Design of a Seamless Grid-Connected Inverter for Microgrid Applications

The objective of this paper is to propose a seamless grid-connected inverter (SGI) for microgrid applications. The proposed SGI is able to operate in the grid-connected mode or the standalone mode. In order to smooth the transfer transition from the grid-connected mode to the standalone mode and to isolate the microgrid from the grid fault immediately via the static switch, three seamless transfer strategies are developed: 1) predetermined sinusoidal waveform detection (PSWD); 2) fast commutation current compensation (FCCC); and 3) predictive transfer voltage control (PTVC). The PSWD strategy is proposed to predict the normal voltage of utility grid for detecting the utility fault in few voltage samples. The proposed FCCC strategy is able to provide the required commutation current for the silicon controlled rectifier (SCR), so the turn-off time of the static switch could be minimized. The PTVC strategy is proposed to prevent the voltage drop or spike as well as to quickly reach the steady-state in the standalone mode. The computer simulations and hardware experimental results from a prototype system are presented to verify the performance of the proposed SGI.