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External Insulation Design and Assessment for Critical Components in Medium Voltage SiC based Converter via Optical Method

Partial discharge measurement
Fig. 1. Partial discharge measurement system and typical time domain PD waveform under PWM excitation.
  For compact and reliable insulation design, electric field (E-field) intensity inside converters should be well controlled to avoid any partial discharge (PD). Usually, if PD happens in defects of solid insulation, it is called internal discharge. In contrast, if PD occurs in air along exposed metal or insulator interfaces, it is called external discharge. Accordingly, methods to explore PD in air should be assigned to the external insulation design and assessment category.
  External insulation design can be very important for power converters. Experimental investigation of PD in air under PWM excitation has been applied, in order to establish proper design and assess- ment guidelines for power converter application. The experimental measurement system and typical time domain PD waveform under PWM excitation are shown in Fig. 1. Based on all the experimental data and mechanism analysis, several external insulation design and assessment guidelines for power electronics application can be summarized. The external insulation and assessment should iterate until the final design achieves an external discharge free state under the normal operation voltage and passes the experimental assessment. A flowchart that demonstrates the iteration process of external insula- tion design and assessment, is provided in the paper
  Targeting on an external, discharge-free, single-turn inductor and transformer in an auxiliary power circuit, used in the self-designed medium voltage modular converter, have been designed, fabricated and tested. As shown in Fig. 2, there is significant E-field crowding at the terminations of the original single-turn inductor de- sign, showing that obvious external PD occurs even under relatively low voltage. After applying high permittivity material on those terminations, the E-field intensity in air reduces to an acceptable level and thus can pass the experimental assessment. The experimental data on representative coupons, design flowchart and examples pro- vided in this paper should help other engineers during the development of MV converters.
Original single-turn inductor design
Fig. 2. Original single-turn inductor design with electric field crowding at terminations and a typical phase-resolved PD pattern.
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