JPH0627184A - Partial-discharge measurement - Google Patents

Abstract

PURPOSE:To realize high-sensitive measurement of partial-discharge level in power cable, etc. CONSTITUTION:A transformer 4 is connected to the near end of a power cable C with a blocking coil 1 interposed. Further, an input unit 4 is also connected to the near end thereof with a capacitor 3 interposed, and the output of the unit 4 is connected with an amplifier 6 through an attenuator 5. The output of the amplifier 6 is connected with a band amplifier 9, that resonates at center frequency of 20kHz, through a wave detector 7 and integrator 8, and the output of the amplifier 9 is connected with an indicator 10. In order to measure partial discharging, the cable C is applied with voltage through the transformer 2 first. When there is any partial discharging, the signal is inputted to the unit 4 by the capacitor 3 and then it is inputted to the amplifier 6 through the attenuator 5 to be amplified. The amplified signal is changed into a narrow-band signal by the amplifier 9 via the detector 7 and integrator 8 and final output can be obtained from the indicator 10.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a partial discharge measuring method for measuring the magnitude of partial discharge of a power cable or the like.

[0002]

2. Description of the Related Art Partial discharge measuring methods include a wide band method (several kHz to several MHz), a tuning method (400 kHz ± 50 kHz) and a low frequency method (several 1) as methods for amplifying a pulse voltage.
0 kHz to 200 kHz) is known, but generally, the latter two are superior in that there is little disturbance of external noise, and the low frequency method is superior in that there is little fluctuation in detection sensitivity due to the difference in the discharge occurrence point in the cable. It is said that.

[0003]

FIG. 3 is a graph showing the frequency spectrum distribution when a partial discharge is artificially generated in the test cable, and FIG. 3 (a) is a measurement system excluding the test cable. Is measured by a spectrum analyzer, and Fig. 3 (b) shows the noise of the line when a test cable of 700 m in length is connected. In the case of this line noise, the center frequency is 50 to 100 kHz. Figure 3 (c) shows the signal obtained at the near end when the pulse generator 2 is connected to the far end of the sample cable and a pulse with a charge amount of 100 pC is repeatedly input. It corresponds to the case. Compared with the line noise in Fig. 3 (b), the components corresponding to the spectrum around 50 kHz, which is a relatively low frequency, are increased. Figure 3 (d)
Represents a signal when a pulse of 1000 pC is repeatedly input from the pulse generator 2, and the components corresponding to the spectrum around 50 kHz or less are greatly increased as compared with FIG. 3 (c).

That is, 50 which is lower than the center frequency of the line.
It can be seen that the relatively low frequency component of kHz or less is largely related to the pulse injection amount, in other words, it is correlated with the discharge amount in the cable.

As described above, the conventional low frequency method is several tens of kHz.
Although it is performed using a frequency in the range of z to 200 kHz, it is a frequency of 50 kHz or less that can actually sufficiently correspond to the discharge amount as can be seen from FIG. Therefore, in the frequency band used in the conventional low frequency method, S /
The N ratio is bad, and accurate measurement cannot be expected.

An object of the present invention is to provide a partial discharge measuring method which enables highly sensitive measurement by using only a relatively low frequency among frequency components used in the low frequency measuring method.

[0007]

The partial discharge measuring method according to the present invention for achieving the above-mentioned object is a method for measuring partial discharge by a low frequency method, in which a power cable based on a frequency component of 50 kHz or less is used. It is characterized in that the partial discharge amount of is measured.

[0008]

The partial discharge measuring method having the above-mentioned structure is 50
The amount of discharge in the power cable is measured based on the magnitude of a relatively low frequency component of kHz or less.

[0009]

1 is a block diagram for applying the method of the present invention to a power cable C. In FIG. The transformer 2 is connected to the near end Ca of the power cable C via the blocking coil 1. Further, an input unit 4 is connected to the near end Ca via a capacitor 3, and an output of the input unit 4 is connected to an amplifier 6 having a center frequency around 100 kHz, for example, via an attenuator 5. The output of the amplifier 6 is connected via the detector 7 and the integrator 8 to the band amplifier 9 which resonates with the center frequency of 20 kHz, and the output of the amplifier 9 is connected to the indicator 10. In addition, 11 is a far end of the cable C for calibration.
It is a pulse generator connected to Cb.

In the measurement, when a voltage is applied from the transformer 2 to the power cable 1, if there is partial discharge, the signal is input to the input unit 4 via the capacitor 3, and then the signal is amplified via the attenuator 5. Input to 6 and amplified. The process up to this point corresponds to the conventional measuring means using the low frequency method.

The signal amplified by the amplifier 6 is narrowed by the band amplifier 9 via the detector 7 and the integrator 8, and the output is obtained by the indicator 10.

Further, the pulse generator 11 allows the far end
If a known charge amount is injected from Cb and measured, and the measured values obtained by the indicator 10 are compared, the previously measured discharge amount can be calibrated.

FIG. 2 (a) is a graph showing the amplified waveform obtained when a pulse generator repeatedly injects 100 pC pulses into a 700 m long test cable, that is, the output waveform of the amplifier 6 in FIG. Yes, Figure 2 (b) is 1
It is a graph figure when a pulse of 000pC is repeatedly injected.

In this way, a reflection waveform is generated on the cable according to the resolution of the waveform determined by the amplification bandwidth of the partial discharge measuring instrument, that is, the frequency of several tens kHz to 200 kHz.
In a cable having a reflection time shorter than the time corresponding to the resolution, the reflection waveform does not separate and becomes one waveform, and such a phenomenon does not occur. The waveform of FIG. 2 is determined by the amplifier of the partial discharge measuring instrument and the cable length, and the repetitive reflections form an envelope.

As can be seen by comparing FIG. 2 (a) and FIG. 2 (b), the envelope of the pulse reflection waveform becomes larger and the enclosed area is larger when 1000 pC is input, that is, when the discharge amount is larger. Therefore, the frequency component lower than the center frequency of the amplifier determined by the envelope also increases.

Therefore, the discharge charge amount can be measured by measuring the low frequency included in the envelope, that is, the low frequency included in the reflection waveform increasing in FIG. Therefore, the band amplifier 9 having a center frequency of 20 kHz shown in FIG. 1 further resonates and amplifies this frequency.

In this way, the low frequency related to the length of the cable is detected, and the partial discharge charge by measuring the frequency lower than the amplification frequency number of the low frequency partial discharge measuring instrument is measured with the long cable.

Therefore, the inclination of this envelope can be measured,
Alternatively, the discharge charge amount can be measured by measuring the area surrounded by the envelope.

[0019]

As described above, the partial discharge measuring method according to the present invention enables highly accurate measurement of the magnitude of partial discharge of a power cable or the like by measuring based on a low frequency component.

[Brief description of drawings]

FIG. 1 is a block diagram for carrying out the method of the present invention.

FIG. 3 is a graph showing an output spectrum of a partial discharge measuring method.

FIG. 2 is a graph showing an amplified waveform of a partial discharge measuring method.

[Explanation of symbols]

 C power cable 1 blocking coil 2 transformer 3 capacitor 4 input unit 5 attenuator 6, 9 amplifier 7 detector 8 integrator 10 indicator 11 pulse generator

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[Procedure amendment]

[Submission date] August 20, 1992

[Procedure Amendment 1]

[Document name to be corrected] Drawing

[Name of item to be corrected] Figure 1

[Correction method] Change

[Correction content]

[Drawing]

Claims (1)

[Claims] 1. A method for measuring partial discharge, which comprises measuring the amount of partial discharge in a power cable based on a frequency component of 50 kHz or less when measuring partial discharge by a low frequency method.