Design and implementation of a single-phase three-wire transformerless battery energy storage system

This paper presents a single-phase three-wire (1φ3 w) transformerless battery energy storage system (BESS). Its power circuit is simple, since it consists of only one power converter. It has three legs to provide 1φ3 w 220/110 V output without using a transformer. For switching control, the BESS is decoupled into differential-mode and common-mode subsystems. Each subsystem has its own inner-loop current-forced switching control mechanism, their current commands at various operating modes are generated by outer-loop controllers. The controllers of differential-mode are designed to control the main voltage (220 V) source, and meanwhile to achieve the desired functions of BESS. As to the common-mode subsystem, its controllers are used to maintain the voltage balance of two subcircuits (110 V circuits) when the utility grid fails, and to keep the current in the neutral line of the utility grid at zero when the utility grid is in a normal condition. The operation principle of the proposed BESS and the design of controllers in various operating modes are described in detail. Some experimental results are provided to show the performance of the proposed BESS