JPH0340348B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement of a conventional method for measuring insulation resistance of a live circuit.

This method is disclosed in Japanese Patent Publication No. 55-19510 as an example of a conventional method for measuring the insulation resistance of a live circuit. In this method, a low-frequency voltage with a frequency considerably lower than the power supply frequency is applied to the object under test of a current-carrying electric device through a reference resistor inserted in series, and the voltage generated across the reference resistor is This method measures the insulation resistance by selectively amplifying only the voltage at which the voltage is applied, but according to this method, (i) it is necessary to insert a resistor in series with the circuit under test when applying the low-frequency voltage; There is. (ii) Even if a low frequency voltage lower than the commercial frequency is applied, the stray capacitance to the ground is unknown, so the influence of the stray capacitance on the measured voltage value is unknown.

The present invention is designed to measure the insulation resistance of a current-carrying electrical circuit by passing the grounding wire of the electrical circuit through a transformer to which a low-frequency voltage is applied or through the core of an oscillation transformer that oscillates a low-frequency voltage (simply through it or by winding it with several turns). ), a low-frequency voltage is applied to the electrical circuit, and this grounding wire is also passed through the zero-phase current transformer, thereby detecting the leakage current that returns via the ground stray capacitance or insulation resistance. The frequency components of the applied low frequency voltage included in the detected leakage current are selectively amplified and the insulation of the live circuit is measured using these voltage values.

This invention will be explained below with reference to Examples. 1st
The figure shows an embodiment of the present invention, in which a high voltage is applied to the primary side of a power transformer A, and a load Z is connected to the secondary side. The insulation resistance of the secondary circuit is indicated by R, and the stray capacitance to ground is indicated by C. The power transformer A is grounded through a second type grounding wire E2. Although the case of a single-phase two-wire electric circuit will be described here, the method of the present invention is not limited thereto, and can be similarly applied to single-phase three-wire, three-phase three-wire, etc.

The ground wire E2 is an oscillation circuit that oscillates a low frequency voltage.
It passes through the core of the OSC's oscillation transformer T or is wound several times through it. Windings N ₁ and N ₂ are for configuring an oscillation circuit. Also ground wire E ₂
also passes through the zero-phase current transformer ZCT, and as a result, ZCT receives commercial frequency components and the transformer T via insulation resistance R and ground stray capacitance C.
The leakage current of the low frequency voltage component can be obtained by The output of the zero-phase current transformer ZCT is amplified by an amplifier AMP, and then the amplifier AMP output is applied to a filter BPF ₁ that passes only the applied low frequency frequency component. By applying the output of the BPF ₁ to the rectifier circuit DET1, a voltage corresponding to the corresponding frequency component can be obtained. This voltage can indicate the insulation resistance. To explain in more detail, if the low frequency voltage applied to the electric line by the transformer T is V ₁ (the voltage between the ground and the electric line), when the relay contacts γl ₁ to γl ₃ are open, the output of the rectifier circuit DET ₁ voltage e ₁ is becomes. Here, the frequency of the low frequency voltage is assumed to be ₀ = ω ₀ /2π.

If the frequency ₀ is made sufficiently low or the stray capacitance C is small, 1/R≫ω ₀ c, and e ₁ can measure the insulation resistance without being affected by the stray capacitance.

Incidentally, the insulation resistance measured as shown in the formula includes an error when the stray capacitance to ground is large, but this error can be removed by the following method.
The grounding wire EL passes through the transformer T and the zero-phase current transformer ZCT as described above, but a new loop connecting wire LINK passes through the grounding wire in a direction opposite to that of the grounding wires. Capacitors C ₁ to C ₃ are connected and terminated to this connection line via relay contacts γl ₁ to γl ₃ .

By the way, when the total insertion capacitance when some of the contacts γl ₁ to _{γl 3} are turned on is C ₀ , since a current of the opposite phase flows through the zero-phase current transformer via the capacitance C ₀ ,
If the output of the rectifier circuit DET1 at this time is e′ ₁ , then becomes.

Therefore, in the formula, e' ₁ is minimum when C=C ₀ . In this way, if the capacitance C ₀ is set so that e' ₁ is close to the minimum value, the output e' ₁ of the rectifier circuit DET ₁ at this time will be V/R, so it is necessary to accurately measure the insulation resistance. I can do it. Then the capacity
To give an example of how to set C ₀ , compare the rectifier circuit output value e′ ₁ after increasing the capacitance C ₀ with the previous rectifier circuit output value e′ 1, and if e′ ₁ decreases after increasing the capacitance C 0, further increase the capacitance C ₀ . If e' 1 increases after C ₀ increases as shown in FIG. 2, e _{' 1 is} minimized by returning the capacitance C ₀ before that point.
In order to compare the values of e′ ₁ “before and after”, switch SW, capacitor CC, and buffer amplifier are used.
Configure a sample holder consisting of BF, and each time the capacitor C ₀ is changed stepwise, the value at that point is
It is controlled by pulses from the clock circuit so that the DET1 output value is memorized. The level comparator COMP compares the output of the rectifier circuit DET1 and the output of the buffer amplifier BF that has already been stored.
When the output is smaller than the buffer amplifier BF output, the clock pulse supplied from the clock circuit CLOCK, the level comparator COMP output, and the logic element.
A logical product is performed using AND, and counting is started using a binary counter COUNT consisting of, for example, 3 bits.
Each bit output of the counter drives the coils of relays RL ₁ to RL ₃ to which the power supply V _s is supplied, and operates the contacts γl ₁ to γl ₃ . That is, for example, when the capacitance of the capacitor connected to relay contact γl ₁ is C ₁ , the capacitance of the capacitor connected to the other relay contacts is C ₂ = 2C ₁ , C ₃ = 2 ² C ₁ = 4C ₁ Counting is continued until the level comparator output determines that the DET1 output is greater than the buffer amplifier BF output, and the value of the capacitor capacitance _C0 is successively increased in steps.
When it becomes larger, the counting is stopped and the counting is decreased by one step.

The output of the rectifier circuit DET1 at this time is the minimum value and corresponds to V ₁ /R according to the theory described above, so by measuring the output value of the rectifier circuit DET1 at this time, the voltage corresponding to the insulation resistance can be obtained. It will be done. Furthermore, if the counter COUNT is reset by the reset signal RESET generated by the clock circuit after a certain period of time, and the same operation is repeated again from the state where the insertion capacitance _C0 is set to 0, the rectifier circuit DET1 is reset by the above operation again. This means that a voltage corresponding to the insulation resistance can be obtained intermittently at the output without being affected by fluctuations in stray capacitance to ground. Further, if the output value of the rectifier circuit DET1 at this time is stored in a separately provided sample holder or the like (not shown), the insulation resistance value can be continuously indicated.

In the above explanation, the number of bits of the counter COUNT is 3.
This is a bit, but this can be expanded as necessary. In addition, although the setting of capacitors C ₁ , C ₂ , and C ₃ is based on the binary system, this is also done using the counter COUNT.
It is clear that the counter can be a decimal counter or the like, but is not limited thereto.

Furthermore, in the above embodiment, the grounding wire passes through the core of the transformer T and the zero-phase current transformer ZCT.
It is also clear that similar results can be obtained even if both or one of them passes through the secondary circuit. Furthermore, if these transformer T and zero-phase current transformer ZCT are of split core type, penetration to the grounding wire can be facilitated.

The method of the present invention not only solves the drawbacks of the conventional methods, but also allows insulation resistance to be measured in a live wire state extremely economically, and has great industrial value.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention, and FIG. 2 is a diagram showing an operation example of the present invention. A: power transformer, T: oscillation transformer,
ZCT: Zero-phase current transformer, AMP: Amplifier, BPF ₁ : Filter, DET ₁ : Rectifier circuit, E ₂ : Ground wire, C ₁ ,
C ₂ , C ₃ : Inserted capacitors.

Claims (1)

[Claims] 1. In measuring the insulation resistance of a current-carrying electrical circuit,
A low frequency voltage is applied to the electric line through a transformer connected to the ground wire, and only the low frequency voltage component is selectively amplified from the output of the current transformer connected to the ground wire and isolated by the voltage value. In a method for measuring insulation resistance of a live circuit that indicates resistance, a loop connecting wire coupled to the transformer is passed through the current transformer in a direction opposite to the grounding wire, and the loop connecting wire is connected to a switch. A simple method for measuring insulation resistance of a live circuit, characterized in that a plurality of series circuits of capacitors are inserted in parallel, and the switch is controlled so that the low frequency voltage output of the current transformer is minimized when measuring insulation resistance. . 2. A method for controlling the switch so that the low frequency voltage output of the current transformer is minimized includes means for rectifying the low frequency voltage output from the current transformer, and storing the DC voltage value obtained by rectification. Claim 1, characterized in that each time the switch is controlled, the voltage value stored before one sampling is compared in magnitude with the current sample value. A simple method for measuring insulation resistance of live circuits. 3. Control the switch to gradually increase or decrease the total capacitance inserted into the loop connection line, and when the magnitude relationship between the stored voltage value one sample before and the current sample value is reversed, 3. A simple method for measuring insulation resistance of a live circuit according to claim 2, wherein the capacitance is set to a capacitance value of .