Measurements of dielectric stress in EHV power transformer insulation

A relatively high failure rate of the internal insulation of EHV power transformers has been reported by public utilities. This prompted a study of the safety margin of EHV transformers in service and under test. In this project, a number of transient overvoltages were digitally recorded in EHV substations and the transient voltage distribution along the HV winding was measured digitally on many untanked transformers. An evaluation of the safety margin indicates that test impulse waveforms specified in the standards do not always represent system-generated transients, which can jeopardize EHV transformer insulation. The insulation can be designed in such a way to keep a similar safety margin in different parts of the winding, and insulating materials can be used more efficiently