An efficient method to assess the insulation condition of rotating machines is online partial discharge monitoring. Difficulties in online monitoring result from various noise sources associated with the machine and from the power system. The paper introduces and discusses the theories, different testing techniques and monitoring methods currently used by Industrial Research Limited and other laboratories. The design and testing of high frequency current transformers for partial discharge online monitoring are introduced. Laboratory and field tests on electrical machines are presented. A database has been developed for efficient insulation monitoring and maintenance. The database allows intra and inter comparisons of partial discharge, tan delta, capacitance between phases in a machine and with other machines easily. The functions of the database enhance the efficiency and provide more information for effective insulation condition assessment.
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