An effective smooth transition control strategy using droop based synchronization for parallel inverters

In this paper a transition control strategy for parallel voltage source inverters (VSIs) operating in a microgrid system is presented. In the proposed control method, VSIs regulate the power flow in standalone (SA) and grid connected (GC) modes through a hybrid control architecture without needing of an external communication link between them. The hybrid controller uses the current control in GC mode and in droop based voltage controller in SA and it switches autonomously and smoothly between them depending on the mode of operation. During the transition periods between the two modes of operation, the current control and droop control participate with different in formulating the inverter output voltage. To ensure high power quality during transition, one of the VSIs is designated as a dispatch unit to facilitate interconnection with utility grid and achieve proper waveform between the modes of operations. The dispatch unit is controlled always using droop control and the parameters of which are adjusted based on the current mode of operation. The proposed control technique provides proper performance, stabile operation, and fast dynamic response in the microgrid system during the transition mode of operation. The performance of the proposed control technique has been verified with simulation and experimental studies.