Virtual Admittance PLL Structure for Grid-forming Power Converters in Microgrids

Synchronization plays a fundamental role in connecting power electronic converters with the grid. Conventionally, the synchronous reference frame phase-locked loop (SRF-PLL) has been widely used to estimate the grid phase angle. Though SRF-PLL is effective under normal grid voltage conditions, its performance degrades drastically under weak grid conditions. This paper presents a control strategy for current controller of grid-connected power converters that contributes in the reduction of the negative effects of SRF-PLL during grid disturbances. The main control concept is based on virtual admittance. This approach is capable of operating effectively power converters when connected to weak grids and permit grid-forming operation. Simulation and experimental results are provided to validate the proposed controller.