Evaluation of equivalent insulation test voltage and reliability of electrical insulation of HTS power devices

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In this research, the equivalent high voltage testing method at room temperature for the promising high temperature superconducting (HTS) power apparatus at the stages of development, manufacturing and shipment was studied. A idea of the equivalent high voltage test in atmospheic air room temperature for the HTS power apparatus with coil structure is proposed to ensure the insulation reliability of the superconducting power apparatus.The proposed method for the evaluation of the equivalent test voltage is based on the idea of no partial discharge occurrence during the operation of the superconducting apparatus. The equivalent test voltage can be obtained from the two medium factors with respect to the potential distribution along the coil winding and the partial discharge (PD) inception voltage. The medium factor relating to the PD inception depends on the coil geometry and the state of the surrounding media at cryogenic temperature. The medium factor was successfully obtained by experiments and calculations for the turn-to-turn coil sample. The experimental values of the medium factor was smaller than the theoretical ones in the present test sample for the turn-to-turn coil structure and therefore it was more desirable to adopt the theoretical value of the medium factor.It was pointed out that the medium factor of the coil-to-coil insulation element should be determined from that of its insulation weakest part, which was triple junction or solid insulation surface. It was recommended for the safest side of the insulation design that the equivalent insulation test voltage at room temperature for the finished HTS apparatus should be determined by the maximum value of the medium factor selected from those of the insulation elements, and then that additional insulation distance is needed in at least one of the insulation elements for preventing discharge activities at room temperature.