Abstract：To address the high-voltage difference between the low-voltage photovoltaic (PV) array and the mid-voltage distribution grid, an input-parallel output-series (IPOS) grid-connected inverter system is employed to reduce reliance on high-ratio step-up transformers. Unlike a single inverter, this modular system introduces multiple control objectives: LCL resonance damping, high-power-factor grid connection, inter-module power sharing, and wireless control. This paper selects the inverter-side inductor current and capacitor voltage as the control variables and proposes a wireless control strategy based on droop control. By optimizing the control structure without adding new control variables, this strategy simultaneously fulfills all four key control objectives. However, the theoretical and modeling analyses demonstrate that the droop coefficient presents a critical design trade-off. A simplified control-loop analysis validates that increasing this coefficient improves power-sharing accuracy in the IPOS configuration. In contrast, subsequent virtual-impedance modeling reveals that droop control inherently introduces an equivalent negative resistance, thereby progressively shrinking the system's positive-damping region. Consequently, beyond a specific critical value, an excessive droop coefficient inevitably introduces right-half-plane (RHP) poles, degrading system stability. Therefore, this paper proposes a virtual-capacitor-based impedance-reshaping method to optimize the system stability margin by adjusting the positive-damping region. The corresponding control block diagram and the equivalent virtual impedance structure incorporating the virtual capacitor control are illustrated in App.Fig.1. In the stability design, besides the positive-damping region, the influence of both the droop coefficient and the virtual capacitance on the system’s actual resonant frequency is taken into account. A transcendental equation for the system is derived, and a three-dimensional graph of the boundaries of the positive-damping region and the actual resonant frequency is plotted. Following a parameter design procedure that prioritizes the droop coefficient before determining the virtual capacitance, key cross-sections of the 3D graph are analyzed. Then, a systematic parameter design guideline is established. Finally, a laboratory prototype of a two-module IPOS grid-connected inverter system was built. The results demonstrate that the constructed droop control strategy and virtual capacitor strategy enable the IPOS inverter system to achieve multi-objective coordinated optimization control. The contribution of this article lies in the following aspects. (1) Construct an interconnection-free droop control strategy by selecting the inverter-side inductor current and capacitor voltage as control variables. (2) Reveal the inherent conflict between power balance and system stability in the droop-controlled IPOS GCI system through mathematical modeling, and an excessively large droop coefficient inevitably leads to resonance instability. (3) Propose a virtual capacitor-based impedance reshaping method to maintain power balance and enhance system stability.
刘承易, 方天治, 金启源, 刘昊, 刘梦雨. 输入并联输出串联并网逆变器系统多重控制目标的下垂策略实现及稳定性优化研究[J]. 电工技术学报, 2026, 41(12): 4176-4188.
Liu Chengyi, Fang Tianzhi, Jin Qiyuan, Liu Hao, Liu Mengyu. Droop Control to Achieve Multi-Objective and Stability Optimization for Input-Parallel Output-Series Grid-Connected Inverter System. Transactions of China Electrotechnical Society, 2026, 41(12): 4176-4188.
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