A transformerless energy storage system based on a cascade multilevel PWM converter with star configuration

This paper describes a transformerless energy storage system based on a cascade multilevel pulsewidth modulation converter with star configuration. The system is intended for power leveling of renewable energy sources, as well as for improving power quality and reliability of a power distribution system. This paper pays attention to active-power control and voltage-balancing control that are indispensable for proper operation of the energy storage system. A 200-V 10-kW 8.8-kJ downscaled laboratory system is designed, constructed, and tested, replacing electric double-layer capacitors with large-capacity electrolytic capacitors. Experimental results obtained from the laboratory system verify the viability and effectiveness of the 6.6-kV energy storage system. Index Terms--Active-power control, cascade multilevel pulsewidth-modulation (PWM) converter, electric double-layer capacitor (EDLC), energy storage system, output-power leveling, renewable energy sources, voltage-balancing control.