A Unilateral Outer-Loop Control for Reduction of Unbalanced and Harmonic Voltages to Negligible Levels in Islanded Inverter-Based Microgrids

Unbalanced and nonlinear loads introduce voltage unbalance and harmonic distortion in islanded inverter-based microgrids, posing significant challenges for sensitive equipment such as induction motors and power electronic devices. Conventional control solutions rely on bilateral band-pass filters with finite gain at harmonic frequencies, offering only limited suppression of unbalanced and harmonic voltages. This paper proposes a novel complex-valued control strategy employing unilateral band-pass filters with infinite gain at selected harmonic frequencies. Implemented as an outer control loop in parallel with the standard droop control and virtual impedance, the proposed control is sequence-selective, targeting only the dominant harmonic and negative-sequence components actually present. This yields two key advantages: first, unbalanced and harmonic voltages are suppressed to practically negligible levels; and second, the compensation task is decoupled from the inner loops, allowing their phase margins to be preserved while the stability of the outer loop is guaranteed independently. The effectiveness of the proposed approach has been validated through experimental testing on a laboratory-scale islanded AC microgrid, including dynamic transitions between balanced, unbalanced, and nonlinear loading conditions with plug-and-play operation. The results demonstrating the ability of the proposed controller to remove negative-sequence and harmonic voltages and to outperform state of-the-art outer-loop compensation methods.