equivalent circuit, it will be as shown in FIG.

In the same figure, L is the inductance of ZCT, R
is the effective resistance and the voltage V ₀ obtained at the terminating resistor R ₀
is V ₀ =I〓/n R ₀ X _L /X _L +R+R ₀ (1) X _L =jωL. Further, the phase difference φ between V〓 ₀ and I〓 is obtained from equation (1) as follows: φ=tan ⁻¹ R+R ₀ /ωL (2). Therefore, if the inductance L changes due to the influence of the hysteresis of the magnetic core of the ZCT due to the current I, the phase φ will change.

Furthermore, although it is clear that R ₀ in equation (2) can be made zero by converting current to voltage, it is difficult to make R zero.

FIG. 5 shows an example of actual measurement of the phase with respect to changes in current I, and it can be seen that in this case, the phase changes over several degrees due to changes in current. In this way, when the range of change in the primary current of ZCT is large, the phase fluctuation also becomes large.

Therefore, by reducing the variation width of the primary current, the phase fluctuation can be reduced.

(Means for Solving the Problem) The present invention suppresses fluctuations in the magnetic core inductance of a current transformer used in the conventional measurement of insulation resistance, etc. of electrical circuits, etc., as described above, and achieves more accurate measurement. It has the following structure.

That is, in order to detect the leakage component of the low-frequency voltage for measurement, which is used in the conventional method of measuring the insulation resistance of electric circuits, a new connecting wire is passed through the current transformer that passes through the grounding wire, and a separate connection wire is installed. A current component having approximately the same level and opposite phase as a frequency current component other than the measurement low-frequency voltage that affects the current transformer is derived by means of the above-mentioned means, and is connected to the current transformer via the connection line. The configuration is such that the voltage is applied to the primary side.

At this time, any means may be used to derive a current component having the same level and opposite phase as the above-mentioned unnecessary current component.

(Function) Since the present invention is configured as described above,
Among the current components flowing back to the grounding wire, components other than the low frequency voltage for measurement are canceled out on the primary side of the current transformer, and these currents do not affect the magnetic core of the current transformer ZCT. Therefore, conventionally, the central inductance of the magnetic core of the current transformer ZCT is swung by the commercial frequency current or other current flowing back to the ground wire as a relatively high current, causing an error in the phase of the low frequency current for measurement to be detected. Further, since these unnecessary voltages are not generated at the input of the amplifier connected to the secondary side of the current transformer, subsequent signal processing can be made extremely easy.

(Example) The details of the present invention will be explained below based on the illustrated example.

FIG. 1 is a block diagram showing one embodiment of the present invention. In the figure, the low voltage line 3 of the power receiving transformer 2
and 4, for example, 4 is connected to the ground via a grounding wire 5, and a transformer 6 to which a signal oscillator OSC is connected in order to apply a low frequency voltage for measurement to the electric line, and the low frequency voltage for measurement A first current transformer ZCT ₁ for detecting leakage components from the electric circuit due to frequency voltage and a second current transformer ZCT ₂ for detecting components other than the measurement low frequency voltage are connected to the ground wire. a bandpass waveform generator that passes through each of the secondary windings of the first current transformer to the amplifier AMP ₁ , and extracts only the low frequency voltage for measurement from the output of the secondary winding of the first current transformer;
It is input to the synchronous detector MUL via the BPF, and the product output of this and a part of the output of the low frequency oscillator OSC obtained through the phase shifter PS is obtained at the terminal 7.

Further, the secondary winding output of the second current transformer ZCT ₂ is sent to the amplifier AMP ₂ , and its output is sent to the band-stop waveform generator.
Components other than the low frequency voltage for measurement are extracted via BEF ₁ , the necessary amplification is performed using AMP ₃ , and this is connected to the resistor.
Connection line 8 coupled to said ZCT ₁ via R ₁ and R ₂
to be applied. At this time, the current applied to ZCT ₁ is made to have approximately the same level and opposite phase as the current of the commercial frequency component flowing through the connection line, so that these currents cancel each other out.

In the circuit for measuring the insulation resistance of an electric path configured in this way, the insulation resistance R ₃ that exists between the electric path and the ground is measured.
and the measurement low frequency voltage V ₁ [V] applied to the circuit via R ₄ and stray capacitances C ₁ and C ₂ and the circuit voltage V ₀
[V] (commercial frequency ₀ ) If each voltage waveform is a sine wave, the leakage current I that returns to the grounding wire 5 is generally expressed by the following equation.

I=(√2V ₀ /R ₁ ) sinω ₀ t+√2V ₀ ω ₀ C ₁ cosω ₀ t+√
2V ₁ {1/R ₃ ) + (1/R ₄ )}sinω ₁ t +√2V ₁ ω ₁ (C ₁ +C ₂ )cosω ₁ t ...(3) Also, from this current, ZCT ₂ and AMP ₂ , BEF ₁
The component i ₀ other than the measurement low frequency voltage ₁ applied to the connection line 8 via the above-mentioned route consisting of AMP ₃ and AMP 3 is calculated and can be expressed by the following equation.

i ₀ = (√2V ₀ a ₀ /R ₁ ) sin (ω ₀ t + θ ₀ ) + √2V ₀ ω ₀ C ₁ a ₀ cos (ω ₀ t + θ ₀ ) ...(4) However, a ₀ ; coefficient, θ _{0 ;} Phase shift due to ZCT ₂ Here, in the above equation (4), AMP ₂ −
Adjust with the system consisting of BEF ₁ - AMP ₃ , and further connect the above-mentioned connection wire 8 in ZCT ₁ so that the current flowing through the ground wire 5 is in reverse phase to detect the low frequency voltage ₁ for measurement. _{The secondary current of current transformer ZCT 1 to _ _ _} {cosω ₀ t−a ₀ cos (ω ₀ t+θ ₀ )} +√2V ₁ {(1/R ₃ +1/R ₄ ) sinω ₁ t +√2V ₁ ω ₁ (C ₁ +C ₂ ) cosω ₁ t …… (5) Here, since a ₀ ≈1 and the phase shift between the two is 1 to 2 degrees, it is possible to make the first and second terms of the above equation (5) small.

Therefore, the commercial frequency component is removed from the output of the circuit that detects the low frequency voltage for measurement, or becomes an extremely small value, which not only facilitates subsequent signal processing, but also makes it possible to reduce the amount of commercial frequency components caused by the comparatively large commercial frequency current. It is possible to accurately detect the phase amount of the measurement signal by removing fluctuations in the magnetic core inductance of the current transformer.

Note that a voltage proportional to 1/R ₁₀ +1/R ₂₀ can be obtained from the above-mentioned synchronous detector MUL output, and the insulation resistance of the electric circuit can be measured. These relationships are detailed in, for example, Japanese Patent Application No. 54-142465, so their explanation will be omitted.

FIG. 2 is a block diagram showing another embodiment of the present invention, and the same symbols as in FIG. 1 have the same meanings. In this embodiment, one ZCT is used to realize the same function as the previous embodiment.

That is, the ZCT ₃ output passed through the ground wire 5 is amplified by the amplifier AMP ₄ , and one of its outputs is
Connect to filter BEF ₂ which removes _one component. This BEF ₂ output is amplified by an amplifier AMP ₅ and coupled to ZCT ₃ passed through the connection line 5 via resistors R ₅ and R ₆ . Thus, the commercial frequency component _{is 0.}
By configuring a feedback circuit for the connection line so that the commercial frequency component flowing through the connection line is in reverse phase with the commercial frequency component flowing through the grounding wire 5.
It is possible to act on the primary current of ZCT ₃ so that the commercial frequency component becomes extremely small, thereby making it possible to reduce the commercial frequency component of the amplifier AMP output. The output terminal of the amplifier AMP ₄ is further connected to _one frequency component for application and measurement in order to detect the frequency component.
In addition to the filter BEF having a passband of only ₁ , the subsequent configuration is the same as the embodiment shown in FIG. 1, and a voltage inversely proportional to the insulation resistance can be obtained at the multiplier MUL output 10.

It should be noted that the present invention is not necessarily limited to the embodiments shown above, and can be applied to any device that uses a current transformer to measure or observe the insulation resistance of an electrical circuit. It will be obvious that the method of carrying out the invention is not limited to the above embodiments, and that other methods may be used.

(Effects) As described above, the present invention provides a method for suppressing fluctuations in the phase of the applied low frequency component in the ZCT output due to the magnitude of the commercial frequency component in the primary current of the current transformer. Since this method proposes a method for suppressing the leakage current of commercial frequency components, it is extremely effective in improving insulation resistance accuracy under live wire conditions.

It is clear that this method also has the effect of eliminating noise in the leakage current that returns to the ground line through the cancellation process.

Note that the method for detecting a voltage that is inversely proportional to the insulation resistance from the applied low-frequency component in the leakage current is not limited to the method described in the embodiments of the present invention, and other methods can also be applied. It is clear that

[Brief explanation of drawings]

FIG. 1 is a block diagram showing one embodiment of the present invention;
FIG. 2 is a block diagram showing another embodiment of the present invention;
Figure 3 is a diagram explaining the operation of ZCT, Figure 4 is
FIG. 5 is a diagram showing an equivalent circuit of ZCT, and is a diagram showing an example of measurement results of the primary current and phase shift of ZCT. ZCT, ZCT ₁ , ZCT ₂ and ZCT ₃ ...Current transformer, R ₃
and R ₄ ... insulation resistance, C ₁ and C ₂ ... ground capacitance, L ... inductance, AMP, AMP ₁ ,
AMP ₂ , AMP ₄ and AMP ₅ ...Amplifier, _R0 , _R1 to _R6 ...Resistor, OSC...Oscillator, 1...Grounding wire,
2...Power receiving transformer, 3 and 4...Electric circuit, 6...
Low frequency voltage application transformer for measurement, BPF... filter, BEF ₁ and BEF ₂ ... ₁ component removal filter,
MUL... Multiplier (detector), 5 and 8... Connection lines.

Claims (1)

[Claims] 1. Commercial frequency is connected to the electric line via the grounding wire of the transformer, etc.
A current transformation in which a low frequency voltage for measurement with a frequency ₁ different from ₀ is applied by electromagnetic induction or series coupling, etc., and a leakage current of the low frequency component ₁ for measurement flowing back to the ground wire is passed through the ground wire. Detected by a device,
In the method of measuring the insulation resistance, etc. of an electrical circuit, a current component other than the measurement low frequency component ₁ flowing back into the ground wire is derived, and a new connection wire is passed through the current transformer, and the Derived frequency ₁
By applying current components other than the measurement low frequency component 1 to the current transformer in reverse phase with respect to the current components other than the measurement low frequency component ₁ flowing through the connection wire and the ground wire, the secondary current component of the current transformer is A method for removing a phase error in a current transformer, characterized in that current other than a low frequency signal for measurement generated in a winding is suppressed. 2. Current transformation according to claim 1, characterized in that the means for deriving the voltage other than the measurement low frequency signal ₁ uses a second current transformer that passes through the grounding wire. A method for removing phase errors in the device. 3. The means for deriving a voltage other than the low frequency signal for measurement ₁ shares the current transformer provided for detecting the low frequency signal for measurement ₁ coupled to the ground wire and the amplifier connected to the current transformer, and the amplifier A part of the output of the current transformer is connected to a second amplifier via a filter that blocks only the low frequency wave for measurement, and this output is applied to a connecting line coupled to the current transformer. A method for removing a phase error in a current transformer according to claim 1.