Multi-loop control algorithms for seamless transition of grid-connected inverter

The grid-connected inverter works as a controlled current source in grid-connected mode, while operates as a controlled voltage source in standalone mode. So in case of utility faults or intentional islanding, the inverter has to change its control strategy from current control to voltage control. This paper first proposed a multi-loop voltage controller with capacitor differential voltage feedback inner loop and voltage reference feedforward for standlone system especially designed to maintain the voltage continuity and decrease the dynamic response time in transition. However, the turn-off characteristics of the SSR which is used as switch here makes the transition last for a long time up to half a cycle. So in order to force the grid currents through the SSR switches to decrease to zero at much less time and make the voltage fluctuates within permissible levels during SSRs turnoff period, the voltage control based voltage amplitude regulation, instantaneous voltage regulation algorithms and current control based zero current regulation algorithms have been adopted in transition. After disconnection from the grid, the inverter will recover its voltage to a rated level. Simulation and experiments are carried out to verigy the proposed controllers and algorithms.