Novel isolated cascaded half-bridge converter for battery energy storage systems

This paper presents a novel isolated cascaded half-bridge converter for medium voltage battery energy storage systems with three phase legs of series connected modules in star connection. Each module consists of a grid-side half-bridge, a battery-side full-bridge and a medium-frequency transformer providing isolation. The presented modulation ensures grid-side zero-current-switching (ZCS) and battery-side zero-voltage-switching (ZVS). The control system consists of a grid current control as well as a state-of-charge (SOC) balancing control for the batteries. Evaluation of the proposed module of a 6.6 kV, 150 kW, 1MWh design example shows an average module efficiency of 97.4% including semi-conductor and transformer losses. Simulation results verify the converter concept including the applied control structures.