JPH05288796A - Method for detecting degree of deterioration of cable insulator - Google Patents

Abstract

PURPOSE:To detect the degree of deterioration of an insulator by a DC voltage electrification or a low-frequency voltage electrification before measuring charge current and then measuring charge current while AC electrification is performed. CONSTITUTION:After a DC voltage electrification or a low-frequency electrification is performed, a commercial frequency AC voltage is applied to a conductor 1 from a terminal part 1a of the cable conductor 1, charge current which flows to a cable insulator 2 is decomposed into a low-frequency component below a commercial frequency, a commercial frequency component, a high-frequency component above the commercial frequency by a variable filter 3 and is measured for each frequency and then each frequency characteristic is measured by an FFT analyzer 4. The collection data is transmitted from the analyzer 4 to a computer 5 and is compared with a database which shows the relationship between the charge current value which is stored in the computer 5 previously and the degree of deterioration of cable insulator. The comparison result becomes a data for diagnosing the degree of deterioration, thus measuring the degree of deterioration.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for detecting the degree of deterioration of a solid insulated cable.

[0002]

2. Description of the Related Art Degradation of the insulation of a power cable causes dielectric breakdown or power failure, so it is necessary to detect the degree of deterioration early and surely before edge breakage. As a method of detecting deterioration, non-destructive measurement of electric current such as DC leak current and tan δ, which reflects the insulation characteristics of a cable, has been put to practical use when a line is out of power.
In addition, when a solid insulator such as a plastic power cable is exposed to water and an electric field, a water tree in which minute traces of local dielectric breakdown develop in a dendritic manner occurs in the insulator, which reduces the thickness of the cable insulator. It is necessary to detect this water tree early and surely because it will cause dielectric breakdown if it progresses so much that it bridges across it. Conventionally, as a method of detecting the deterioration of the insulator due to the water tree under a hot line, a method of measuring a direct current component related to the water tree portion has been proposed.

[0003]

When the degree of deterioration of the insulator is small and the length of the water tree relative to the insulator is short, for example, when the length of the water tree is 70% or less of the insulator thickness,
Since the DC component, the low frequency component, and the high frequency component of the charging current component do not remarkably increase, the conventional method could not obtain the electric signal due to the water tree and could not obtain the information.

Therefore, the conventional method for detecting the deterioration of the cable insulator under the hot line can detect only when the deterioration in which the water tree bridges the cable insulator has progressed significantly.

The object of the present invention is to solve the above-mentioned problems,
An object of the present invention is to provide a measuring method capable of detecting the degree of deterioration of a cable insulator even when the degree of deterioration of the insulator is small and the length of the water tree is short.

[0006]

In order to achieve the above-mentioned object, a method for detecting the degree of deterioration of a cable insulator according to the present invention provides insulation of a power cable by measuring a charging current component under an AC voltage flowing through the cable insulator. In the method for detecting the degree of deterioration of characteristics, DC voltage or low-frequency voltage is applied before charging current measurement, and then the charging current is measured while AC charging is applied. It is characterized by detecting the degree of deterioration of the insulator by decomposing it into a low frequency component below the commercial frequency, a commercial frequency component, and a high frequency component above the commercial frequency and measuring the magnitude of each current value and frequency characteristics. To do.

[0007]

[Function] The charging current i of the solid insulated cable is i = IDC +
It is expressed as IAC + IETC. Where i; charging current, IDC;
DC component, IAC; AC component, IETC; Current component related to terminals and local batteries.

When the degree of deterioration of the insulator is small and the length of the water tree is short, the direct current component, the low frequency component and the high frequency component of the charging current component do not increase and an electric signal due to the water tree cannot be obtained. , DC voltage or low frequency voltage (0.01HZ to 25HZ) is applied before measuring the charging current, space charge due to the water tree is forcibly injected, and this charge moves when AC is applied. The degree of deterioration is detected by observing the situation by measuring the charging current. The polarity of the DC voltage applied may be positive or negative. Thereby, even when the length of the water tree is short, an electric signal due to the water tree can be obtained.

[0009]

Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 schematically shows an apparatus for measuring a charging current by the method for detecting the degree of deterioration of a cable insulator according to the present invention, 1 is a conductor of a power cable, 1a is a terminal portion, 2 is a shielding layer, and 3 is a filter box. The variable filter, 4 is an FFT analyzer, 5 is a personal computer for controlling the filter 3 and FFT analyzer 4, and 6 is a control GDI.
B.

A direct current voltage or a low frequency constant voltage of 0.01 Hz to 25 Hz is applied to the conductor 1 from the terminal portion 1a of the power cable conductor 1 for a certain period of time. The DC voltage value and time to be applied vary depending on the degree of deterioration of the cable insulator to be detected. The smaller the degree of deterioration of detection, the larger the applied voltage value and the longer the time. The same applies to the case of applying a low frequency voltage, and in order to detect a slight degree of deterioration, it is necessary to apply a high voltage at a lower frequency for a long time.

After applying the DC voltage or the low frequency voltage, the commercial frequency AC voltage is applied from the terminal portion 1a of the cable conductor 1 to the conductor 1, and the charging current flowing in the cable insulator at that time is supplied to the filter box. The variable filter 3 accommodated therein decomposes it into a direct current component, a low frequency component below the commercial frequency, a commercial frequency component, and a high frequency component above the commercial frequency, and measures each frequency, and the FFT analyzer 4 etc. Measure the property. The collected data is transmitted from the FFT analyzer 4 to the computer 5, and is compared with a database stored in advance in the computer 5 and showing the relationship between the charging current value (size, frequency characteristic) and the degree of cable insulator deterioration. The result of this comparison serves as data for diagnosing the degree of deterioration of the cable insulator.

With this, the charging current is decomposed into a direct current component, a low frequency component below the commercial frequency, a commercial frequency component, and a high frequency component above the commercial frequency, and the degree of deterioration is measured by the magnitude of each current value and the frequency characteristic. In addition, even when the water tree is short and the degree of deterioration of the insulator is small, an electric signal due to the water tree can be obtained.

[0013]

As described above, according to the method for detecting the degree of deterioration of the cable insulator of the present invention, the DC voltage or the low frequency voltage is applied before the charging current is measured. Even if the water tree is light and the length of the water tree is short, the DC component, low frequency component, and high frequency component of the charging current component can be increased to obtain an electrical signal due to the water tree, and the water tree bridges the cable insulator. The conventional problem that could be detected only when such deterioration has progressed extremely is solved, and it is possible to appropriately deal with the problem before the cable dielectric breakdown.

[Brief description of drawings]

FIG. 1 is a diagram showing a charging current measuring device in the method of the present invention.

[Explanation of symbols]

 1; Cable conductor 2; Shielding layer 3; Variable filter 4; FFT analyzer 5; Control computer

─────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] June 6, 1992

[Procedure 3]

[Document name to be corrected] Drawing

[Correction target item name] All drawings

[Correction method] Change

[Correction content]

[Figure 1]

Claims (1)

[Claims] 1. A method of detecting the degree of deterioration of insulation characteristics of a power cable by measuring a charging current component under an AC voltage flowing through a cable insulator, wherein a DC voltage or a low frequency voltage is applied, and then an AC voltage is applied. The charging current is measured by applying electricity, and the measured charging current is decomposed into a direct current component, a low frequency component below the commercial frequency, a commercial frequency component, and a high frequency component above the commercial frequency, and the magnitude and frequency of each current value. A method for detecting the degree of deterioration of a cable insulator, which comprises detecting the degree of deterioration of the insulator by measuring the characteristics.