JPH06308189A - Method for detecting partial discharge of cable - Google Patents

Abstract

PURPOSE:To make it possible to identify an external noise and to detect the insulation defect of a cable by a precursor cut-off method by repeating the steps for increasing an applied voltage to the cable and for stopping the voltage application when a partial discharge signal exceeds a reference voltage, and judging the partial discharge signal based on an electric tree when the voltage at the stop of the voltage application becomes one half of the voltage at the previous time. CONSTITUTION:A high-voltage transformer 3 for applying a high voltage is connected to one end of a cable 1, and a power-supply cut-off device 4, which shorts and cuts off the power supply of the transformer 3 is connected. Partial-discharge detecting resistors 5 are connected so as to hold an isolating part 2 at the central part of the cable 1 and further connected to a partial-discharge detector 6. The detector 6 automatically cuts off the power supply through the cut-off device 4 when the charge quantity of the discharge pulse exceeds the reference value. The application and the cut-off of the voltage to the cable 1 are repeated, and the voltage at the cut-off is recorded. When the cut-off voltage becomes approximately one half of the previous cut-off voltage, it is judged that the previous cut-off is the initial partial discharge as the precursor of breakdown. The position of the partial discharge is spotted, and the defect is detected.

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 partial discharge of a cable such as a cross-linked polyethylene insulated cable by the so-called precursor cutoff method.

[0002]

2. Description of the Related Art It is well known that dielectric breakdown occurs due to defects such as voids, foreign substances, and semiconductive layer protrusions existing in a cable insulator, and it is not possible to detect such defects. , It is extremely important to improve the performance of the cable and to know the remaining performance of the cable.

Conventionally, such a defect is generally detected by observing a dielectric breakdown path with a microscope, but the size of the defect is reduced as the manufacturing technique is improved, and the dielectric breakdown is destroyed. Even after that, the breaking holes were burned out due to the flowing current, and it became difficult to detect the trace of the defect. Therefore, a method has been developed for observing the starting point of the breakdown before the insulator is destroyed, that is, in the state where the electrical tree comes out from the defective portion. That is,
This method is called the pre-breaking method, which utilizes the phenomenon of partial discharge before the insulator breaks down, and conducts a dielectric breakdown test of the cable while measuring the partial discharge signal and detects the discharge. Immediately stop applying electricity and leave the insulator in a partially destroyed state.

By the way, the problem here is noise such as air corona and switching noise which comes from the outside of the cable when measuring the partial discharge signal. It is necessary to clearly distinguish between such external noise and partial discharge that occurs as a precursor phenomenon of breakdown, that is, breakdown precursor discharge.

Conventionally, the above problem has been dealt with by attaching a device for suppressing and removing noise to the measurement system and the voltage application system. However, such measures alone can completely eliminate the influence of external noise. It was difficult to remove.
Therefore, it is often the case that the external noise is erroneously recognized as a partial discharge generated from the cable insulator and the defect cannot be finally detected.

[0006]

As described above, in the prior art, it is difficult to distinguish between the pre-breakdown discharge signal and the external noise, and it is often mistakenly recognized. It was

The present invention has been made in consideration of such a point, so that the breakdown precursory discharge signal and the external noise can be clearly discriminated, and the insulation failure of the cable can be detected reliably and efficiently by the precursory interruption method. The purpose is to provide a method.

[0008]

As a result of intensive studies to achieve the above-mentioned object, the inventors of the present invention have found that when the voltage application is stopped due to the pre-breakdown discharge, the voltage is gradually and again gradually reduced. It was found that the voltage is stopped at a voltage of about 1/2 of the previous voltage, and this phenomenon serves as an index that can clearly distinguish the pre-breakdown discharge signal and external noise. The present invention has been completed.

That is, the present invention is a method of detecting a partial discharge of a cable in which a high voltage is applied to the cable to generate an electrical tree in the insulator, and when the partial discharge signal generated from the insulator reaches a reference value, the voltage is stopped. In, while gradually detecting the partial discharge signal, gradually increase the voltage applied to the cable,
When the detected value of the partial discharge signal exceeds the predetermined reference value, the process of immediately stopping the power supply is repeated, and the voltage when the power supply is stopped is almost 1% of the voltage when the power supply is stopped before that.
When it becomes / 2, the partial discharge signal is determined to be a partial discharge signal based on the electric tree.

It is preferable that the reference value of the partial discharge signal for interrupting the charging is a partial discharge charge amount of 120 to 200 pC. If it is less than 120 pC, the progress of the electrical tree will be insufficient, and if it exceeds 200 pC, the entire road will be destroyed, which may make it difficult to detect defects.

[0011]

In the method of the present invention, since the partial discharge signal generated as a precursor phenomenon of destruction can be clearly identified, it is possible to reliably and efficiently detect the partial discharge of the cable without misconception as in the conventional case. . Especially, the reference value of the partial discharge signal to stop charging is set to the partial discharge charge amount 120 to 20.
When it is set to 0 pC, the partial discharge of the cable can be detected more reliably.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A C according to an embodiment of the present invention will now be described with reference to the drawings.
A method of detecting a partial discharge of the V cable will be described. FIG. 1 is an electric circuit diagram of a device that applies an AC voltage to a test cable, detects a partial discharge pulse that occurs, and shuts off the power supply based on the partial discharge pulse.

In FIG. 1, reference numeral 1 denotes a sample CV cable having both ends subjected to predetermined terminal processing. This test CV cable 1 has a metal shielding layer on the outer semiconductive layer,
Edges are cut off at three places near the center and both ends. 2 shows these edge cuts. A high voltage transformer 3 is connected to one end of the sample CV cable 1 so that a high voltage is applied, and a power cutoff device 4 such as a thyristor for short-circuiting and cutting off the power supply of the high voltage transformer is connected. . On the other hand, the partial discharge detection resistors 5 and 5 are connected to the central portion of the sample CV cable 1 with the edge cut portion interposed therebetween, and are connected to the partial discharge detector 6. The partial discharge detector 6 is connected to the power cutoff device 4 and sends a signal to the power cutoff device 4 automatically when the detected charge quantity of the partial discharge pulse exceeds a predetermined reference value, for example, 200 pc. The power is cut off.

Therefore, in the present invention, by applying the voltage to the CV cable to be tested and interrupted by such a device, the voltage at the time of interruption is recorded, and the interruption voltage is the interruption voltage before that. When it becomes about 1/2 of the above, it is judged that the interruption before that is the initial partial discharge of the destruction precursor, and
Based on the conventional method, the position where the partial discharge occurs is located and the defect is detected.

That is, if the partial discharge of destruction precursor is confirmed as described above, for example, as shown in FIG. 2, the sample CV cable 1 is divided into the A side and the B side. A similar circuit is formed and an attempt is made to detect a partial discharge that is a precursor of breakdown. If partial discharge is detected,
It is judged that the defective portion is on the A side, and the A side is the A ₁ side and the A side.
_It is divided into two, and one side, for example, the A ₁ side, is tried to detect the partial discharge of the destruction precursor (If the partial discharge is not detected on the A side, it is judged that the defective portion is on the B side,
The B side is further divided into a B ₁ side and a B ₂ side, and an attempt is made to detect a partial discharge of destruction precursor on _one side, for example, the B ₁ side. ). In this way, while detecting the partial discharge of destruction precursor, gradually narrowing down the occurrence site,
When the length is narrowed down to about several cm, the insulator at the narrowed portion is sliced and observed under a microscope. As a result, the defective portion can be detected extremely efficiently and reliably.

An example in which a defect of the 66V class CV cable is detected by the above embodiment will be described below. FIG. 3 shows a partial discharge pulse generated at each of the following interruptions. As shown in Fig. 3, the cable was cut off immediately after boosting the voltage to 135kV, and when the voltage was started again from low voltage, it was cut off at 215kV. Here, it was judged that the previous cutoff was a malfunction due to noise, and when the voltage was again applied from a low voltage, it was cut off at 100 kV. Since this cutoff voltage is almost half of the previous cutoff voltage (215 kV), it was determined that these cutoffs were due to destruction precursors, and the location of the partial discharge pulse was specified by the above-mentioned narrowing down. After that, when the insulator of the specified portion was sliced and observed under a microscope, it was observed that an electrical tree of 2000 μm was developed from an inner conductive tree of 700 μm in length.

[0017]

As is apparent from the above embodiments, according to the cable partial discharge detection method of the present invention, the partial discharge due to the destruction precursor can be clearly discriminated from the external noise, so that the detection by false recognition can be performed. Loss of time and work is eliminated, and the partial discharge of the cable can be detected reliably and efficiently.

Particularly, if the reference value of the partial discharge pulse for stopping the voltage application is set within the range of the partial discharge charge amount of 120 to 200 pc, the electric tree is cut off when it has properly advanced, so that the partial discharge of the cable is detected. Can be made more reliable and efficient.

[Brief description of drawings]

FIG. 1 is an electric circuit diagram of an apparatus used in an embodiment of the present invention.

FIG. 2 is a diagram for explaining a method of narrowing down the occurrence position of partial discharge of a cable using the method of the present invention.

FIG. 3 is an example of partial discharge pulse at break measured according to an embodiment of the present invention.

[Explanation of symbols]

 1 ………… Test CV cable 3 ………… High voltage transformer 4 ………… Power cutoff device 5 ………… Partial discharge detection resistor 6 ………… Partial discharge detector

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Miyuki Ogishima 2-1-1 Oda Sakae, Kawasaki-ku, Kawasaki City, Kanagawa Prefecture, Showa Electric Cable Co., Ltd. No. 1-1 No. 1 Showa Electric Cable Co., Ltd.

Claims (2)

[Claims] 1. A method for detecting partial discharge of a cable, wherein a high voltage is applied to the cable to generate an electrical tree in the insulator, and when the partial discharge signal generated from this reaches a reference value, the power is stopped. Repeating the process of gradually increasing the voltage applied to the cable while detecting the discharge signal, and immediately stopping the power supply when the detected value of the partial discharge signal exceeds a predetermined reference value,
When the voltage at the time of stopping the voltage application becomes almost half of the voltage at the time of stopping the voltage application before that, the partial discharge signal is determined to be a partial discharge signal based on the electrical tree. Partial discharge detection method for cable. 2. The method for detecting partial discharge of a cable according to claim 1, wherein the reference value of the partial discharge signal for stopping charging is set to a partial discharge charge amount of 120 to 200 pC. Detection method.