JPH0312257B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a method of measuring insulation resistance of an electrical circuit under live conditions.

By passing the grounding wire of the transformer through the core of the oscillation transformer that oscillates the low-frequency voltage that is the measurement signal or the transformer to which low frequency is applied, the low-frequency voltage is applied to the electrical circuit through the grounding wire. There is a method for measuring the insulation resistance of electrical circuits by using a zero-phase current transformer to detect the leakage current of the low frequency component that feeds back to the current. Consists of electric current. Therefore, to measure insulation resistance, it is necessary to detect the effective current in the leakage current.
Generally, the reactive current is significantly larger than the active current, which causes a large error in detecting the active component. The present invention proposes a method of suppressing such reactive current in an extremely simple manner.

FIG. 1 shows a method for measuring insulation resistance. Let the load of transformer T be Z. Here, for ease of explanation, a single-phase, two-wire electric circuit is shown, but the principles described below can be similarly applied to single-phase, three-wire and three-phase, three-wire cases. Let the insulation resistance of the electric path be R, and the stray capacitance to ground be C.

The grounding wire EL is an oscillation transformer OT that oscillates the frequency f ₁ (different from the commercial frequency f ₀ ), which is the measurement signal.
or through the core of the transformer to which a voltage of frequency f ₁ is applied. The voltage at frequency f ₁ induced in the ground wire at this time is V ₁ (volt). (Although the measurement signal may be a sine wave or a rectangular wave, it is treated as a sine wave here.) The present invention will be explained below based on examples.

When the secondary voltage of the transformer T is V ₀ (volt), when the leakage current i _G is detected by the zero-phase current transformer ZCT passing through the grounding wire EL, ω ₁ = 2πf ₁ , ω ₀ = 2πf ₀ I _G =√2V ₁ /Rsinω ₁ t+√2ω ₁ CV ₁ cosω ₁ t +√2V ₀ /Rsinω ₀ t+√2ω ₀ CV ₀ cosω ₀ t. If this is added to the filter FIL that detects only the frequency f ₁ component, its output i _g becomes i _g =√2V ₁ /Rsinω ₁ t+√2ω ₁ CVcos ₁ t.

Let the output voltage of the oscillation circuit OSC be e ₁ , and use the multiplier
If the product with i _g is calculated in MULT1, it becomes i _g ×√2e ₁ sinω ₁ t=e ₁ V ₁ /R +e ₁ V ₁ ω ₁ C・sin2ω ₁ t−e ₁ V ₁ /Rcos2ω ₁ t. Therefore, when the output of the multiplier MULT1 is passed through a low-pass filter to obtain the DC component of i _g ×√2e ₁ sinω ₁ t, a voltage proportional to e ₁ V ₁ /R is obtained at the low-pass filter output OUT ₁ . That is, if e ₁ and V ₁ are constant, the insulation resistance can be measured by knowing the low-pass filter output OUT ₁ .

By the way, the calculation of the formula is generally performed using a multiplier or a synchronous detection circuit, but if the second term in the formula, which represents the reactive current, is significantly larger than the first term, which represents the effective component, the calculation of the formula is performed. Therefore, it becomes difficult to obtain e ₁ V ₁ /R accurately. For example, if insulation resistance R = 200KΩ, stray capacitance to ground C = 8μF, and ω ₁ /2π = 20Hz, then from the formula, reactive current / active current = ω ₁ CR = 2π × 20 × 8 × 10 ^-6 × 200 ×10 ³ 190, which shows that the reactive current is significantly large.
A method of suppressing the influence of such ground stray capacitance has already been proposed by the same inventor and the same applicant, but a method for realizing suppression even more easily will be described here. Generally, the impedance 1/ω ₁ c ₁ due to the ground stray capacitance is lower than the insulation resistance R. In addition, stray capacitance to ground mainly depends on the wiring structure of the electrical circuit and the load, and is relatively constant over time unless the load is turned on and off.On the other hand, insulation resistance changes with the time of day and night (depending on the load conditions). ) It has been found through actual measurements that there has been a considerable change.

The broken line in FIG. 1 shows a method for measuring the floating capacitance to the ground. By adding the output of the oscillator circuit OSC to the 90° phase shifter ps, the output becomes √2
We get e ₁ cosω ₁ t. By adding the output of the phase shifter ps and the output of the filter FIL to the second multiplier MULT2, the output of the multiplier is i _g ×√2e ₁ cosω ₁ t=ω ₁ cv ₁ e ₁ +e ₁ v ₁ /Rsin2ω ₁ t＋e ₁ v ₁ ω ₁ c ₁ cos2ω ₁ t. Generally, as can be seen from the above explanation, the reactive current is larger than the active current, so there is little error in calculations using the formula, and therefore the multiplier
Low pass filter LPF2 connected to MULT2
The DC component of the equation ω ₁ cv ₁ e ₁ is obtained as the output, and the stray capacitance to the ground can be measured accurately. In the method of the present invention, in order to sufficiently suppress the stray capacitance to ground, the capacitance C is first known from the low-pass filter output OUT2. Based on this value, it is suppressed using a simple device.

FIG. 2 shows the ground stray capacitance suppression method of the present invention. That is, there is a zero-phase current transformer ZCT passing through the grounding wire EL and a coil OT of an oscillation transformer for applying the measurement signal, but these ZCT and OT are connected by a loop connecting wire CL to which a capacitor C ₀ is inserted and connected. .

For example, the loop connection line CL is passed through the OT in the same direction as the ground line, but in the opposite phase to the ZCT.

When connected in this way, the current flowing through the capacitor C ₀ due to the voltage of the oscillation transformer output will be applied to ZCT in the opposite phase, so if the leakage current in this case is i _G ′, then i′ _G = √2V ₁ ／Rsinω ₁ t＋√2ω ₁ CV ₁ cosω ₁ t−
√2ω ₁ C ₀ V ₁ cosω ₁ t+√2V ₀ /Rsinω ₁ t+√2
ω ₀ CV ₀ cosω ₀ t, and the output i′ _g of the filter F ₁ becomes i′ _g =√2/RV ₁ sinω ₁ t+√2ω ₁ V ₁ (C−C ₀ )cosω ₁ t. Therefore, if C ₀ is set to a value close to the value of C measured using the method shown in Figure 1, the second term in the equation will be sufficiently smaller than the second term in the equation, and as in the equation i By calculating ′ _g ×√2e ₁ sinω ₁ t, insulation resistance can be measured without the influence of stray capacitance to ground.

The ground stray capacitance measurement circuit (dashed line) in Figure 1 is not always used in equipment that continuously monitors insulation resistance, so it should be separated from other circuits and set only during installation. You may run it. By doing so, the insulation resistance monitoring device can be manufactured economically.

By employing the method of the present invention, it is not only possible to simply monitor the insulation resistance of a circuit with a large stray capacitance to the ground in a live state, but also to economically realize a significant improvement in the measurement accuracy. It is clear that the above description can be applied not only to single-phase, two-wire lines, but also to AA, three-phase, three-wire, and other lines.

Although omitted in the above explanation, the filter FIL
Patent application No. 55-39926 by the same applicant including the same inventor, which determines the characteristics of -47324) has been confirmed to be even more effective when used in combination.

In Fig. 1, the grounding wire is passed through the zero-phase current transformer ZCT and the oscillation transformer OT, but it is clear that similar measurements can be made even if the electrical lines 1 and 2 are passed through both.

The present invention is a method of achieving a remarkable effect of suppressing stray capacitance to the ground through extremely simple measures, and can be said to have great industrial value.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a method for measuring insulation resistance and stray capacitance to ground. FIG. 2 is a diagram showing an embodiment of the present invention. T: transformer, Z: load, C: stray capacitance to ground,
R: insulation resistance, ZCT: zero-phase current transformer, OT: oscillation transformer core, OSC: oscillation circuit, FIL: filter, MULT1, 2: multiplier (synchronous detection circuit),
LPF1, 2: Low pass filter, EL: Ground wire,
IN: Transformer primary side, CL: Loop connection line, C ₀ :
Ground stray capacitance suppression capacitor, 1, 2: Secondary circuit.

Claims (1)

[Claims] 1. By passing a grounding wire of the transformer through the core of an oscillation transformer that oscillates a low-frequency voltage that is a measurement signal or a transformer to which a low-frequency voltage is applied, a low A method in which a frequency voltage is applied, a leakage current of the low frequency signal is detected by a current transformer passed through the grounding wire, and an effective component of the leakage current is used to measure the insulation resistance of the electrical circuit, The current transformer and the transformer are connected together so that the current transformer is terminated with a capacitor having a value close to the ground stray capacitance value between the electric line and the earth, and a current having a phase opposite to the current flowing through the ground wire is applied to the current transformer. By providing a new loop connection line that is passed through or coupled, the reactive component caused by the ground stray capacitance of the low frequency voltage component contained in the leakage current detected by the current transformer is removed or suppressed. Insulation resistance measurement method.