Voltage phase regulated seamless transfer control strategy for utility-interconnected fuel cell inverters with an LCL-filter

This paper proposes a new control strategy for the transition between grid-tied mode and off-grid mode of the utility interconnected fuel cell inverters with an LCL filter. It is operated in current-controlled mode, when the inverter is grid-interconnected. And in the stand-alone mode, it is operated in the voltage-controlled mode. For sensitive and mission critical local loads, a seamless transition between the two operating mode of the fuel cell inverter is very valuable, extraordinarily for the transition from grid-tied mode to stand-alone mode in the case of a utility fault. In the proposed control strategy, for the transition from grid-tied mode to stand-alone mode, the inverter control is switched to voltage-controlled mode from the beginning of the transition, using inverter output voltage phase regulation to reduce the output grid current of the inverter quickly, so the static transfer switch is turned off immediately, result in a short transition time and minimum voltage fluctuation on the load. The algorithm is described and discussed in detail, along with their transition performances. Simulations and experiments on a 5kW fuel cell inverter prototype are provided to validate the algorithm.