Insulation Design and Assessment Considerations to Eliminate Partial Discharge in SiC based Medium Voltage Converters
For compact and reliable insulation design, partial discharge (PD) must be eliminated to achieve the desired lifetime of the insulation system and guarantee the safe operation of the converter system. With a PD-free design, lifetime-based insulation design can be further applied. For eliminating PD in converter systems, power electronics engineers, who try to design and evaluate their converter insulation, usually need to answer four major questions: 1) What are the critical components or interconnections for insulation design and assessment in SiC-based modular converters; 2) How can the experimental investigation on representative insulation structures help the design of the converter prototype; 3) What can be the practical ways for electric field (E-field) management; 4) How can experimental insulation assessment be applied properly in converter application for acceptance test or improvement verification purposes.
With a self-designed SiC-based medium-voltage modular converter as shown in Fig. 1, a systematic review and summary are provided to fully analyze the critical insulation regions inside such converters. Then, experimental investigation data for PD under PWM excitation is summarized and discussed. Admittedly, due to the effectiveness of measurement methods, most of the experimental data today is based on simple representative coupons and may lack a physics-based explanation; this can still give application engineers a sense for the design. Based on the design guidelines from experimental investigation, further E-field management can help improve the insulation in converters. General E-field management methods and their possible adverse effects on other design aspects of the power converters are demonstrated with examples. Finally, in order to verify the insulation improvement or provide some ways for acceptance tests, experimental insulation assessment methods under 60 Hz ac, dc and PWM excitation are discussed and compared. Depending on the purposes, conventional 60 Hz ac and dc tests may be more effective and practical than tests under PWM excitation. With the summary of related CPES experience and results published by other researchers, this paper should apply a better perspective on the four questions mentioned above and provide a framework for related research work in the future.