Virtual Capacitor Strategy to Coordinate Stability and Power Balance in Droop-Controlled Input-Parallel–Output-Series Grid-Connected Inverter System

To address the high voltage difference between the low-voltage photovoltaic (PV) array and the mid-voltage distribution grid, the input-parallel output-series (IPOS) grid-connected inverter system is employed to reduce the reliance on high-ratio step-up transformers. This article focuses on the droop control strategy for IPOS systems, where a precise mathematical model reveals the dual effects of the droop coefficient: while increasing the coefficient enhances power balancing, excessive values introduce a negative damping effect that degrades system stability. To resolve this trade-off, this article proposes a virtual capacitor-based impedance reshaping method to optimize stability margins by adjusting the positive-damping region. Theoretically, rigorous derivation establishes quantitative design criteria for both droop coefficient and virtual capacitance parameters, providing comprehensive guidance for parameter tuning. Finally, experimental results based on a two-module IPOS prototype demonstrate that the proposed strategy maintains system stability while achieving power-sharing accuracy, validating both theoretical soundness and practical feasibility.