JPH0528350B2 - - Google Patents

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 cross-linked polyethylene (XLPE) cables for power distribution. [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. ing. but
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 treeing. This water tree grows with use,
This progresses and reduces the dielectric strength of the cable, 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 the insulation deterioration caused by the water tree of the lever and prevent insulation breakdown of the cable, we have traditionally measured insulation resistance, DC leakage current, and dielectric loss angle (tanδ).
Water tree deterioration diagnostic tests have been carried out using characteristic items such as water tree deterioration. New water tree diagnostic methods such as the potential attenuation method and the DC transient current method, which were developed with a focus on decreasing insulation resistance, 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 that uses the DC emission current or the amount of emission charge based on the residual charge in the insulator as an index for determining deterioration will now be described. After applying a DC voltage to a cable with water trees and a healthy cable with no water trees, we shorted the cable electrodes and measured the time characteristics of the discharge current. As shown in Figure 1, water trees were present. A current with a longer decay time is observed in a deteriorated cable than in a healthy cable. This is thought to be due to the fact that charges generated by the 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 reverse absorption current method described above, 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, the specific resistance of polyethylene is very high.
Since it is more than 10 ¹⁶ Ω-cm, the residual charge Q measured by this method is considered to be part of the accumulated charge in the water tree deteriorated part, and most of it remains in the insulator even after measurement. It is expected to be. Therefore, if all the accumulated charge in the water tree deteriorated part can be released and measured,
There is a possibility that the degree of water tree deterioration can be diagnosed with high judgment 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. [Objects and Features of the Invention] The present invention improves the conventional method in which the DC emission current or the amount of discharged charge based on the residual charge in the cable insulator is used as an indicator for deterioration judgment as described above, and improves the judgment accuracy. By applying an alternating current voltage as external energy to release the residual charge, the residual charge can be easily released sufficiently and in a short time, thereby preventing insulation deterioration with high accuracy. It is characterized by realizing a diagnostic method. Next, the embodiment will be described in detail using the drawings. [Embodiment of the Invention] FIG. 2 is a diagram showing an example of a diagnostic measurement circuit implementing the present invention, and the measurement method thereof will be explained below. First, switch 3 is grounded, then switch 2 is turned on to the DC power supply side, and DC voltage is applied to the cable 1 under test. For 6KV class cable, the DC voltage is
Appropriate voltage is 10-20KV and application time is 10 minutes. Next, switch 2 is set to the ground side to preliminarily discharge residual charges with short relaxation times (charges that flow out without external energy). A suitable discharge time is 1 minute. Next, switch 2 is switched to the AC power supply side, that is, the test transformer 5 side, and switch 3 is opened. Then, slide transformer 6 is adjusted to increase the voltage at a constant speed from zero voltage to the maximum applied voltage (6.6KV for 6KV class). boost the pressure. Appropriate rate of increase in voltage is about 0.5 KV/sec. As the AC voltage increases, 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 its alternating current component is removed by a low frequency converter 8 and introduced into the Y terminal of an XY 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 10.The output voltage of the transformer 5 is adjusted from zero voltage to the maximum voltage by adjusting the slide transformer 6. When the voltage is increased and then further lowered to zero voltage, a relationship curve as shown in FIG. 3, for example, is drawn on the recording paper. Table 1 also shows four types of
This is a comparison of the AC field emission current (indicating the maximum value) according to the present invention measured on a 6KV class XLPE cable and the results obtained by the conventional water tree deterioration diagnosis method. Test cable No. 3 in Figure 3 , No.
4 is the same as that 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.

〔Effect of the invention〕

As is clear from the above explanation, when using the method of the present invention, (1) water tree deterioration can be easily diagnosed with high accuracy and in a short time; (2) There is no cable length effect (reduction in characteristic values due to cable length) like in tanδ characteristics. (3) It is capable of diagnosing bowtie-like water tree deterioration that occurs and progresses with microvoids etc. as nuclei in the insulator intermediate layer, improving the reliability of water tree deterioration diagnosis.
This greatly contributes to the prevention of insulation breakdown of XLPE cables.

[Brief explanation of the drawing]

Figure 1 is an example of the time characteristic of the discharge current when the cable is short-circuited after applying a DC voltage between the core wire and the jacket of a sound cable, and Figure 2 is an example of a diagnostic measurement circuit implementing the present invention. FIG. 3 is an example of 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 device, 9...X-Y recorder, 10...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 period of time, then apply an AC voltage, and calculate the maximum value or time integral of the DC component of the charging current. A method for diagnosing insulation deterioration of a cross-linked polyethylene power cable, the method comprising determining the degree of insulation deterioration due to water tree by measuring a value.