JPH0243409B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for monitoring the insulation condition of an electrical circuit and generating an alarm signal when the insulation deteriorates.

(Prior art) Conventionally, the grounding wire of a power receiving transformer with one end on the secondary side is grounded, and the low frequency voltage that is the measurement signal is passed through the oscillation transformer or the core of the transformer to which the low frequency voltage is applied. Alternatively, by cutting the grounding wire and connecting it in series with an oscillator that applies a low-frequency voltage, a low-frequency voltage can be applied to the electrical circuit through the grounding wire, and the leakage current that returns to the grounding wire via insulation resistance and stray capacitance to the ground can be suppressed. Insulation resistance is measured by detecting the effective low-frequency component (component in phase with the applied voltage) of this leakage current using a current transformer, and an alarm signal is generated when this value exceeds a predetermined value. A method is proposed for this to occur. In the method for detecting insulation deterioration as described above, the measurement signal applied to the electrical circuit is generally about 10 V or less, and the insulation resistance of the electrical circuit in a normal state is several hundred kilohms or more, so the leakage current of the low frequency component that returns is detected. is weak. Further, the value for generating an insulation deterioration alarm is generally set to several tens of kilohms, and the configuration is generally such that an alarm signal is generated when the insulation resistance of the electrical circuit deteriorates below this value.

However, if the insulation of the electrical circuit deteriorates abnormally and, in extreme cases, approaches a ground fault condition, the leakage current of the commercial frequency component of the electrical circuit naturally increases significantly, and the leakage current of the component due to the measurement signal also increases significantly.

However, since the circuit that detects the active ingredient normally operates to accurately measure the insulation resistance with a weak leakage current, when such a large leakage current is input, the active ingredient detection circuit may become saturated or incorrectly measure the insulation resistance. There was a drawback that it was difficult to maintain the output of the measurement results and that an alarm signal could not be generated.

(Objective of the Invention) The present invention has been made to eliminate the drawbacks of the conventional insulation deterioration alarm method as described above, and when an abnormally large leakage current is detected, An object of the present invention is to provide a method for generating an insulation deterioration alarm signal that accurately warns of an abnormality due to deterioration of the insulation of an electrical circuit by generating an alarm signal based on the magnitude of leakage current.

(Summary of the Invention) In order to achieve the above object, the present invention applies a low frequency voltage as a measurement signal to an electric line, and predetermines the effective portion of the low frequency leakage current that returns to the grounding line. A first alarm signal is generated by comparing the leakage current returned to the grounding wire with a predetermined second alarm setting value; It is characterized in that it has two alarm signals, and a signal obtained by calculating the logical sum of the first alarm signal and the second alarm signal is used as the alarm signal.

(Example) Hereinafter, the present invention will be explained in detail by referring to an example shown in the drawings.

FIG. 1 is a diagram showing an embodiment of the present invention. Although this example shows a single-phase, two-wire electrical circuit for ease of explanation, the method described below is not limited to this, and can be applied to secondary, one-end grounded electrical circuits such as single-phase, three-wire, three-phase, and three-wire. It is applicable.

In this figure, the load of the power receiving transformer T is Z, the insulation resistance of the electric lines L ₁ and L ₂ is R, and the stray capacitance to ground is C. The grounding wire EL is a transformer OT that oscillates the measurement signal at frequency ₁ (different from commercial frequency ₀ ).
Or it is configured to pass through the core of the transformer to which frequency ₁ is applied. Let the low frequency voltage of frequency ₁ induced in this grounding wire EL be V ₁ (volts) (the measurement signal may be a sine wave or a rectangular wave, but it is treated as a sine wave here). The zero-phase current transformer ZCT that passes through the grounding wire EL detects leakage current, and converts the commercial frequency voltage of the electrical circuits L ₁ and L ₂ into Vo
(volt), the leakage current is detected as Ig=√Z/−Vo/R−sinωot+√2ωoCVocosωot +√2/−V ₁ /Rsinω ₁ t+√2ω ₁ CV ₁ cosω ₁ t Ru. Add this to the amplifier AMP ₁
In addition to the filter FIL that amplifies and detects the frequency ₁ component, if the leakage current of the commercial frequency ₀ component is removed, the output ig of the filter FIL is ig=√2/−V ₁ /Rsinω ₁ t+√2 ₁ CV ₁ Cosωot can get. Here, ω ₁ =2π ₁ .

Also, let the output voltage of the oscillation circuit CSC be e ₁ [volt], and use the multiplier MULT or the synchronous detector to calculate
If we take the product of ig and the OSC output voltage, we get ig×√2e ₁ sinω ₁ t=e ₁ V ₁ /R+e ₁ v ₁ ω ₁ csin2ω ₁ t −e ₁ v ₁ /Rcos2ω ₁ t. Therefore, when the output of the multiplier MULT is passed through the low-pass filter LPF to obtain the DC component of ig×√2e ₁ sinω ₁ t, the output of the low-pass filter LPF is
OUT, that is, the effective component is OUT=e ₁ v ₁ /R, and if e ₁ and v ₁ are constant, by knowing the value of OUT, a voltage value that is inversely proportional to the insulation resistance can be obtained. Therefore, the output of the low pass filter LPF
Input OUT to the voltage comparator COMP ₁ and compare it with the reference voltage e _s . If the effective portion OUT becomes larger than the reference value e _s , that is, if it becomes a value lower than the insulation resistance determined as the alarm value, the voltage comparison is performed. The output of the device becomes logic "1" and an alarm signal ALM ₁ is generated.

On the other hand, when the insulation resistance R is significantly lower than the normal degree of insulation deterioration or the stray capacitance C to ground becomes abnormally large, Ig in the above equation is an abnormal value (several A to
100A). In this case, it is natural for the system from the highly sensitive AMP ₁ to MULT to be in a saturated state, and at this time, the effective portion OUT is in a state where it cannot output the correct value. Therefore, the alarm signal ALM ₁ may not be generated correctly. But current transformer ZCT output level detector
The DET is circuitized so that it can detect currents up to several hundred A, and the level detector DET output is added to a second voltage comparator COMP ₂ whose reference voltage is e _p , and this reference voltage is used to generate a leakage current of, for example, 1 A. If the input to the level detector DET exceeds this value, the second alarm signal ALM ₂ can be obtained. Thus alarm signal ALM ₁ and alarm signal ALM ₂
By using the logical sum output as the alarm signal ALM, it is possible to generate an alarm signal even if insulation deterioration is more significant than normal insulation deterioration, and it is possible to constantly monitor the insulation deterioration of the electrical circuit under live wire conditions. becomes.

The reason for detecting insulation deterioration using the effective part of the measurement signal without using the commercial frequency component as in the present invention is that in a single-phase three-wire electrical circuit, when there is insulation deterioration in two ungrounded electrical circuits, the commercial frequency component Leakage current is ground wire
The phase of the leakage current of both ungrounded circuits returning to the EL is
This is because they cancel each other out because they differ by 180°, making it impossible to set the insulation resistance of the electrical circuit correctly.

Further, when the second alarm signal ALM ₂ is generated, there is a possibility that a commercial current is reversely applied to the oscillator OSC due to a large leakage current, causing damage to the oscillator OSC. In order to prevent this, for example, the second alarm signal ALM ₂ may be used to operate the relay REL via the relay driver D to disconnect the oscillator OSC and the oscillation transformer OT.

Note that if the insulation returns to normal, the relay REL will return to normal operation and the measurement signal will continue to be applied to the ground line EL.

Furthermore, if it is anticipated that the highly sensitive current transformer ZCT will become saturated and the level detector DET will be unable to detect large currents when the current becomes abnormally large, a second large current transformer will be used as shown in Figure 2. Current transformer for current detection
A grounding wire BL is passed through ZCT ₂ , and its output is connected to the level detector along with the ZCT output of the other current transformer.
It can be used as an input for DET.

Also, instead of separately providing this second current transformer ZCT ₂ , a second winding COIL is added to the oscillation transformer OT as shown in Figure 3, and the current is detected using the output of this winding. You may also do so.

In the above embodiment, a current transformer is used to detect the current, but the grounding wire EL is cut off, a low resistance is connected in series with it, and the current is detected using the voltage across this resistance. It is also possible to disconnect the grounding wire EL when applying low frequency voltage to the electrical circuit.
It is also possible to insert and connect an oscillator OSC in series to this.

(Effects of the Invention) Since the present invention is configured and functions as explained above, it ensures the safety of electric circuits in factories and the like where accidents such as ground faults in addition to normal insulation deterioration are likely to occur. It has a remarkable effect on the above.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a configuration for realizing the method for generating an insulation deterioration alarm signal according to the present invention, and FIG.
3 and 3 are partial circuit diagrams showing different other embodiments. T...Transformer, EL...Grounding wire, OSC...Low frequency oscillator, OT...Oscillating transformer, e _s ...First alarm set value, ALM ₁ ...First alarm signal, e _p ...
Second alarm set value, ALM ₂ ...Second alarm signal,
COMP ₁ and COMP ₂ ...first and second comparators,
D...Rele driver, REL...Rere.

Claims (1)

[Claims] 1. A low-frequency voltage, which is a measurement signal, is applied to the electric circuit through the grounding wire of the transformer by electromagnetic induction or series coupling, and the effective component in the low-frequency leakage current that returns to the grounding wire is determined. A first alarm signal generated by comparing a first alarm set value determined in advance, a leakage current that returns to the grounding wire, and a second alarm signal determined in advance.
and a second alarm signal that is generated by comparing the alarm set value of the first alarm signal and the second alarm signal. Features: Insulation deterioration alarm signal generation method. 2. A low-frequency voltage, which is a measurement signal, is applied to the electric circuit through the grounding wire of the transformer by electromagnetic induction or series coupling, and the effective portion of the low-frequency leakage current that returns to the ground is A first alarm signal generated by comparing the first alarm set value with a second alarm signal generated by comparing the leakage current returned to the grounding wire with a predetermined second alarm set value. and an insulation deterioration alarm signal generating method in which the alarm signal is a signal obtained by performing a logical sum of the first alarm signal and the second alarm signal, wherein the second alarm 1. A method for generating an insulation deterioration alarm signal, characterized in that when this occurs, the application of a low frequency voltage for measurement applied to a grounding wire is cut off.