JPH04109179A - Defect section detecting method for cable insulator - Google Patents

Abstract

PURPOSE:To detect a defect section of a cable insulator without omitting it by preliminarily charging the high voltage with the component at the specific frequency or above to a cable, then charging the commercial-frequency AC voltage. CONSTITUTION:Before AC is charged to a plastic-insulated power cable, the shock wave high voltage with the frequency component about 1 KHz or above is preliminarily charged, if a defect section exists in a cable insulator, the bud of a tree is generated at this portion in advance. The discharge charge quantity is increased when a partial discharge occurs, it is amplified to a large charge quantity about 5-10 pc from the start, and the discharge signal generated by the partial discharge of the insulator defect section exceeds the partial discharge detection sensitivity. It can be detected before the insulator defect section is broken, and an omen of the defect section breakdown is obtained. The partial discharge charge quantity is measured while the commercial-frequency AC voltage is charged, the breakdown omen of the cable insulator is detected, and a charge is immediately stopped before the cable insulator is broken.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for detecting insulator defects that cause cable breakage in order to manufacture defect-free cables.

Conventional technology] Conventionally, in order to evaluate defects in the insulator in plastic electrical cables, a high voltage was applied to the cable to destroy the defective part of the cable insulator, and the broken point was dismantled and the defective part was removed. We investigate defects such as foreign objects, voids, and protrusions in the conductive layer to find out the cause of breakdown in the defective part of the insulator, evaluate what kind of defects should be removed, and improve the cable manufacturing method based on this. We have been working to improve electrical performance.

[Problems to be Solved by the Invention] As mentioned above, in the conventional method of destroying a defective part in a cable insulator, a large fracture hole is created in the insulator due to heat and chemical changes when the defective part of the insulator is destroyed. Since the defective portions that are formed often disappear due to destruction, it has been difficult to investigate the defective portions and identify the cause of insulation breakdown.

Therefore, partial discharge measurements are performed at the same time as the AC voltage application test, and the partial discharge that occurs due to the formation and progression of a resin-like failure path called a tree, which occurs starting from a defective part in the cable insulation, is measured. By electrically detecting a defective part immediately before it breaks down and stopping the application of electricity before the destruction of the defective part reaches full destruction, the defective part can be detected while leaving the defective part of the insulator without disappearing. A method has been developed to make this possible.

According to this method, defective parts of the cable insulation can be detected without disappearing, but the discharge signal due to the partial discharge that occurs just before the breakdown of the defective part of the insulator is extremely small, so it is difficult to detect this due to measurement accuracy. The problem was that it was not easy.

The detection sensitivity of this partial discharge signal is 3 to 5 DC in discharge charge amount.
10~20p depending on the relationship with noise and surrounding conditions.
c, and the amount of partial discharge charge gradually increases in the process from partial destruction of the defective part of the cable insulation to complete circuit destruction, but as the detection sensitivity deteriorates, the time until destruction shortens, and the power supply is turned off. There will not be enough time to shut it off, and it will no longer be possible to prevent the destruction in advance.

For this reason, detection of partial discharge occurrence often fails, resulting in the destruction of materials.

In detecting a defective portion of a cable insulator, the present invention is capable of reliably detecting a defective portion of a cable insulator by detecting a discharge signal generated by the aforementioned partial discharge without causing the defective portion to disappear. The purpose of this invention is to provide a detection method.

[Means for Solving the Problems] In order to achieve the above object, the cable defect detection method of the present invention includes applying IKI (frequency components of z or higher) to the cable before applying an alternating current voltage to the plastic electric cable. Then, while applying a commercial frequency AC voltage, partial discharge is measured to detect signs of cable insulation breakdown, and the above-mentioned voltage is applied before the cable insulation breaks down. By stopping the power supply, defects in the cable insulator can be detected without disappearing.

[Operation: If there is a defect in the cable insulation, tree buds are generated in the defective part by pre-charging with the high voltage as described above.
In this preliminary electrification, by applying an impulse voltage wave having a frequency component of IKHz or higher, tree buds are effectively generated without completely destroying the defective portion of the insulator.

After this preliminary energization, the amount of partial discharge charge is measured while applying a commercial frequency alternating current voltage, and a signal that the amount of partial discharge charge increases, that is, a sign that a defective portion of the cable insulator will be destroyed is detected.

By detecting an increase in the amount of partial discharge charge, which is a sign of destruction of the insulator defective portion, and interrupting the application of electricity before the insulator is destroyed, destruction and disappearance of the insulator defective portion can be avoided. This makes it possible to detect a defective portion of the cable insulator without destroying or destroying the defective portion.

[Example] The present invention will be explained in detail below.

By using a VCV cable (insulation thickness: 3111) as a plastic insulated power cable, before applying AC power, first pre-charge the cable with a high voltage of an impulse voltage wave having a frequency component of IKHz or higher. If there is a defective part in the cable insulation, tree buds are generated in the defective part in advance.

The relationship between this frequency and the tree generation voltage is as shown in the figure.

By generating tree buds in advance in the defective part of the insulator through the preliminary charging described above, the amount of discharge charge when a partial discharge occurs increases, and the amount of discharge charge is 5 to 10% from the beginning.
This means that the amount of charge increased to pc and was amplified. As a result, a discharge signal generated by a partial discharge in the insulator defective portion, that is, a partial discharge signal, exceeds the partial discharge detection sensitivity, so that it becomes possible to immediately detect the insulator defective portion before it breaks down.

This increase in the amount of partial discharge charge is a sign that the defective portion of the cable insulator will break down.

As mentioned above, tree buds are generated from the defective part of the insulator by preliminary energization, and then, while applying a commercial frequency AC voltage, the amount of partial discharge charge is measured to detect signs of breakdown of the cable insulator. However, as soon as this sign of destruction is detected, the application of electricity is stopped immediately before the cable insulator is destroyed.

According to the experiment, the following results were obtained.

The results of experiments on sample A without pre-charging using a 6 V C ■ cable (insulation thickness 31 m) and sample B with pre-charging of 30 V shock voltage 10 times are shown in the table below. That's right.

This will greatly increase the amount of information needed to improve the pull characteristics, making it possible to improve the electrical performance of the pull.

[Brief explanation of drawings]

The figure is a diagram showing the relationship between frequency and tree generation voltage. In the pre-energized sample, the amount of partial discharge charge increased, and the voltage applied to the sample was cut off before the entire circuit was destroyed, and the defective part was successfully detected. Please make sure that the defective part will not be destroyed if only the pre-voltage voltage is applied. [Effect of the invention]

Claims (1)

[Claims] By pre-applying a high voltage with a frequency component of 1 KHz or more to the cable, and then measuring the partial discharge while applying a commercial frequency AC voltage, the amount of partial discharge charge, which is a sign of failure of a defective part of the cable insulation, can be determined. A method for detecting a defective part of a cable insulator, characterized in that the defective part of a cable insulator is detected by stopping the voltage application before the defective part of the insulator is destroyed.