A Novel 6.5 kV Reverse Asymmetric Bypass Thyristor (RAByT) for VSC HVDC Systems

Modern modular multilevel converter (MMC)-based voltage source converter (VSC) high voltage direct current (HVDC) systems increasingly employ 6.5 kV trench-gate IGBT modules. These systems incorporate a dedicated bypass thyristor connected across the antiparallel fast recovery diode (FRD) to provide resettable protection against DC ground surge current faults. At voltage ratings of 6.5 kV, standard thyristor designs use relatively thick silicon, leading to high on state dynamic forward voltage, thus limiting their protection capability in this application. This paper presents a 6.5 kV reverse asymmetric bypass thyristor (RAByT), designed with an N-buffer diffusion located on the cathode side of the device enabling significantly thinner silicon. Furthermore, an optimised interdigitated gate-arm structure gives a low anode pick-up voltage, providing further enhanced protection. Finally, the high-resistivity N-base design improves the device’s reliability (cosmic ray FIT rating), making the RAByT an even more robust protection solution.