JPS6240663B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for measuring live wire insulation resistance. (Prior Art) A measuring device called a megger is generally used to measure the insulation resistance of electrical equipment, power transmission lines, etc., but it cannot be used to measure live lines. A conventional method for measuring insulation resistance in a live line state is to superimpose an AC voltage at a frequency lower than the commercial frequency onto the circuit under test and detect the feedback current.
Since the feedback current includes a component that passes through stray capacitance, it cannot be applied to live circuits with large stray capacitances, and the adoption of this method is limited to non-grounded circuits. FIG. 1 is a block diagram showing a conventional measuring device. Regarding the method of the prior art, for example, there is Patent Application Publication No. 53-79578, which will be explained as a conventional example. In FIG. 1, TR1 is a transformer connected to an AC power supply, and PWS is a DC power supply circuit for operating each component. OSC is an oscillator that generates low frequency voltage for measurement, TR2 is a transformer, RO is a current detection resistor, F1 is a filter, AP1 is an amplifier,
M indicates indicator. The secondary side of the current detection resistor RO and the transformer TR2 is inserted and connected to the grounding line EL of the grounding system. The grounding line of a grounding system means a line used for grounding in a 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. In Figure 1, the output of the oscillator OSC1 is sent to the grounding line EL of the grounding system as a low-frequency measurement voltage through the transformer TR2, and the current I flowing through the grounding line EL is detected by the resistor Ro, and the measured voltage is The insulation resistance of the electrical circuit is measured by the magnitude of the output of the filter F1 that passes only frequency components. However, the current I includes a current due to stray capacitance C in addition to the current flowing through the insulation resistor R, and the conventional technique shown in FIG. 1 has the disadvantage that measurement accuracy deteriorates significantly when there is a large amount of stray capacitance. (Objective of the Invention) The present invention provides an insulation resistance measurement method that is capable of measuring insulation resistance in a live wire state, regardless of ground system stray capacitance, and that also enables measurement of ground system stray capacitance if necessary. The purpose is to (Summary of the Invention) In order to achieve this object, in the present invention, two frequency signals (W ₁ and W ₂ ) different from the commercial frequency are applied to the electric line via a grounding wire connected to the electric line, By detecting the voltage proportional to the leakage current of each frequency component and using the DC components e ² ₁ and e ² ₂ obtained by _square ^- law detection of each ^voltage _,
The calculation ² ₁ e ² ₂ is performed, and the insulation resistance of the electrical circuit, which is related to the ground stray capacitance, is measured based on the result. (Example) Hereinafter, the present invention will be described in detail based on the illustrated example. FIG. 2 is a block diagram showing one embodiment of the present invention. In the figure, the same symbols are used for the same components and objects to be measured as used in FIG. 1. In the present invention, two low frequency oscillators, OSC1 and OSC2, are used. Let the respective angular frequencies be W ₁ and W ₂ . At the output end of transformer TR2, the amplitudes of both frequency components are assumed to be equal, the low frequency signal voltage of frequency W ₁ sent from transformer TR2 is e ₀ sinω ₁ t, and the impedance to ground consisting of insulation resistance R and capacitance C is If Z ₁ , the low-frequency signal voltage V ₁ across Ro is generally |Z ₁ |≫Ro, so it can be approximated as V ₁ 〓Ro/|Z ₁ | e ₀ sin (ω ₁ t + φ ₁ ) …(1) can. Here, Z ₁ =R/1+jW ₁ CR φ ₁ =-tan ^-1 W ₁ CR ...(2). Therefore, since the output signal of filter F1 with central angular frequency W ₁ is expressed by equation (1), its amplitude component e ₁
is e ₁ = Roe _p / | Z ₁ | = Roe _p / R√1 + ( ₁ ) ² ... (3) Similarly, the output signal of filter F2 with center angular frequency W ₂ is V ₂ = Roe _p / | Z ₂ | sin(W ₂ t + φ ₂ ) (here Z ₂ = R/1 + jW ₂ CR, φ ₂ = - tan ^-1 W ₂ CR), so its amplitude component e ₂ is e ₂ = Roe _p / | Z ₂ |=Roe _p /R√1+( ₂ ) ² ...(4). Amplitude components e ₁ and e ₂ in equations (3) and (4) are obtained by rectifying the outputs of both filters. For example, if we square e ₁ and e ₂ and take the difference, we get If W ₁ 〉W ₂ When calculating by substituting equation (5) into equation (3) or (4), Therefore, the insulation resistance R can be measured. In this embodiment, the low frequency signal of the filter F1 output is
By passing V ₁ directly through the square-law circuit SQ1 and passing the DC component obtained by square-law detection through LPF1, e ² ₁ is equivalent to rectifying the filter output as described above and squaring the DC component e ₁ . We are looking for a voltage proportional to . Similarly, the low frequency signal of filter F2 output is converted to square circuit SQ2.
By passing the direct current through LPF2, e ² ₂
can be found. By multiplying the thus obtained e ² ₁ by a constant (W ² ₂ ), W ² ₂ e ² ₁ is obtained, and e ² ₂
By multiplying e 2 2 by a constant (W ² ₁ ), we get W ² ₂ e ² ₁ , and by multiplying e ² ₂ by a constant (W ² ₁ ), we get W ² ₂ e ² ₂
is obtained. Subtraction circuit SU for the difference between W ² ₂ e ² ₁ and W ² ₁ e ² ₂
When taken at B, W ² ₂ e ² ₁ − ^{W 2} ₁ e ² ₂ is obtained as the SUB output,
By passing this SUB output through the square root circuit Root, √ ² ₂ ² ₁ − ² ₁ ² ₂ is obtained. And this Root
By dividing the "constant value" Roe _p √ ² ₁ − ² ₂ by the output, the right side of equation (8) is calculated, so the insulation resistance R of the electric circuit can be determined. Also, W ² ₂ e ² ₁ − W ² ₁ e ² ₂ of the subtraction circuit SUB output is 1/R ²
It is clear from equation (7) that the value is proportional to . As explained above, according to the method of the present invention, the measured insulation resistance is not affected by stray capacitance at all. Also, as can be seen from equation (6), LPF1 output e ² ₁ and
Take the difference ^{e 2 1} _{- e} ² ₂ from the LPF2 output e 2 2, find the square root of it, and multiply it by a constant.

It is also possible to obtain the stray capacitance by using the following formula. In the above embodiment, the low frequency signals having angular frequencies W ₁ and W ₂ are sent out simultaneously via the TR 2, but this need not be done simultaneously. A similar calculation becomes possible by adding a circuit that alternately sends W ₁ and W ₂ for a certain period of time and holds the detected signal. Furthermore, although analog processing has been shown in the above embodiments, it is clear that the present invention can be realized by digital arithmetic processing. (Effects of the Invention) According to the method of the present invention, insulation resistance can be measured with high precision without the influence of stray capacitance C.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a conventional insulation resistance measuring device. FIG. 2 is a block diagram of the insulation resistance measuring device of the present invention. TR1...Transformer, PWS...Power supply circuit, TR
2...Transformer, _Rp ...Current detection resistor, F1,
F2...Filter, AP1...Amplification, M...Indicator, SQ1, SQ2...Squaring circuit, LPF1, LPF2
...Low pass filter for DC component detection, OSC1,
OSC2...Low frequency oscillator, W ² ₂ , W ² ₁ , R ₀ eo√ ²
₁
-W ² ₂ ...Constant circuit (amplifier or attenuator), R...
...Insulation resistance, C...Stray capacitance, EL...Grounding wire for grounding system.

Claims (1)

[Claims] 1. A frequency different from the commercial frequency is connected to the power line via the grounding wire.2.
two low-frequency signals (angular frequencies W ₁ , W ₂ and W ₁ 〉
W ₂ ) is applied; the angular frequency components are detected through a filter that extracts each of the angular frequencies W ₁ and W ₂ from the current flowing through the ground wire by the low frequency signal, and these currents are squared. Detection is performed to obtain DC voltages e ² ₁ and e ² ₂ for each frequency component, and further W
² ₂ e ² ₁ - W ² ₁ e ² _{2 An insulation resistance measuring method characterized in that the insulation resistance of the electric circuit is measured by performing the calculation: 2 2 e 2 1 −W 2 1 e 2 2} .