The Dual-Mode Combined Control Strategy for Centralized Photovoltaic Grid-Connected Inverters Based on Double-Split Transformers

Abstract

Centralized photovoltaic (PV) grid-connected inverters (GCIs) based on double-split transformers have been widely used in large-scale desert PV plants. However, due to the large fluctuation of short circuit ratio (SCR) under high-penetration PV power plants, the stability of GCIs controlled in current source mode (CSM) is seriously affected. Reducing the bandwidth of the phase-locked loop (PLL) will improve the stability of CSM-controlled GCI, but the dynamic response performance will be degraded. Even if the PLL bandwidth is designed to be very low, the CSM-controlled GCI still cannot operate stably in very weak grids. The voltage source mode (VSM) controlled GCI can operate stably in very weak grids, but is not suitable for strong grids. Therefore, this article proposes a dual-mode combined control strategy: When SCR gradually decreases, the GCIs operate in full CSM (FCSM), hybrid mode (HM), and full VSM (FVSM) in turn, which effectively improves the stability of GCIs when SCR fluctuates greatly. The stability region of GCIs under the constraints of multiperformance-index is obtained, and the switching boundaries between the above three modes are also quantitatively analyzed in this article. Finally, the correctness of the proposed control strategy is verified by experiments.