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

Chengjun Shen,Jose Ortiz-Gonzalez,Olayiwola Alatise,Avinash Aithal,Phil Mellor,Saeed Jahdi

doi:10.1109/ojpel.2021.3060942

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.

Highlights

T HYRISTORS are arguably the most rugged power semiconductor devices
Silicon Carbide (SiC) thyristors can alleviate the electro-thermal stress on the silicon thyristors following bypass of a DC fault current
The reverse voltage on all thyristors is kept at its minimum, especially when the thyristors suffer from a high junction temperature due to bypassing a significant proportion of surge current

Summary

INTRODUCTION

T HYRISTORS are arguably the most rugged power semiconductor devices. They are capable of conducting currents as high as tens of kiloamperes while blocking voltages as high as several kilovolts. The forward bias of the upper diode means that the entire voltage of the capacitor (which up to the moment of fault was at its nominal value) collapses on the thyristor in the reverse polarity, while the thyristor still suffers from a very high junction temperature This is risky since the thermally-excited carriers have higher mobility and could skip the narrow bandgap of silicon, which could possibly initiate an avalanche breakdown and a potential failure by high electro-thermal stress. The cosmic ray failures in SiC is 10 times less than that of silicon devices [45] This can be improved in silicon thyristors at the cost of thicker drift regions, it would increase the on-state drop, resulting in higher share of fault current in diode. Enhancing the carrier lifetime results in a lower forward voltage enabling a better protection of diodes by diverting more current into the thyristor The higher maximum junction temperature of SiC provides some flexibility in managing this until thyristors with full-scale SiC wafers become available

PERFORMANCE ANALYSIS OF THYRISTORS AT DC FAULTS

Findings

CONCLUSION