JP2000009788A - Deterioration diagnosis method of cables - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a most appropriate method for diagnosis of insulation deterioration in a hot-line of a high voltage power cable. SOLUTION: In this diagnosis method, an alternating voltage is applied from an AC power E to a cable 1, a passing band region of a band-pass filter 7 is adjusted to a frequency (fundamental frequency) of the power source E and a frequency component of the AC power E outputted from the filter 7 is canceled by a resistance adjustment of a variable resistance R1, and a capacity adjustment of a variable capacitor C1 and is adjusted to be a minimum. The size of a harmonic current with respect to a basic frequency of the current released at the time is measured. The measurement of the harmonic current is carried out regarding the AC voltage having a different voltage value, and a deterioration degree of the cable is decided by a ratio of the harmonic current detected in an application of each voltage. Since correlation exists between the ration of the harmonic current and the rupture voltage of the cable, the degree of deterioration for the cable can be diagnosed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for diagnosing deterioration of cable insulation under a live line.

[0002]

2. Description of the Related Art As a conventional method for diagnosing insulation deterioration of a cable under a live line, the method described in Japanese Patent Application Laid-Open No. 9-184866 is known. In this method, a bridge circuit including a resistor, a capacitor, and a cable insulator is provided between a conductor of a cable to which an AC power supply voltage is applied and ground, and a circuit that extracts a potential difference between bridge neutral points is used. . At the time of the deterioration diagnosis, the resistance value of the resistor and the capacitance of the capacitor are adjusted, and the frequency component of the AC power output through the bandpass filter of the circuit for extracting the potential difference is canceled to zero. Then, in this state, the degree of insulation deterioration is determined from the magnitude of the output of the other frequency component extracted from the band-pass filter.

[0003]

In the case of the above-mentioned diagnosis method, deterioration can be diagnosed by the magnitude of the harmonic current when a single voltage is applied with a 6 kV CV cable. This is a 6kV CV
This is because, in the cable, the length and the frequency of occurrence of the water tree are proportional to the harmonic current value, and the harmonic current value at only one applied voltage has a correlation with the deterioration state. However, with a 22 kV CV cable, the length and occurrence frequency of the water tree do not correlate with the harmonic current value, so that it is difficult to diagnose insulation deterioration with the harmonic current at one applied voltage.

Accordingly, it is a primary object of the present invention to provide an optimal method for diagnosing insulation deterioration, especially in high-voltage power cables.

[0005]

In order to solve the above-mentioned problems, according to the present invention, a harmonic current is measured by applying a plurality of AC voltages having different voltage values, instead of applying a single AC voltage. Is performed, and the deterioration state is diagnosed based on the ratio of the harmonic current values at each voltage application.

That is, according to the degradation diagnosis method of the present invention, an AC voltage is applied to a cable, the fundamental frequency component of a current flowing through the cable is adjusted to be a minimum, and a harmonic of a current emitted at that time with respect to the fundamental frequency is adjusted. In the method for diagnosing deterioration of a cable for measuring a magnitude of a wave current, a plurality of types of the AC voltages having different voltage values are used, and a degree of deterioration of the cable is determined based on a ratio of harmonic current values detected in applying each voltage value. It is characterized by doing.

The degree of insulation deterioration is determined by calculating the ratio between the harmonic current values obtained for each applied voltage value and the relationship between the magnitude of this ratio and the degree of deterioration in advance. It is good to judge by comparing the magnitude of the ratio.

In the method of the present invention, the measurement of the harmonic current is carried out in the same manner as in the invention described in JP-A-9-184866. First, in order to adjust the fundamental frequency component of the current flowing through the cable (the same frequency as the frequency of the AC power supply) to a minimum, a resistor, a capacitor, and a cable are connected between the conductor of the cable to which the AC voltage is applied and the ground. It is preferable to use a circuit in which a bridge circuit made of an insulator is formed and a potential difference between the neutral points of the bridge is extracted through a bandpass filter. By adjusting the resistance value of the resistor and the capacitance of the capacitor, the bridge is balanced, the charge current is canceled, and the fundamental frequency component of the current output through the bandpass filter is minimized.

Then, it is sufficient to measure the magnitude of the other frequency components in the harmonic current extracted from the band-pass filter with the fundamental frequency component minimized. Thus, the capacitance component, the resistance component, and the AC power supply waveform of the cable are canceled. Therefore, even if the AC power supply contains harmonic components, most of them are cancelled.

In this canceled state, if the cable is sound and the current-voltage characteristics of the resistance component are linear, the output voltage should be zero. However, if the cable is deteriorated by the water tree and the current-voltage characteristic is non-linear, only the voltage due to the current generated thereby is output.

In the present invention, after canceling the fundamental frequency component, the band-pass filter is switched to another frequency to detect the other frequency component included in the resistance component of the cable.
Therefore, the current output from the band-pass filter after the cancellation is only the current generated by the deterioration of the cable insulator, and the influence of the harmonic component included in the AC power supply can be eliminated.

The other frequency components extracted from the bandpass filter after canceling the frequency components of the AC power supply are as follows:
The frequency is preferably an odd multiple of the frequency of the canceled AC power supply. If the other frequency component extracted from the bandpass filter is an odd multiple of the frequency of the AC power supply, the resistance component detected from the water-tree-degraded cable contains many harmonic components that are odd multiples of the frequency of the AC power supply. For,
A stable output is obtained. For example, if the frequency of the AC power supply is 60 Hz, the other frequency may be 180 Hz.

Then, the same harmonic current value is measured by changing the applied voltage, and the ratio of the obtained current values is determined to perform the deterioration diagnosis. The breakdown voltage of a cable degraded by a water tree is determined by the maximum length of the water tree. The ratio of the harmonic current under different applied voltages increases as the length of the water tree increases. Therefore, the ratio of the harmonic current and the breakdown voltage of the cable have a correlation, and the degree of deterioration of the cable can be detected.

[0014]

Embodiments of the present invention will be described below. FIG. 1 is a circuit diagram of a deterioration diagnosis device used in the method of the present invention.

When the insulation resistance and the insulation capacitance between the conductor of the cable 1 to be diagnosed and the shielding layer are represented by an equivalent circuit, they are expressed as a parallel connection of a resistance component rx and a capacitance component Cx. This device has an AC power supply E having a load terminal 2 and a ground terminal 3. The power supply E is configured to be able to apply a plurality of voltages having different voltage values,
Is connected to the conductor of the cable. A standard capacitor Co is connected to the conductor of the cable 1, and a standard capacitor Co is connected in parallel to the variable resistor R via the first connection point 4.
1 and the variable capacitor C1 and grounded.

A resistor R is connected between the shield layer of the cable 1 and the ground via a second connection point 5. And the first connection point 4
A circuit is provided which amplifies a potential difference between the first and second connection points 5 by a differential amplifier 6 and passes the amplified signal through a bandpass filter 7.

With such a circuit configuration, the pass band of the band-pass filter 7 is adjusted to the frequency of the AC power source E, and the frequency component of the AC power source output from the filter 7 is adjusted by adjusting the resistance value of the variable resistor R1 and adjusting the variable capacitor C1. And the minimum (zero).

The balance condition of the bridge circuit is when the following equation is satisfied. At that time, the fundamental frequency component of the AC power supply is removed from the output of the band-pass filter 7. C1 · R · R1 + Cx · rx · R = Co · rx · R1 ω 2 · C1 · Cx · R1 · rx = 1 However ω = 2πf f: Power Frequency

Next, when the pass band of the band-pass filter is adjusted to another frequency (preferably an odd multiple of the canceled power supply frequency) in this state where the frequency component of the power supply is cancelled, the deterioration of the cable insulator is caused. Only the frequency component of the voltage due to the current caused by the current is output.
Such harmonic current measurement is performed by changing the voltage of the AC power supply, and the ratio of the measured current value at each voltage value is obtained. Then, the magnitude of the ratio is compared with data indicating the relationship between the ratio obtained in advance and the degree of insulation deterioration to determine the degree of deterioration.

(Test Example) Using a 22 kV CV cable as a test cable, the relationship between the third harmonic current value and the cutoff voltage based on a single voltage value was examined (Comparative Example). The applied voltage of the AC power supply is 12.7 kV. The result is shown in FIG.
As is clear from this graph, there is no correlation between the third harmonic current value and the cutoff voltage, and it can be seen that deterioration diagnosis cannot be performed.

Next, 12.7 kV and 2
An AC voltage of 0 kV was applied, the third harmonic current value was measured at each voltage value, the ratio of the current values was determined, and the relationship with the cutoff voltage was examined (Example). The result is shown in FIG. As is clear from this graph, the ratio of the third harmonic current value when different voltages were applied was correlated with the cutoff voltage, and it was found that the cable deterioration diagnosis was possible.

[0022]

As described above, according to the deterioration diagnosis method of the present invention, even in a high-voltage cable, the insulation deterioration can be surely achieved by utilizing the ratio of the harmonic current values when different voltages are applied. The condition can be diagnosed. Also,
Even if harmonics are superimposed on the power supply connected to the cable, the adverse effects of the harmonics are eliminated, so that the degree of deterioration of the cable can be accurately determined, and the time to replace the cable in use is optimized or under development. It is useful for collecting highly reliable evaluation data from a cable.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a diagnostic device used in the method of the present invention.

FIG. 2 is a graph showing a relationship between a third harmonic current value and a cutoff voltage when a single voltage is applied.

FIG. 3 is a graph showing a relationship between a ratio of a third harmonic current value and a cutoff voltage when two types of voltages are applied.

[Explanation of symbols]

1. Cable 2. Load terminal 3. Ground terminal 4. First connection point 5. Second connection point 6. 6. Differential amplifier Bandpass filter

 ──────────────────────────────────────────────────の Continuing on the front page (72) Kenichi Hirotsu 1-3-1 Shimaya, Konohana-ku, Osaka City Inside the Osaka Works, Sumitomo Electric Industries, Ltd. (72) Miji Motoi 1-chome, Shimaya, Konohana-ku, Osaka-shi 1-3, Sumitomo Electric Industries, Ltd., Osaka Works (72) Inventor Takashi Motoharu 3-2-2, Nakanoshima, Kita-ku, Osaka-shi Kansai Electric Power Co., Ltd. (72) Inventor Hiroyuki Okamura 3, Nakanoshima, Kita-ku, Osaka-shi 3-3-22 Kansai Electric Power Co., Inc. F-term (reference) 2G015 AA27 BA04 CA05 CA20

Claims (2)

[Claims] An AC voltage is applied to a cable, the fundamental frequency component of the current flowing through the cable is adjusted to be a minimum, and the magnitude of a harmonic current with respect to the fundamental frequency of the current emitted at that time is measured. A method of diagnosing deterioration of a cable, wherein a plurality of types of AC voltages having different voltage values are used, and a degree of deterioration of the cable is determined based on a ratio of harmonic current values detected in application of each voltage value. 2. A bridge circuit comprising a resistor, a capacitor, and a cable insulator is formed between a conductor of a cable to which an AC voltage is applied and ground, and a potential difference between bridge neutral points is formed via a band-pass filter. An extraction circuit is provided, and by adjusting the resistance value of the resistor and the capacitance of the capacitor, the fundamental frequency component of the current output through the bandpass filter is minimized. In this state, the harmonic current extracted from the bandpass filter is reduced. 2. The method according to claim 1, wherein the magnitude of the other frequency component is measured.