Minimizing inverter self-synchronization due to reactive power injection on weak grids

Power converter operation on high-impedance grids can result in oscillatory behavior as a result of “self-synchronization” of an inverter's phase detection unit (PDU). This phenomenon is particularly of concern in renewable energy applications and it occurs when the converter's injected current changes the voltage angle at the point-of-common coupling (PCC) and synchronizes to itself through its PDU. One of the most prevalent PDUs in industry is based on the synchronous-reference frame (SRF) phase-locked loop (PLL). In this work the large signal stability limit of the SRF-PLL is modified to include the angle of reactive current injection, the phenomena of reduced gain margin as a result of self-synchronization is explained and a method to prevent the effects of self-synchronization is proposed and demonstrated. The proposed method is referred to as the grid-sync (GS) PLL and requires a real-time estimation of the grid impedance. The GS-PLL is shown to improve small-signal stability in order to reduce phase-margin erosion.