JPS585677A - Detecting method for partial discharge of power cable - Google Patents

Abstract

PURPOSE:To detect partial discharge correctly by applying correlation processing to a partially-earthed discharge current via a band-pass filter and to a commercial supply signal whose pulse is transformed into a rectangular-wave pulse, and thereby preventing the intrusion of outside noises. CONSTITUTION:A secondary current in accordance with a magnetic field corresponding to the partial discharg caused by the insulation inferiority of a power cable is detected by a current transformer 3 and turned into a partial discharge signal freed from outside noises such as a commercial frequency and a radio freuency through the intermediary of BPF4. This signal and a commercial supply signal whose pulse is transformed into the rectanguar by a transformer 11' are subjected to correlation processing by a correlator 5. When the signal from BPF4 becomes large, the partial discharge is indicated and detected correctly by an abnormality indicator 6 without being affected by the outside noises, since the signal from the transformer 11' has a constant level.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a method for detecting partial discharge in a power cable.◇ When an insulation failure occurs in a power cable, the resulting partial discharge current is transferred to the metal sheath 1it1b, 1C.
flows to ground M2 connected to. Therefore, by detecting this ground current, it is possible to detect the partial discharge, but as shown in FIG. 1, conventionally the metal sheath la of the cable.

Poor insulation in the power cable was detected by measuring the current flowing through the grounding wire 2 connected between 1b and lc and the earth G through a transformer or shunt resistor using a cathode ray tube oscilloscope (not shown). However, This method has the disadvantage that if external noise such as radio frequency current enters the cathode ray tube oscilloscope in addition to the partial discharge signal, it may be difficult to accurately measure insulation defects. .

The present invention has been made in order to eliminate the above-mentioned drawbacks, and provides a partial discharge detection method using an electric canal pull, which can accurately detect a partial discharge signal without allowing external noise to enter the partial discharge signal. It is something.

The present invention will be explained below based on examples.

In FIG. 2, when the insulator of the live wire power cable becomes defective, a partial discharge occurs and this partial discharge signal is generated. The partial discharge signal current generates a magnetic field in the iron core 3a of the current transformer 3, and the magnetic field generates a secondary current in the secondary lead wire 3b of the current transformer 3. As shown in Figure 3, the secondary current is a partial discharge signal 8 superimposed on a commercial frequency signal T consisting of three-phase unbalanced current and sheath induced current, and an external dose (not shown) such as radio frequency. )
In this case, the iron core 3 may be provided with an opening sufficient to allow the grounding 32 to pass therethrough, if necessary. 2 by a pandonosas filter 4 (for example, a series circuit of a capacitor 41 and a coil 4b).
The low and high range of the secondary current (for example, 20 KHg or less 110
The commercial frequency signal T and the radio frequency current are cut off, resulting in an output waveform 9 of the bandpass filter as shown in FIG.

Next, we will explain the correlator 5 that calculates the correlation function between the partial discharge signal and the rectangular pulse wave.The output from the commercial power supply 11 is converted from the sine wave 7 in FIG. The signal of this rectangular pulse wave 10 is converted into a rectangular pulse wave 10, and the product of the signal of this rectangular pulse wave 10 and the output of the band pass filter is calculated in the correlator 5 of FIG. 9' is the average value waveform r. (For example, this is done using a known AND circuit and a smoothing circuit.)
Next, the operation of the partial discharge signal will be explained. That is, assuming that the output level of the rectangular pulse wave signal 10 from the commercial power supply is constant, the output level 9 from the bandpass filter 4
increases, and the abnormality indicator 6 issues a warning that the insulation of the power cable is defective.

As explained above, the secondary side of the current transformer is connected to the grounding wire 2 via the iron core 3, and the signal obtained by converting the commercial power signal into a rectangular pulse wave via the bandpass filter 4 is connected to the secondary side of the current transformer. Since the correlation (correlation function) with the output of the pass filter is calculated and the output signal of the correlator is input to the abnormality indicator, insulation defects in the power cables of live electric wires can be detected with a high signal-to-noise ratio. It has the effect of being able to

[Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between the power cable and the grounding wire, Figure 2
The figures are circuit diagrams explaining the invention in detail, Nos. 3.4, 5, and 6.
．． FIG. 7 is a diagram showing waveforms at each step of an embodiment of a method for calculating a correlation function between a partial discharge signal and a rectangular pulse wave. 2 -----1--Grounding wire 3--------1 Current transformer 4--Band pass filter 5--------
--- Correlator 6 ------------One-time display board I Y-----One sine wave rectangular pulse wave converter Fig. 1 Fig. 2 Fig. 3

Claims (1)

[Claims] 1. Introducing the grounding current of the power cable taken out through a current transformer to a band-pass filter, and guiding the obtained signal to an abnormality display to detect a partial discharge signal generated at the defective insulation point of the power cable. In the partial discharge detection method for a power cable, a correlator is connected between the bandpass filter and the abnormality indicator to calculate a correlation coefficient between the partial discharge signal and a commercial power signal converted into a rectangular pulse wave. A method for detecting partial discharge in a power cable, characterized by: