On-site diagnostic tests on medium voltage, XLPE-insulated power cables

The dc depolarization current measurement technique was used to test 20 medium voltage, underground XLPE-insulated cables in the local utility power distribution network between January and March 2006 and then in March 2007. The cables varied in length (30 m to ~1600 m), voltage rating (9 to 28 kV), and age (4 to 36 years). Unlike a typical depolarization current measurement where only a low frequency portion of the current is monitored with an electrometer, the described measurements have also included the high frequency component of the depolarization current, occurring right after switching off the poling voltage. The measurements in 2007 also included the dielectric loss factor measurements between 10 mHz and 500 Hz to correlate the tan delta values with the parameters of the depolarization currents. A reasonable correlation was found between cable age and other operating conditions and the depolarization current characteristics. A one year period between 2006 and 2007 produced noticeable changes in the depolarization current characteristics for some cable and almost no change in others. No consistent time effect on the insulation of various cables could be identified.