A Droop-Based PI-QPR Control Strategy for Islanded Parallel-Inverter Microgrids

Parallel-inverter microgrids are prone to PCC voltage distortion during islanded operation with nonlinear and unbalanced loads. Virtual-impedance methods can reshape inverter output impedance, but they also add control complexity and may introduce extra voltage drops. This paper proposes a droop-based PI-QPR control strategy to improve PCC voltage quality in islanded parallel-inverter microgrids. The droop scheme generates the fundamental voltage and frequency references, and the PI-QPR voltage outer loop regulates the fundamental, negative-sequence, and dominant low-order harmonic voltage components in the dq synchronous reference frame. The PI regulator is used for the fundamental component, while QPR branches at 2ω0 and 6ω0 compensate the negative-sequence component and the dominant fifth- and seventh-order harmonics. No additional virtual-impedance loop is introduced. Two-inverter hardware-in-the-loop (HIL) tests were conducted under nonlinear and unbalanced load conditions. Compared with the traditional voltage controller, the proposed controller reduces the measured three-phase PCC-voltage THD from 5.79–6.02% to 2.41–2.79%, confirming improved PCC voltage quality in the tested islanded condition.