Diagnostic testing of oil-impregnated paper insulation in prorated power transformers under accelerated stress

Abstract

In the recent past, a considerable amount of work has been published on the dynamics of aging of oil impregnated paper (OIP) insulation in transformers. The insulation is always subjected to stresses of different types at the same point in time. Among the more important stress are, electrical, thermal and their combination. Insulating properties of both solid (cellulose paper) and liquid (mineral insulating oil) phases deteriorate with time due to an electrothermal process called aging. The status of insulation at a given point in time can therefore be assessed by monitoring such of those properties of insulation which are sensitive to the amount of aging, so as to be able to take and appropriate corrective measures to avoid/postpone impending insulation failure. This procedure, called diagnostic testing and condition monitoring (DTCM) aims to avoid catastrophic failure. The aim of the present investigation is to try and determine the most sensitive parameters which can serve as indices of aging and hence as a reasonably accurate indicator of possible imminent failure of apparatus insulation in service. To this end, carefully controlled laboratory aging experiments have been performed on about eight nominally identical and scaled-down models of prorated units incorporating all the essential features of actual power transformers under normal operating electric stress and accelerated thermal stress. The paper details the DTCM program designed to acquire aging data.