Transient Stability Analysis and Coordinated Phase Control Method for Multiparallel PLL-Synchronized Inverters During Grid Fault

Abstract

To utilize renewable energies, such as solar energy, distributed power generation systems are widely used, where multiple string inverters are parallelly connected to the grid and often adopt the phase-locked loop (PLL) for grid synchronization. However, under severe voltage sags caused by grid faults, the PLL-synchronized inverter system is susceptible to transient instability, which manifests as loss of synchronization. Although such transient instability caused by interactions between the inverter and the grid has been well studied in state-of-the-art research, the instability due to interactions among different inverters is not fully considered. Thus, to investigate this issue, this article establishes the modeling of an <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i>-parallel PLL-synchronized inverter system for transient stability analysis while considering the parameter difference. It is revealed that the inter-inverter interaction introduces the transient instability risk, even though every grid-connected inverter is separately designed to keep synchronization during the grid fault. Subsequently, this article proposes a coordinated phase control method to enhance the transient stability of the multiparallel inverter system during grid faults. Finally, utilizing the RT-LAB OP5707XG platform, a real-time simulation model of a 3-parallel PLL-synchronized inverter system is established and tested to verify the theoretical analysis and the proposed method.