Development of a 500-kW Modular Multilevel Cascade Converter for Battery Energy Storage Systems

Renewable energy sources such as wind turbine and photovoltaic power generators may make the power grid unstable due to their output fluctuations. Battery energy storage systems (BESSs) are being considered as a countermeasure for this issue. A modular multilevel cascade converter (MMCC) is expected as a power conversion circuit for BESSs because each bridge cell can control the state of charge of a battery unit, independent of that of another one, and the harmonic current generated is low enough to eliminate the ac filter, normally installed on conventional two-level converters, from the MMCC. This paper describes the development of a real-scale (500 kW) single-star-bridge-cell-based MMCC for BESSs and reports successful test results obtained from a downscaled grid model and a 6.6-kV real-scale distribution line.