Assessment of thermal aging of HVDC converter transformer insulation

An HVDC converter transformer was removed from service and salvaged after 24 years of service. It had not experienced an in-service failure, although others from the same family had failed as a result of overheating. The salvage of the transformer provided an opportunity to observe signs of overheated cellulose, to take paper samples for DP measurements, and to assess the mechanical strength of the bonded CTC.