A 6.6-kV Transformerless Battery Energy Storage System Based on a Cascade Multilevel PWM Converter: Experimental Verification by a 200-V, 10-kW, 3.6-kWh Laboratory Model

Renewable energy sources such as wind turbine generators and photovoltaics are intermittent in nature, thus resulting in fluctuating electric power. Leveling of their electric power is indispensable to realize large-scale renewable energy sources, and to avoid severe power quality problems when they are interconnected into the grid. A battery energy storage system (BESS), combining batteries with a power converter and digital control, should be installed in the vicinity of an intermittent energy source. The fluctuating power is compensated by appropriately controlling an active power stored in, or released from, the battery. This paper describes a 6.6-kV transformerless battery energy storage system based on a cascade multilevel PWM (pulse-width-modulation) converter, with focus on a control method for active power and SOC (state-of-charge) balancing. A down-scaled 200-V, 10-kW, 3.6-kWh (13-MJ) laboratory model using nine NiMH (Nickel Metal Hydride) battery packs is designed, constructed, and tested to verify the viability and effectiveness of the 6.6-kV system.