JPH04109180A - Nondestructive test method for power cable - Google Patents

Abstract

PURPOSE:To measure a partial discharge while suppressing the infiltration of noises by connecting partial discharge detecting resistors at both-side shield layer portions of a cut slit section provided on a sample cable, heating the cable, and applying AC high voltage. CONSTITUTION:A sample cable A is inserted into an excitation heating AC unit 12, a conductor is excited and heated, and a partial discharge is measured while the AC high voltage of the secondary output of a test transformer 2 is applied to the cable A. When a partial discharge occurs on the cable A, the partial discharge is correctly detected and measured 10 with good sensitivity without being affected by noises by partial discharge detecting resistors 9a, 9b on both sides of a capacitor formed by a cut slit section 8. A partial discharge logical judgment device 11 judges the magnitude of the partial discharge based on the output signal and turns off GTO thyristors 5a, 5b of a primary side power source cutoff section 5 before an electrical tree is progressed and the breakdown of a defect section occurs, the applied voltage is cut off at a high speed, and the defect section can be disassembled, investigated and analyzed.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for non-destructively testing power cables.

[Conventional technology] Conventionally, testing at room temperature has been a common method for non-destructive testing of plastic insulated power cables, but when testing at room temperature, the breakdown voltage (partial discharge inception voltage) tends to be higher than when testing at high temperature. If the breakdown voltage was high, partial discharge measurements could not be carried out efficiently due to the occurrence of atmospheric discharge from the high voltage section, or the test itself could not be completed successfully due to destruction of the terminal section.

Therefore, it is possible to experiment by heating the cable to a high temperature of about 90 degrees Celsius to lower the dielectric breakdown value.
In this case, heating methods include a method using external heating such as a heater, a method of providing a lead on the high voltage side and energizing a conductor, and heating with Joule heat.

The method of measuring partial discharge by heating the cable from the outside with a heater is as shown in the second diagram.
0, the partial discharge detection resistor 23 is connected to the shielding layer of the cable, the heater winding 25 connected to the heating control power source 24 is wound around the outer circumference of the cable, and the Electricity is applied to heat the cable to a high temperature and partial discharge is measured.

The method of measuring partial discharge by heating the cable by energizing the conductor is as shown in Figure 3.
1. Test transformer 20 connected to partial discharge detection resistor 22
, a partial discharge detection resistor 23 is connected to the shielding layer of the cable, a current-carrying bar 26 is connected to each conductor end at both ends of the cable, and a current-carrying current transformer is connected to a current-carrying control power source 27 . 28, the cable conductor is energized and heated to measure partial discharge.

[The problem that the invention aims to solve fJ! ] To perform partial discharge measurements in non-destructive tests, it is necessary to prevent external noise from entering as much as possible, but in methods that use external heating, the heater wire wrapped around the cable acts as an antenna to receive external noise radio waves, causing noise. Also, with the method of energizing the conductor, it is easy for noise to enter the closed loop consisting of the energizing lead of the circuit on the high voltage output side of the test transformer 20 and the external coupling capacitor. When applying a high voltage while heating and measuring partial discharge, there is a problem that noise intrusion has a negative effect on the partial discharge measurement.

Therefore, the present invention aims to provide a non-destructive testing method that makes it possible to measure partial discharge while keeping the cable in a high-temperature state while minimizing the intrusion of noise.

[Means for Solving the Problems] In order to achieve the above object, a non-destructive testing method for power cables according to the present invention involves cutting off the shielding layers A2 and A3 of the sample cable A, which is a plastic insulated capeple, to form a capacitor. A slit portion 8 is provided, and an impedance 9a for partial discharge detection is provided in the shielding layer portion on both sides of the separation slit portion.
. .

[By creating a capacitor with a cut-off slit in the cable shielding layer, there is no need to use an external coupling capacitor to measure partial discharge, and this prevents noise from entering the high-voltage output side of the test transformer.] A closed loop consisting of the current-carrying lead and external coupling capacitor is not formed to allow for

By heating the cable by energizing the conductor and heating it with Joule heat, there is no longer a heater wire wrapped around the cable that acts as an antenna to pick up external noise radio waves.

Therefore, partial discharge can be measured without the influence of noise intrusion, and the occurrence of partial discharge can be detected with good sensitivity.

Partial discharge is measured while applying AC high voltage with the conductor of the sample cable energized. If partial discharge occurs in the defective part of the cable, it is detected by the partial discharge detection impedance on both sides of the capacitor formed by the cut-off slit part. Based on the partial discharge detection signal, the applied AC high voltage is immediately cut off at high speed before the defective part of the cable is destroyed.

This makes it possible to conduct disassembly and analysis without destroying the defective part.

[Example code] Embodiments of the present invention will be described below with reference to FIG.

In the figure, 1 is a commercial frequency power supply, 2 is a test transformer, 3 is its primary winding, and 4 is its secondary winding. This is a next-side power cutoff section, and 5a and 5b are GTO thyristors.

A is a sample cable of plastic insulated cable, A1 is its conductor, A2 and A3 are its shielding layers, 6.
6 is a test terminal of sample cable A.

This test terminal 6.6 is connected to the high voltage side energizing lead 7 and is also connected to the secondary winding 4 of the test transformer 2.

The above-mentioned shielding layer is cut at an appropriate location, separated into a shielding layer A2 portion and a shielding layer A3 portion, and then separated into these separated portions.
A y-to-portion 8 is provided. This separation slit portion 8 forms a capacitor in which the edges of the shielding layer portion A2 and the shielding layer portion A3 face each other with the slit interposed therebetween.

Partial discharge detection resistors 9a and 9b are connected as partial discharge detection impedance to the shield layer portions ^2 and shield layer portions ^3 on both sides of the separation slit portion 8, and the partial discharge detection resistors 9a and 9b are used for partial discharge measurement. Connect to the device 10.

A partial discharge logic determining device 11 is installed on the output side of the partial discharge measuring device 10! Subsequently, the output terminal of this partial discharge logic determining device 11 is connected to the GTO thyristors 5a and 5b of the primary r#J'@ source cutoff section 5.

12 is a current transformer for energization heating, and 13 is its energization current control device.

The above sample cable A is inserted into the current transformer 12 for energization and heating, and the conductor is heated by Joule heat, which is generated while applying the AC high voltage of the secondary output of the test transformer 2 to the sample cable A. Measure partial discharge.

The heating of this sample cable differs from the conventional method in that there is no heating wire wrapped around the outer periphery of the cable, which acts as an antenna to pick up external noise radio waves, and the high voltage output side of the test transformer 2 does not allow noise to enter, as in the conventional case. Since a closed loop consisting of the current-carrying lead and the external coupling capacitor is not formed, partial discharges can be measured efficiently with less noise intrusion even when AC voltage is applied, so partial discharges can be detected with good sensitivity. can be detected.

As mentioned above, the occurrence of partial discharge is investigated while heating the sample cable A. If a partial discharge occurs in the sample cable A, the noise is suppressed by the partial discharge detection resistors 9a and 9b on both sides of the capacitor formed by the separation slit part 8. A partial discharge logic determining device 11 that is accurately detected without being influenced and measured by a partial discharge measuring device 10 and receives its output signal.
determines the size of the partial discharge and outputs a power cutoff signal before the electrical tree develops significantly and causes damage to the defective part.
The GTO thyristors 5a and 5b of the next power cutoff section 5 are turned off. As a result, when a partial discharge occurs in a defective part of the sample cable A, the applied voltage is immediately cut off at high speed before the defective part is destroyed, making it possible to disassemble, investigate, and analyze the defective part.

According to the experiment, a 1 x 1501n" CV cable with a thickness of 31m was used as the sample cable A as described above, and the temperature directly above the conductor was maintained at 90°C by energizing the conductor. Electric voltage 70KV 1
0 minutes, then 10 V, step increase for 10 minutes, while energizing the cable, measure the partial discharge at the same time. In this state, 5 minutes after applying the applied voltage 150 V, Occurrence of partial discharge was observed, and the applied voltage was immediately cut off. The maximum partial discharge charge amount immediately before shutoff was 29 pC. When the defective part was subsequently dismantled and investigated, approximately 35
Approximately 2.71 mm on both sides of the electric field from a μm-sized metal foreign object
An electrical tree of length occurred, and the applied voltage was quickly cut off just before the sample cable insulation was bridged.

It should be noted that in addition to the GTO thyristor, an appropriate large current interrupting device may be used for the above-mentioned primary ll@power interrupter 5, and inductance or the like may be used instead of the resistors 9a and 9b as the partial discharge detection impedance. .

[Effects of the Invention] As described above, the non-destructive testing method for power cables of the present invention heats the cable by energizing the conductor when detecting partial discharge, and heats the cable by energizing the cable without using an external coupling capacitor. Since a capacitor is formed by providing a separation slit, it is possible to detect partial discharges that occur at cable defective parts while they are relatively small without noise intrusion, and therefore the electrical tree can significantly advance due to partial discharges. The AC high voltage applied to the cable can be quickly cut off before the defective part is destroyed, and failure of the cable non-destructive test can be prevented.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an embodiment of the present invention, and FIGS. 2 and 3 are diagrams showing a conventional example. A: Sample power cable A2, A3: Shielding layer 8: Separation slit part 9a
9b: Partial discharge detection impedance special fraud consultant Furukawa Electric Co., Ltd. agent Patent attorney Oka 1) Kikuji

Claims (1)

[Claims] A capacitor is formed by providing a separation slit in the shielding layer of the sample power cable, a partial discharge detection impedance is connected to the shielding layer on both sides of the separation slit, the cable is heated by conductor current, and an alternating current is applied to the cable. A method for non-destructive testing of power cables, characterized in that partial discharge is measured while applying a high voltage, and the AC high voltage applied to the cable is cut off based on a partial discharge detection signal.