JPS6214072A - Analysis of deterioration in insulation of crosslinked polyethylene power cable - Google Patents

Abstract

PURPOSE:To enable diagnosis of a highly accurate analysis of deterioration in the insulation, by applying an AC voltage as external energy in the release of the residual charge to ensure the release thereof sufficiently and in a short time. CONSTITUTION:After a switch 2 is set to the DC power source position to apply a DC voltage of 10-20kV to a sample cable 1 for about 10 minute and then, set to the ground position to discharge the residual charge for one minute. Then, the switch 2 is changed over to the test transformer 5 position to adjust a slide transformer 6 so that the voltage will boost to the maximum from zero at a fixed speed. Then, a fine direct current flowing through a cable 1 is detected with a detection resistance 7 and after the removal of AC components with a reduction filter 8, it is inputted into a Y terminal of an X-Y recorder 9 while a tertiary voltage of the transformer 5 is inputted into the X terminal of the recorder 9 through an AC transducer 10. Then, the output voltage of the transformer 5 is raised to the maximum from zero by adjusting the slide transformer 6 and then, lowered to zero to measure the released current value of an AC electric field. This enables simple diagnosis of the deterioration in the water tree at a high accuracy and in a short time.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field to which the Invention Pertains) The present invention relates to a method for diagnosing insulation deterioration of 6KV to 30Kv class power distribution cross-linked polyethylene (XLPE) cables.

[Prior art and its problems]

XLPE cables have superior dielectric properties compared to conventional oil-impregnated paper cables, and are easier to install and maintain, so they are currently used in cable lines from 6KV class to 275KV class. However, when XLPE cables are used for many years in a flooded environment (flooded pipes, submarine cables, etc.), water penetrates into the insulation, which causes insulation deterioration called water tree.

This water tree grows and develops with use, reducing the dielectric strength of the cable and eventually leading to dielectric breakdown of the cable. For this reason, the problem of preventing water tree deterioration or diagnosing water tree deterioration is currently an important issue in the field of insulation research.

Water trees are generated due to the interaction between water and a localized steep electric field in an insulator, and are formed in a dendritic or butterfly-like manner in the direction of the electric field, with protrusions on the interface of the insulator, minute voids, and foreign matter contained in the insulating layer as the nucleus. This is a form of deterioration that progresses in a tie-like manner. Microscopically, this is observed as a collection of water voids. However, in order to understand insulation deterioration caused by water trees on levers and prevent insulation breakdown of cables, water tree deterioration diagnostic tests have been conducted using characteristic items such as insulation resistance, DC leakage current, and dielectric loss angle (tanδ). Recently, the reverse absorption current method and residual voltage method have been developed to focus on the increase in residual electric field due to the development of water trees, or the decrease in insulation resistance due to water tree deterioration. New water tree diagnostic methods such as the potential decay method and the DC transient current method are being proposed one after another. However, these new and old diagnostic test methods each have advantages and disadvantages in terms of accuracy and field applicability, and have not yet achieved sufficient reliability.

A conventional diagnostic method using the DC emission current or the amount of emission charge based on the residual charge in the synthetic fiber edge as an index for determining deterioration will be described. After applying a DC voltage to a cable with water trees and a healthy cable without water trees, short-circuit the cable electrodes and measure the time characteristics of the discharge current.
As shown in Figure 1, a deteriorated cable with a water tree produces a current that takes a longer time to decay than a healthy cable. This is thought to be because charges generated by an electric polarization phenomenon when DC voltage is applied accumulate (remain) in the water tree degraded area, and this is observed as a current with a long decay time (relaxation time) during discharge. In the above-mentioned reverse absorption current method, this discharge current is measured for 3 to 30 seconds after the start of discharge, and the value Q/C obtained by dividing the integral amount Q of the current by the capacitance C of the cable during that time is used as the deterioration judgment amount. It is. However, since the specific resistance of polyethylene is very large, 1OI6Ω-1 or more, the residual charge IQ measured by this method is considered to be a part of the accumulated charge in the water tree deteriorated area, and even after measurement, most of the residual charge IQ remains in the insulation. It is expected that it remains in the body. Therefore, if all the accumulated charges in the water tree deteriorated portion can be released and measured, there is a possibility that the degree of water tree deterioration can be diagnosed with high measurement accuracy.

However, in order to do this, it is necessary to release the accumulated charge to the outside in a short time, and a commonly known method is to apply external energy such as electricity, light, or mechanical shock to the insulator to release or annihilate the charge. However, this problem has not yet been resolved.

[Purpose and characteristics of the invention]

The present invention aims to improve the accuracy of the determination by improving the conventional method that uses the DC discharge current or the amount of discharged charge based on the residual charge in the cable insulation as an index for determining deterioration. The present invention is characterized in that by applying an alternating current voltage as external energy when releasing the charge, the residual charge can be easily released sufficiently and in a short time, thereby realizing a highly accurate method for diagnosing insulation deterioration. It is something to do. Next, the embodiment will be described in detail using the drawings.

[Embodiments of the invention]

FIG. 2 is a side view of a diagnostic measurement circuit implementing the present invention, with reference to which the measurement method will now be explained. First switch (3)
ground, then turn on switch (2) to the DC power supply side,
Apply DC voltage to the test cable (1).

DC voltage is 10-20K for 6KV class cable.
V and an appropriate application time of 10 minutes. Next, switch (2
) to the ground side to pre-discharge residual charges with short relaxation times (charges that flow out without external energy). A suitable discharge time is 1 minute. Next, switch the switch (2) to the AC power supply side, that is, the test transformer (5) side, open the switch (3), and then adjust the slide transformer (6) from zero voltage to the maximum applied voltage (for 6KV class). 5.6KV)
The voltage is increased at a constant rate until The rate of increase in voltage is 0.
Approximately 5KV/sec is appropriate. As the AC voltage rises, a charging current begins to flow through the test cable, and superimposed on this, a small DC current flows. This is detected by a detection resistor (7), and the AC bone is removed by a low-frequency p-wave device (8) and introduced into the Y terminal of an X-Y recorder (9).

On the other hand, the tertiary voltage of the test transformer (5) is applied to the X-axis terminal of the X-Y recorder through the AC transducer Q (11).The output voltage of the transformer (5) is applied to the slide transformer (6).
When the voltage is increased from zero voltage to the maximum voltage by adjusting the voltage and then lowered to zero voltage, a relationship curve as shown in FIG. 3, for example, is drawn on the recording paper. In addition, Table 1 compares the AC field emission current (indicates the maximum value) according to the present invention measured for four types of 6KV class XLPE cables with the results obtained using the conventional water tree deterioration diagnosis method. The test cables 11h3 and 11h4 in the figure are the same as those in Table 1. As is clear from the results shown in the test example of FIG. 3, it can be seen that the residual charge in the insulator is easily and reliably released within a short time by the alternating current electric field. Table 1 also clearly shows the comparison between the conventional diagnostic method and the method of the present invention.

Table 1 [Effects of the Invention] As is clear from the above explanation, when the method of the present invention is used, (11) water tree deterioration can be easily diagnosed in a short time with high accuracy. There is no length effect (reduction in characteristic values due to cable length).(3) It can diagnose bowtie-like water tree deterioration, which occurs and progresses with microvoids in the intermediate layer of the insulator as a core, increasing the reliability of water tree deterioration diagnosis. This greatly contributes to the prevention of dielectric breakdown of XLPE cables.

[Brief explanation of drawings]

Fig. 1 is an example of the time characteristic of discharge current when the cable is short-circuited after applying a DC voltage between the core wire and the outer sheath of a sound cable, and Fig. 2 is an example of a diagnostic measurement circuit implementing the present invention. 3 are graphs showing the relationship between emission current and AC voltage measured by the method of the present invention. (1)...Test cable, +2), (3)...Switch, (4)...DC power supply, (5)...Test transformer, (6)...Slide transformer , (7)...Detection resistor, (8)...Low frequency p wave device, (9)...XY recorder, Ql...AC voltage transducer.

Claims (1)

[Claims] (1) Apply a DC voltage between the core wire and the jacket of the test cable, perform a ground discharge for 1 minute after a predetermined time (about 10 minutes), then apply an AC voltage, and apply the DC voltage of the charging current. A method for diagnosing insulation deterioration of a cross-linked polyethylene (XLPE) power cable, the method comprising determining the degree of insulation deterioration due to water trees by measuring the maximum value of minutes or the time-integrated amount (emitted charge).