Optimal Medium-Voltage Cascaded H-Bridge Converters for High-Power Distribution System Applications

Medium-voltage (MV) cascaded H-bridge converter (MV-CHBC) provides a transformerless connection to MV distribution system applications such as grid-connected battery energy storage systems (G-BESSs). An MV-CHBC consists of multiple series-connected sub-modules (SMs) forming a wye-connected three-phase topology. The blocking voltage of the utilized power semiconductor modules impacts many converter parameters such as the number of required SMs. Thus, a stepwise design methodology is proposed to select the most suitable high-voltage (HV) module for voltages ranging from 4.16 kV to 35 kV. Considering that the converter current is constant and independent of the regarded voltage level, 4.16kV/2.5MVA, 13.8kV/8.5MVA, 25kV/15MVA, and 35kV/21MVA MV-CHBC systems are designed considering HV silicon (Si) IGBT and silicon carbide (SiC) MOSFET power modules rated 1.7 kV up to 10 kV. These designs are evaluated per criteria such as power losses, power density, system complexity, and number of parallel-connected modules. A multi-attribute decision-making (MADM) technique is applied to evaluate these designs to select the optimal one according to weights for each criterion. For the 4.16kV/2.5MVA MV-CHBC system, the 3.3 kV SiC MOSFET-based design is the most suitable one. The 6.5 kV SiC MOSFET-based designs are the optimal ones for the 13.8kV/8.5MVA and 25kV/15MVA MV-CHBC systems. For the 35kV/21MVA MV-CHBC system, 3.3 kV and 6.5 kV SiC MOSFET-based designs are the most suitable ones. Experimental results of a 3.3 kV SiC MOSFET-based SM are demonstrated to validate the proposed methodology and MV-CHBC simulations under PLECS environment.