In the last two decades, some new methods, based on the characterization of the dielectric response of transformer insulation in both time and frequency domains are being used by power utilities for assessment of the condition of power equipments. From fields and laboratory investigations, these techniques were found to be highly operating conditions dependant. During normal operating conditions, the temperature inside a transformer is much higher than ambient, depending upon the operating condition. Because field measurements last hours after switching off the transformer, the final temperature may be much lower than the initial. Thus at onsite measurements the water migration is commonly running, the transformer is in a non equilibrium state. This transient can lead to mistaken interpretation of insulation condition. In the current research work, a systematic investigation on the influence of thermal transients on the results of polarization and depolarization current measurements is presented. A series of experiments have been performed under controlled temperature conditions on an oil impregnated bushing model. Precautions that can be taken to minimize thermal transient effects are discussed.
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