Highly Reliable Insulation Techniques for HTS Cable under Dynamic and Composite Stress Environment

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High temperature superconducting (HTS) cables have been developed, and their electrical insulation design techniques have being established. However, the electrical insulation performance under operational condition of HTS cables has not yet been clarified. For the reliable and rational insulation design of HTS cables, partial discharge (PD) characteristics of liquid nitrogen (LN_2)/polypropylene (PP) laminated paper composite insulation system for HTS cables should be understood and systematized. From the above viewpoint, we investigated PD generation characteristics under different operational conditions of HTS cables as follows:(1) Quench condition of HTS cables:The thermal stress due to quench of HTS cables would be transiently superimposed on the electric field stress under the operating voltage of HTS cables. Such electrical insulation characteristics under thermal/electrical combined stresses can be referred to as the “quench-induced PD characteristics" of HTS cables. Experiment … More al results revealed that the quench品nduced PD could be generated, even under the intrinsic or static PD inception voltage without the quench. The criteria of the thermal/electrical combined stress level for quench-induced PD inception were quantified for different LN_2 pressures and fault durations under sub-cooled condition.(2) Lightning surge invasion into HTS cables:The impulse voltage would be transiently superimposed on the operating ac voltage of HTS cables. Experimental results revealed that the ac/impulse superimposed voltage could trigger PD in the electrical insulation layers of HTS cables, which would also cause consecutive PD under the subsequent ac voltage. Such PD may be attributed to the residual gas bubbles in LN_2 generated by thermal energy of PD under the ac/impulse superimposed voltage. The consecutive ac PD generation characteristics were evaluated in terms of both the ac/impulse superimposed instantaneous voltage and the subsequent ac voltage. As the result, a criterion to discriminate the consecutive ac PD was identified for different ac/impulse superimposed voltages. Less