This paper describes a 6.6-kV battery energy storage system based on a multilevel cascade PWM (pulse-width-modulation) converter with star configuration. It discusses design concepts with and without a line-frequency transformer for grid connection. The control system consists of SOC(state-of-charge)-balancing control and fault-tolerant control. The former is indispensable for effective utilization of battery energy while the latter is required for maintaining continuous operation during a converter-cell or battery-unit fault. A 200-V, 10-kW, 3.6-kWh laboratory system combining a three-phase cascade PWM converter with nine NiMH (nickel-metal-hydride) battery units is designed, constructed, and tested to verify the validity and effectiveness of the proposed control system.
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