Enhanced Operation of Grid-Following VSCs with Alternating-Voltage Control in Ultra-Weak 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. However, asymmetric d-q 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 d-q coupling, stability region, and voltage dynamics are co-investigated. To enhance weak-grid operations, a stability enhancement control is further proposed to reversely compensate for the negative coupling effects induced by the phase-locked loop and AVC. The proposed scheme enables rated power operations of the VSC in ultra-weak grids. Experiments confirm the validity of the proposed method.