Location of Water-tree Degradation in XLPE Cables by DC Leakage Current Measurement

Abstract

In general, the degradation span of 6.6kV XLPE cable with plural connections is determined by DC leakage current measurement. This method needs to disconnect and reconnect the shielding layer and the outer semi-conducting layer at the cable connecting part, and hence it takes a lot of time and cost.  We investigated a new method to locate the water tree degradation by DC leakage current. In 6.6kV XLPE cables the pulse-like leakage current called “kick” due to water tree degradation is often observed. Consequently, we detected the currents at the DC applying terminal and another terminal, and transformed them to the signals like a sine-wave having the phase difference exactly proportional to the time difference of arrival (TDOA) by using ceramic filters at 455kHz. The position of the water tree is calculated from TDOA obtained by the phase difference. In the experiments to locate the water tree using 6.6kV XLPE cable samples up to 85m long, the mean squared error was 2.2m. From the results, it is possible to locate the water tree in 6.6kV XLPE cable 2km long with an error of about 20m by optimizing the center frequency of the ceramic filter.