Design and Testing of the Auxiliary Circuit and Cooling System in Medium Voltage Converter

10 kV SiC MOSFET is a promising power semiconductor device for high performance medium voltage (MV) converters especially in grid applications. With outstanding device characteristics, 10 kV SiC MOSFET brings a variety of challenges in the design of the auxiliary circuit, cooling system and test of MV converter.

To tackle these challenges, this dissertation focuses on designing an auxiliary power supply (APS) that provides more than 10 kV insulation and sufficient EMI immunity. The protection scheme for the APS is investigated to isolate faults between different outputs. Testing with gate driver in a MV converter offers insight to designing with higher EMI immunity and more reliable startup.

10 kV SiC MOSFET temperature sensitive electrical parameters (TSEP) have not been identified. They are crucial to monitoring power module temperature and state of health. Potential TSEPs are selected based on static test and double pulse test (DPT). The impact of gate resistance and load current are analyzed, so that TSEP calibration is effective under different switching conditions. 10 kV SiC MOSFET discrete device TSEP is also presented, which shows similar thermal features as the modules.

At the converter level, the cooling system is required to be compact and offer high voltage insulation between the power components and cooling component. In liquid cooling, detailed design procedure considering coolant temperature, air velocity and pump pressure is specified. Air cooling design investigates the air velocity propagation among different air ducts. And the electric field distribution of the cooling system is simulated to determine insulation weak point.

The mechanical layout of a 20 kV 8-level flying capacitor converter is challenging because of stacking multiple components in a tight space while maintaining its insulation integrity. It is successfully tested to 20 kV, which demonstrates the overall robustness of the design.