The effects of long term operation and system conditions on the dielectric capability and insulation coordination of large power transformers

The dielectric design and insulation coordination required for a transformer are predominantly a function of the requirements of the user. Applicable standards provide both a methodology and criteria through which the industry can then construct and proof test a unit. This permits the user to coordinate the insulation of the transformer with its application in a power system. However, the transformer is then subjected to the actual conditions and operating practices at its installed location. These conditions, and the passage of time, present challenges to the dielectric design different from those of the factory. This paper discusses those differences, and their effect on the operation of large power transformers.