A transformerless battery energy storage system based on a multilevel cascade PWM converter

Renewable energy sources such as wind turbine generators and photovoltaics produce a fluctuating electric power. A battery energy storage system (BESS) should be installed in the vicinity of these sources. The fluctuating power is compensated by appropriately controlling an active power stored in the battery. This paper describes a feasible circuit configuration of a 6.6-kV transformerless battery energy storage system based on a multilevel cascade PWM (pulse-width-modulation) converter, with focus on a control method for active power and SOC (state-of- charge) balancing. A 200-V, 10-kW, 3.6-kWh (13-MJ) laboratory system combining a multilevel cascade PWM converter with nine NiMH (nickel metal hydride) battery units is designed, constructed, and tested to verify the viability and effectiveness of the 6.6-kV system.