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

Abstract

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 the4.16kV/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 and25kV/15MVA MV-CHBC systems. For the 35kV/21MVA MVCHBC system, 3.3-kV and 6.5-kV SiC MOSFET-based designs are the most suitable ones. Experimental results of a 3.3-kV SiCMOSFET-based SM are demonstrated to validate the proposed methodology and MV-CHBC simulations under PLECS environment.