Prospects and Challenges of 4H-SiC Thyristors in Protection of HB-MMC-VSC-HVDC Converters

Abstract

Pole-to-pole DC faults on HB-MMC-VSC-HVDC schemes impose significant risk of cascade failure on IGBT/diode pairs. Other novel topologies with fault blocking capability, i.e. AAC converters, and DC circuit breakers are not yet fully matured. Therefore, silicon thyristors are used to bypass the DC faults until AC breakers activate. However, silicon thyristors are also at risk of failure due to the capacitor voltage collapse at high junction temperatures caused due to imbalanced reverse recovery current conduction. Hence, the recovery cycles are included as part of IEC standard 62 501 HVDC type-test program. Emergence of commercial Silicon Carbide (SiC) thyristors has the potential to tackle this risk. This paper investigates such opportunities and challenges by accurately modeling the performance of thyristors at fault. It was seen that SiC thyristors with acceptable surge current and reverse blocking capability can eliminate the failure mode of silicon thyristors due to minimal recovery stored charge, resulting in an equal share of reverse voltage on all thyristors.