Post-Failure Evaluation of Dielectric Performance of Winding of 38-y.o. Transformer Enhanced by On-Line Moisture Monitoring

摘要

A 38-year old 20/27 MVA 66/11 kV utility zone substation transformer experienced a HV winding failure in February of 2005. Following replacement of the damaged HV component of this winding at a local workshop, the transformer was re-installed and loaded a few months later. Since then to the present day the transformer has been operational. During 2013-2014, the transformer was dried using an on-line dryout unit. A failure investigation conducted in 2005-2006 revealed that the transformer was extremely wet at the moment of failure. Traces of water were found at the bottom of the damaged winding. However, no free water was found during the fault investigation and inspection. The failure investigation also revealed that an arcing discharge within the winding between two neighbouring discs was taking place during a few hours. It was not a full arc that would bridge the winding and the earth: there was no through fault in the transformer. Numerous melted copper balls were found at the bottom of the damaged winding next to the traces of water. The following assumption was made during the failure investigation: the fatal discharge in the winding had been triggered by oversaturated (foggy) oil due to the excess of dissolved moisture. To prove this assumption, a number of on-line moisture and temperature monitors were installed on the transformer in 2007. The on-line data collected between 2007 and 2016 proved the assumption correct. This paper reveals the mechanism of the winding failure that involved condensation of micro droplets of water from the foggy oil onto the surface of winding paper and inception of a corona discharge on the wet paper surface that escalated into a disc-to-disc arcing discharge. The condition of the winding insulation and oil in the transformer, as well as unusual loading, weather and dielectric stresses on the day of failure, were additional important factors contributing to the failure occurrence.