Design considerations of 6.5kV enabled three-level and 10kV enabled two-level medium voltage SST

The advent of medium voltage silicon carbide (SiC) power semiconductor devices (6.5kV and 10 kV) has opened up the possibilities of looking into different converter topologies for the MV grid interfaced applications. A medium voltage mobile utility support equipment-based three-phase solid-state transformer (MUSE-SST) system is one such application aimed to interconnect a three-phase 4160 V/60 Hz grid to a three-phase 480 V/60 Hz grid to provide a shore-to-ship power interface for naval vessels. The system can be realized by both 10 kV SiC MOSFET and 6.5kV SiC MOSFET employing a two-level and three-level architecture respectively. The aim of this paper is to understand the thermal challenges and provides detailed design considerations of the two MV device-based architectures for a system scale-up to 500kVA rating. Device characteristics for both 6.5kV and 10kV SiC MOSFETs have been evaluated from experimental results. Based on these experimental data, the thermal performance of these devices enabled converter architecture is compared using elctro-thermal simulation-based loss comparison.

I. Introduction

MV high power converters are conventionally built using silicon (Si)- based insulated-gate bipolar transistors (IGBTs) [1], [2]. These silicon-based devices limit the switching frequency to less than 1 kHz due to their slow switching speeds. With the recent advancement of wide bandgap semiconductors, power devices such as SiC MOSFETs with blocking voltages up to 15 kV are now available to realize the MV power converters [3]. SiC-based devices can operate at higher switching frequencies ( kHz) with hard-switching PWM, without considerable switching losses. A high switching frequency helps reduces the size of magnetic components, hence improving the power density of the MV converters. The advantages of these medium voltage SiC power semiconductor devices have opened up different areas of applications, including high-speed MV motor drives, medium voltage high power EV battery charging [4], [5] and solid-state transformers interfaced to medium voltage AC grids [6]–[9].