Water Tree Detection in Medium Voltage XLPE Cables

During the period of 1993-1995 Manitoba Hydro investigated the claim by a service provider that water trees could be detected and located in service aged medium voltage XLPE cables by means of time domain reflectometry (TDR) following conditioning of the cable dielectric with direct voltage. In order to avoid subsequent short time in-service failures, it was necessary to limit the conditioning voltage to 0.5 per-unit. With this restriction, no conditioning related surge impedance anomalies were found in cable that subsequently was determined as containing substantial vented water trees. However, it was established that the measurement of leakage current at 0.5 per-unit negative polarity direct voltage did correlate with the magnitude of subsequent 0.1 Hz breakdown voltage. Beginning in 2014 Manitoba Hydro has been assessing service aged medium voltage XLPE cables using Tangent Delta measurements at 0.1 Hz voltages in accordance with IEEE Std 400.2. However, the notion that water tress can be detected and located as a result of conditioning with direct voltage is still of interest as is indicated by publications that have appeared in the literature in recent years, but the related conditioning utilizes direct voltages in excess of 0.5 per-unit.