Linear Active Disturbance Rejection Control Strategy Based on Improved Quasi-Proportional Resonance for the Inner Current Loop of a Grid-Connected Inverter

Nonlinear and asymmetrical loads in microgrids cause severe current distortion and phase unbalance, presenting critical challenges for grid-connected energy storage inverters. Conventional dq-frame Linear Active Disturbance Rejection Control (LADRC) suffers from two key limitations: bandwidth coupling between the observer and controller, and insufficient AC disturbance attenuation. To address these issues, an improved Quasi-Proportional Resonant-LADRC (QPR-LADRC) strategy is proposed for the inner current loop. First, by reconfiguring the disturbance feedback structure within the Linear Extended State Observer (LESO), observer bandwidth is decoupled from controller bandwidth, eliminating parameter tuning conflicts. Second, parallel QPR units tuned at 2ω0 and 6ω0 are integrated into the LESO state-error feedback loop to construct a QPR-LESO. This provides high selective gain for dq-frame AC ripples, enabling zero steady-state error tracking for negative-sequence components and dominant 5th/7th harmonics. Validated via MATLAB/Simulink and StarSim Hardware-in-the-Loop (HIL) testing, the proposed strategy restricts grid-connected current Total Harmonic Distortion (THD) below 2.7% (2.61%, 2.54%, and 2.63% for phases A, B, and C) under severe nonlinear and unbalanced loads. Compared with conventional PI and conventional LADRC schemes, it achieves maximum THD reductions of 1.68% and 3.43%, respectively, while maintaining three-phase current unbalance below 1.1%.