On-line condition monitoring of power transformers: A case history

This paper describes how a hydrogen on-line monitor (Hydran®) accompanied with specialist help can assist Asset Managers/ Operational Managers make good decisions regarding critical plant and identify the warning signs of a potentially catastrophic failure in a large power transformers. In October 2006, a hydrogen monitor was installed in a 500 MVA, 19/230 kV step-up transformer. This monitor showed the presence of about 45 ppm of hydrogen, but readings were very stable with time. Unexpectedly, in June 2010, hydrogen readings started to increase at a very high rate in terms of milliliters per hour. Oil was sampled and dissolved gas analysis was carried out in the laboratory indicating a thermal fault type T2 (temperature between 300 °C and 700 °C) according to standard IEC 60599. For safety reasons, it was recommended to remove the transformer from service. Failure investigation showed bad connections in the conductor leads between the LV windings and the bushings. The molding of individual strands was not properly done and with time, the connection has degraded and reached high temperatures. Connections were repaired and the transformer put back in service again.