A Three-Phase Isolated Building Block for High-Power Medium-Voltage Grid Applications

Abstract

High-voltage (HV) wide bandgap (WBG) silicon carbide (SiC) power modules, capable of higher switching frequencies and thermal stresses than their silicon-based counterparts, are well suited for the next generation of highly efficient power converters for medium-voltage (MV) grid applications. HV SiC power modules allow for simpler two-level HV converter topologies, reducing control and system-level complexity while increasing power density, efficiency, and reliability; furthermore, modular designs are also possible for multilevel converters at 15 kV-class distribution voltages and beyond. Both 10 kV and 6.5 kV SiC MOSFETs are leading candidates for developing multiple MV grid applications. This article presents the design and test of a two-level dc–dc solid-state transformer (SST) based on third-generation 10 kV SiC power modules as a building block for MV power conversion. The converter configuration is based on a three-phase dual-active bridge (TPDAB). An integrated leakage methodology is used to meet the series inductance requirements for maximum power transfer and optimize the transformer volume. Because of the transformer stray capacitances and high dv/dt of the SiC MOSFETs, high-frequency oscillations (HFO) become a significant challenge to avoid unwanted voltage spikes, current resonances, and losses. The HFO amplitudes can be minimized by considering transformer winding arrangements with lower stray capacitances, leading to higher oscillation frequencies. Hence, design considerations and boundaries for the power converter and the high-frequency (HF) transformer are derived and presented. Experiments on a 150 kW TPDAB are conducted to validate the proposed design, yielding efficiencies above 98% at 20, 40, and 100 kW power levels, and demonstrate significant mitigation of HFO issues.