Investigation of EHV Current Transformer Failure by Dielectric Frequency Response Technique

HV and EHV systems operate in a reliable and safe manner thanks to a proactive operational strategy and qualified expert judgement taken by asset management specialists and equipment manufacturers. Nevertheless, failure during operation is a potential risk that may derive in catastrophic events. In the field, there is a large amount of HV and EHV equipment such as bushings and instrument transformers with paper and mineral oil as main insulation media. For these equipment, oil sampling might be an option but it is not desirable, and sometimes, not even recommended by manufacturers. Aiming to avoid any catastrophic event, Hydro Quebec has relied on a variety of testing techniques to evaluate the condition of instrument transformers and bushings. Hydro Quebec incorporated dielectric frequency response (DFR) for condition assessment of oil-paper and resin-paper insulation systems. Technology now is not limited to low voltage DFR but it incorporates high voltage DFR (HVDFR) and the individual temperature correction (ITC) algorithm which have opened the opportunity for qualitative and quantitative evaluation. HV DFR and ITC present an alternative to intrusive diagnostic techniques for condition assessment and failure analysis of the main insulation of HV and EHV bushings and instrument transformers. The dielectric responses obtained from “normally aged” and “faulty” HV and EHV Current Transformers (CT) are discussed throughout this document, in addition with the analysis of the ITC algorithm as applied to MV, HV and EHV OIP CTs. The work covers a 765 kV rated OIP type CT removed from service and fully dissected to correlate DGA, DFR and actual findings inside the insulation system of the CT.