Control algorithm research on seamless transfer for distributed resource with a LCL filter

Generally distributed resource (DR) with LCL-based grid inverter works as controlled current source in grid-connected mode, while operates in controlled voltage source in island. So in case of utility faults or intentional islanding, DR has to change its control strategy from controlled current source to controlled voltage source. This paper analyzes the transition between controlled current source and controlled voltage source on single-phase inverter and three-phase inverter, respectively, and then presents a novel control strategy based on controlled voltage mode whether connected to the grid or in stand-alone. Once the grid failure, this control strategy regulates the inverter output voltage amplitude quickly, which forces the grid currents through the SCR switches to decrease to zero at much less time and during this SCRs turn-off period, the voltage fluctuates within permissible levels. After disconnection from the grid, the inverter will recover its voltage amplitude to a rated level. The algorithm is discussed in this paper detailedly. Computer simulation and experiments proved the feasibility of this control scheme.