Double synchronous frame current regulation of distributed generation systems under unbalanced voltage conditions without sequence current separation

The usual strategy for current regulation of distributed generation sources in an unbalanced three phase system is to separately control the positive and negative sequence currents using independent positive and negative synchronously rotating frame current regulators. This approach requires the measured phase currents to be separated into positive and negative sequence components before transformation into their respective synchronous rotating frames, which is computationally expensive, potentially introduces errors and compromises dynamic and transient performance (particularly for weak grids). This paper presents a simpler approach for controlling unbalanced three phase currents that does not require separation of the measured currents into their individual sequence components, and thus avoids these problems. The approach is combined with classical synchronous frame PI regulators with maximized gains, to achieve excellent unbalanced current control with a minimum of complexity. Simulation and experimental results are provided to verify the strategy.