Variable speed generating unit for stand-alone microgrids

Variable speed operation has been proposed in literature as a solution to efficiency and pollution problems for diesel generating units, as well to improve efficiency and power conversion in rotating systems for renewable energy sources. In general, different topologies of prime-movers, like internal combustion engines, wind turbines and hydro turbines, etc. can be used to move an electric generator at variable speed. In variable speed generating systems, the electric output frequency is disengaged from the mover speed through a power electronic converter. This paper deals with a newly-conceived integral-resonant single-loop output voltage control arrangement for a three-phase four-leg VSI, to be used in stand-alone microgrids, for a variable speed generating units using static conversion to provide the constant electric frequency required by the loads. The control structure is discussed and experimentally verified by means of a 25 kVA rated power converter prototype. It is shown that the proposed voltage control allows simplifying the overall control configuration, achieving zero-overhead power factor regulation and fast compensation of load unbalances. Further to that, a complete current limiting strategy is described and tested under different load conditions.