Abstract：Parallel inverter systems are widely used in AC microgrids, renewable energy generation, UPS, and electric vehicles. Achieving reasonable power sharing among inverters is a key technology for the stable and reliable operation of parallel systems. Traditional droop control can achieve active power sharing but struggles to ensure equitable distribution of nonactive power (including reactive, unbalanced, and harmonic power) due to inverter parameter mismatches or line impedance differences, potentially leading to inverter overloading or system instability. This paper proposes a dual-frequency-droop control strategy without communication or line impedance measurements. Reactive, unbalanced, and harmonic power all represent nonactive current components, and their distribution depends on the inverter output impedance. Equalizing the equivalent line impedance through virtual impedance enables simultaneous sharing of these power components. Following IEEE 1459-2010 standards that define nonactive power as encompassing reactive, unbalanced, and harmonic power, this strategy treats nonactive power as the unified control target. The dual-frequency-droop control includes conventional active power- fundamental frequency droop control for active power sharing and novel nonactive power-specific harmonic frequency droop control for nonactive power sharing. The method injects a specific harmonic signal (e.g., 300 Hz low-amplitude AC voltage) into the inverter output. Thus, a droop relationship between harmonic frequency and nonactive power, as well as a positive correlation between harmonic active power and virtual impedance, can be established. It forms a complete control loop for adaptive virtual impedance adjustment, compensating line impedance differences and ultimately achieving accurate nonactive power sharing. A simulation model was developed in Matlab/Simulink featuring two parallel inverters with equal capacity connected to the PCC through different line impedances (predominantly inductive), supplying balanced, unbalanced, and nonlinear loads simultaneously. An identical experimental setup was constructed. In simulation, compared with conventional droop control and small-signal AC injection methods, the dual-frequency-droop control maintains accurate nonactive power distribution and improves current sharing among inverters. Dynamic response tests under various load disturbances show that the proposed method quickly restores stable operation while maintaining proper power sharing during balanced, unbalanced, and nonlinear load transients. The dual-frequency-droop control achieves effective nonactive power sharing and current distribution. The following conclusion can be drawn. (1) The proposed dual-frequency-droop control is suitable for complex microgrids with high renewable energy penetration, particularly in systems with significant line impedance variation, high harmonic distortion, and three-phase unbalance. (2) In microgrids with mixed linear loads, nonlinear loads, single hybrid supply, three-phase hybrid supply, and multiple parallel inverters, this method simultaneously achieves accurate sharing of fundamental active power, reactive power, harmonic power, and unbalanced power. Compared to conventional droop control, it maintains communication-free operation while improving adaptability to complex load conditions.
刘宝谨, 张彬荣, 裘子豪, 何家盛, 郑峰. 基于双频率下垂的逆变器并联系统非有功功率均分控制方法[J]. 电工技术学报, 2025, 40(22): 7349-7362.
Liu Baojin, Zhang Binrong, Qiu Zihao, He Jiasheng, Zheng Feng. A Dual-Frequency-Droop Based Nonactive Power Sharing Control Method for Inverter-Parallel Systems. Transactions of China Electrotechnical Society, 2025, 40(22): 7349-7362.
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