Enhanced Operation of Grid-Following VSCs With Alternating-Voltage Control in Ultraweak Grids

Alternating-voltage control (AVC) is preferred in weak grids to enhance voltage stiffness and active power transfer capability for grid-following voltage source converters (VSCs) with a phase-locked loop (PLL). However, asymmetric dq coupling of control loops is intensified with the AVC due to reactive currents, which leads to undesired interactions and thus risks weak-grid instability. To tackle stability challenge considering the AVC, a parameter tuning approach is first proposed to design the AVC according to the system rating. Based on that, the impact of the AVC on the intensified dq coupling, stability region, and voltage dynamics are co-investigated. To enhance weak-grid operations, a stability enhancement control (SEC) is further proposed to reversely compensate for the negative coupling effects induced by the PLL and AVC. The proposed scheme enables rated power operations of the VSC in ultraweak grids. Experiments confirm the validity of the proposed method.