Control of a Three-phase Four-wire Inverter

In this paper, a three-phase four-leg voltage source inverter operating in island mode is described. The four-leg inverter is implemented by using a delta/wye or ZigZag transformer to meet isolation requirement. The control scheme includes an inner current loop providing the capability of fast current limiting and outer voltage loop. Digital sliding mode control is used for the inner current loop which requires higher bandwidth. The voltage loop is implemented in synchronous frame with selected harmonics cancellation for both positive and negative sequence components. Simulation and test of a 125 kW inverter at various operation conditions are presented to verify the validity of the control method. Index Terms—Three-phase, Island mode, Harmonics cancellation, Current control, Voltage control, Uninterruptible power systems, Pulse width modulation, Inverters, Synchronous frame, Stationary frame.