Power-Quality Enhancement of a Grid-Following Inverter Under Grid Voltage Unbalance and Impedance Uncertainty in Modern Energy Systems

Abstract

Grid-following inverters play a vital role in integrating renewable energy systems into modern power grids. However, their performance deteriorates under grid voltage unbalance and uncertain grid impedance, resulting in current distortion and double-frequency active-power oscillations. This paper proposes a robust dual-sequence current control strategy for a three-phase grid-following inverter operating under these adverse conditions. The inverter model explicitly incorporates grid impedance uncertainty and is represented using a polytopic uncertainty model. Positive-and negative-sequence current controllers are independently designed based on a state-feedback integral structure, where the control gains are synthesized using linear matrix inequality (LMI) optimization to guarantee robust stability over the entire uncertainty range. In addition, sequence-based reference-current generation is employed to suppress the negative-sequence current and eliminate double-frequency power oscillations under unbalanced grid voltages. The effectiveness of the proposed approach is evaluated through simulation studies and compared with a previous robust power controller. The results suggest that the proposed controller significantly suppresses the 120 Hz active-power oscillation despite the presence of grid voltage unbalance and impedance uncertainty. Compared with a previous controller, the magnitude of the 120 Hz component is reduced by approximately 91%, demonstrating improved power-quality performance under grid voltage unbalance and impedance uncertainty.