HK electric´s experience of VLF diagnostic testing on distribution cables

The 11kV distribution cables in HK Electric system are installed underground by direct burial method. PILC (Paper Insulated Lead Covered) cables were commonly installed prior to 1976, before the adoption ofXLPE (Cross Linked Polyethylene) cables. Cables fail due to various reasons, in particular when they have been disturbed or damaged by external activities, leading to delayed cable failure or accelerated degradation in cable insulation. In most cases, high voltage withstand test alone could not identify the degraded component(s). With these challenges, it is important to select an effective means and formulate a condition based maintenance strategy to assess cable condition with an aim to remove the weak components before failure for proper management of the cable asset. A number of testing techniques for distribution cables were studied and field trialed starting in 2009. Very low frequency (VLF) sine-wave withstand test together with tangent delta (TO) and partial discharge (PD) mapping technique were finally selected as the main stream techniques. The main reason is that this method has proved effective in accurately identifying weak/problematic components or sections of a cable circuit. As the experience on VLF diagnostic testing was built up, in-house cable condition assessment guidelines by referring to international practices/testing and manufacturer recommendations were drawn up in early 2011 for classification of the severity of the insulation degradation. Through skillful interpretation of VLF TD results, PD mapping and PD patterns together with the use of cable installation details and the in-house guidelines, 89 weak components were identified after conducting 1,029 numbers of testing in 28 months. Through analyzing the cable fault statistics and VLF TD & PD testing results, our cable condition based assessment strategy was further fine tuned in early 2012. The details of this testing regime and the experience gained are elaborated in the paper.