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. With this information, a transient induced at (and across) every disc of the winding by a system generated overvoltage can be calculated and compared to a transient appearing at the same winding section during the impulse test. The pertinence and severity of the standard impulse test can be assessed from this comparison. Subsequently, a dielectric stress in the winding insulation was calculated with the aid of an electric field plotting program. A critically stressed path was then selected in the winding insulation, and a cumulative stress computed along this path: The insulation safety margin was subsequently determined by comparing the cumulative stress to a withstand reference curve for paper-oil insulation. The reference curve for the bulk oil insulation, and for the creep stress along a spacer or barrier was derived from published works. 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.