PDC极化去极化电流法测电缆绝缘

Insulation Detection and Analysis of XLPE Medium-Voltage Cables

Abstract:The polarization-depolarization current (PDC) method is used to detect the insulation status of medium -voltage cables and the testing data are analyzed. Thelow frequency component of depolarization current, which is relatively less sensitive to the cable length, is used to overcome the impact of cable length. Cable aging status is determined through analyzing the value range of the depolarization current ’s low frequency components,the average discharge rate and the impact of different polarization time. Finally, this method is applied in repaired cables and it is verifiedthat the characteristics of the repaired cable is close to that of new cable with same length, which shows that the method can be used for identification of the cable’s aging status.

Key words:medium-voltage cable; insulation detection; polarization and depolarization (PDC) method

0 introduction

Cross-linked polyethylene (XLPE) power cable has been widely used due to its realiable dielectric and mechanical properties[1].The insulation of the cable is greatly damaged by electricity tree and water tree generated in the long operation[2].Power outage caused by insulation aging could cause great economic losses, which requires the detection of the cable insulation condition.The PDC method is a method of detecting a large segment of the overall performance of the cable[3-4]. The measuring instrument used is simple, lightweight, and the method can effectively detect the cable insulation performance in a variety of reasons.

1 Fundamentals of Polarization - depolarization current method

Charging the tested cable by a constant voltage source,then a charging current ich is generated, after a long charge time we can obtain a steady-state current idc. After a time of t, switch off the voltage power to discharge, and a discharge current idisch is generated(idisch 'is merely the discharge current with polarity reversed and shifted backward in timeline by t), and then after a sufficiently long discharge, the current will be reduced to zero.We know the charging and discharging process are related to electrical characteristics of the power as well as the supply voltage, so we can analyze ich and idisch to determine the electrical characteristics of the cable, and then to determine whether the tested is excessively aging. Current variation in PDC measurement shown in FIG. 1

2 Measuring circuit and measurement procedures

PDC measurement circuit structure shown in Figure 2.Charging the tested cable by connected the high-voltage interrupter switch to the voltage source. After a period of polarization switch to connect KEYTHLEY 6485. We can use KEYTHLEY648 to observe and record depolarizing current, then transmit the information to a computer for analysis.

Fig.1 Current variation Fig.2 Circuit schematic Fig.3 The depolarization currents