JPH0618605A - Defect detecting method for macromolecule insulated power cable - Google Patents

Abstract

PURPOSE:To detect the defects of a cable by casting laser pulses on the outer surface of the cable, on which DC voltage is applied beforehand, and analyzing the generated induced current. CONSTITUTION:A cable conductor 1 is provided at the central part of a macromolecule insulated power cable A, and an inner semi-conducting layer 2, a cable insulator 3 and in outer semiconducting layer 4 are sequentially provided thereon. A laser pulse generator B emits laser pulses on the outer surface of the cable A. The induced current generated by the emission is detected and amplified with an amplifier C. The waveform is read with an oscilloscope D. For example after the DC of about 60kV is applied on the cable A for about 10 minutes, the conductor 1 and the semiconducting layers 2 and 4 are grounded. The laser pulses are emitted, and the induced current in the conductor 1 is detected. When a charge layer is present in the insulator when the pressure wave caused by the laser pulses is propagated in the cable insulator, an induced current flows through an external circuit. The induced current is proportional to the amount of the charge in the insulator. When projections and voids are present in the insulator 3, the peculiar current waveforms, which are not observed in the normal cable, are observed in the generated induced current. The defect in the insulator is detected ny observing the induced current.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for detecting defects in power cables, especially polymer insulated cables.

[0002]

2. Description of the Related Art The performance of power cables, especially polymer insulated cables, is significantly degraded by the presence of voids, foreign matter, or protrusions of the semiconductive layer in the insulator. Therefore, it is important to detect these defects in advance and judge the quality of the cable before shipping.

Conventionally, as such a defect detection method before shipment, a defect detection by X-ray, a breakdown voltage test by commercial frequency or DC voltage of the entire product, and a defect detection by corona discharge are adopted. Other than that, it is not the full length of the product,
A defect detection method is also used in which the existence of voids, foreign matter, and protrusions on the cable insulator at the front and rear positions in the product is actually measured, and the defect status of the product is estimated based on this.

[0004]

In the conventional detection method as described above, the sensitivity is low and the defects that can be detected are
It is limited to those with a certain size or more, therefore,
It could never be said that it was sufficient as a shipping test. Therefore, a defect detection method having excellent sensitivity has been demanded, but in reality, a detection method corresponding to this has not been developed. The present invention provides a novel defect detection method that meets such a demand.

[0005]

According to the present invention, as a cable defect detecting means, an induced current generated by applying a laser pulse to an outer surface of a cable to which a direct current voltage is applied in advance is analyzed and analyzed. This makes it possible to detect defects with extremely high sensitivity.

[0006]

1 shows a measuring circuit of a defect detecting method according to the present invention. In the figure, A is a polymer-insulated power cable, in which the inner semiconductive layer 2, the cable insulator 3, and the outer periphery of the cable conductor 1 provided in the central portion are provided.
The external semiconductive layer 4 is provided in the order of layers.
The polymer-insulated power cable A shown in the figure merely represents an example of a conventional cable of this type, and the detection target of the detection method according to the present invention is not limited to the cable shown in the figure.

Reference numeral B is a laser pulse generator for irradiating a laser pulse from the outer surface of the power cable A described above. C is an amplifier for detecting and amplifying the induced current generated based on the irradiation of the laser pulse, and D is an oscilloscope for reading the induced current amplified by the amplifier C as a waveform.

[0008] By the way, it has been conventionally confirmed that when a defect such as a foreign substance, a void, or a protrusion is present in the cable insulator, and when a DC voltage is applied to the cable, space charges are accumulated around these defects. ing. Then, in the charge storage region, the charge is accumulated more than twice the size of the defect, that is, even if the protrusion is 30 μm, the charge is accumulated up to about 60 to 100 μm. The detection method according to the present invention utilizes such facts.

The specific embodiment of the present invention is as follows. In the state shown in Fig. 1, a polymer insulated power cable (XLPE cable) having an insulation thickness of 4 mm and a conductor size of 100 mm ² was applied with DC 60 KV (conductor negative polarity) for 10 minutes, and then the cable conductor 1 and the internal semiconductor layer 2 And the external semiconductor layer 4 was grounded. In this state, a laser pulse is emitted as shown in FIG. 1 to detect the current induced in the cable conductor 1. When a pressure wave caused by a laser pulse propagates in a cable insulator, an induced current flows in an external circuit if a charge layer exists in the insulator. This induced current is proportional to the amount of charge inside the insulator.

FIG. 2 shows the waveform of the induced current in a normal cable. According to this, it is confirmed that in the normal insulator 3, only a small amount of negative charge exists in the entire region of the insulator, and there is no abnormality.

FIG. 3 shows an induced current waveform of a cable having protrusions on the inner semiconductive layer 2. In the figure, a large negative charge is found in the vicinity of the inner semiconductive layer 2, and after measuring the induced current, the laser pulse irradiation position was cut and disassembled and investigated. Was confirmed to exist.

FIG. 4 shows the induced current waveform of a cable having voids. According to the figure, the positive. Presence of both negative charges is seen. When the cable was disassembled and investigated in the same manner as above, insulator 3
It was confirmed that there was a void in the center of the.

As described above, when there are protrusions or voids in the insulator 3, a peculiar current waveform which is not found in a normal cable is observed in the induced current generated by the laser pulse irradiation. Therefore, by observing this induced current, defects existing in the insulator of the cable will be detected.

As the insulation thickness of the cable increases, the pressure pulse is attenuated and the sensitivity deteriorates. However, a defect of about 5 μm can be detected in a group having an insulation thickness of 4 mm. And the DC voltage applied in advance is 12 ~ 15KV
/ mm is sufficient, and by irradiating the laser pulse after the DC withstand voltage test, two tests will be performed simultaneously.

Further, in detecting defects over the entire length of the cable, a direct current voltage of 10 to 15 KV / mm is applied to the cable in advance, the cable is swept, and laser pulses are sequentially irradiated while rotating the laser pulse. Inspection runs over the entire circumference and length of the cable are enabled.

[0016]

According to the present invention, since the induced current generated by applying a laser pulse to the outer surface of a cable to which a direct current voltage is applied in advance is analyzed to detect defects in the cable, the entire length of the cable is detected. Not only is defect detection achieved without destroying the cable, the charge storage area will be displayed at more than twice the actual size of the defect, so its defect detection capability is The sensitivity is remarkably increased.

[Brief description of drawings]

FIG. 1 is a measurement circuit diagram showing a defect detection method according to the present invention.

FIG. 2 is a waveform graph showing an example of measurement of induced current based on laser pulse irradiation on a normal cable.

FIG. 3 is a waveform graph showing an example of measurement of an induced current based on laser pulse irradiation for a cable having protrusions.

FIG. 4 is a waveform graph showing an example of measurement of an induced current based on laser pulse irradiation for a cable having a void.

[Explanation of symbols]

 A Polymer-insulated power cable 1 Cable conductor 2 Inner semiconductive layer 3 Cable insulator 4 External semiconductive layer B Laser pulse generator C Amplifier D Oscilloscope

Claims (1)

[Claims] 1. A defect detection of a polymer-insulated power cable, characterized in that an induced current generated by applying a laser pulse to an outer surface of a cable to which a direct current voltage is applied in advance is analyzed to detect a defect in the cable. Law.