Dynamic stability improvement scheme for dual-sequence PLLs in VSC based renewable energy generation system during asymmetrical LVRT

Abstract

This paper studies the influence introduced by the application of dual-sequence phase-locked loops (PLLs) structure for dynamic stability of the voltage source converter (VSC)–based renewable energy generation system (REGS) during asymmetrical low voltage ride through (LVRT) in the weak grid. The result unveils that applying the dual-sequence PLLs structure will increase the numbers of modals with weak damping and changes the unstable modal during the asymmetrical LVRT, compared with the single-sequence PLL structure, because it brings novel and complex dynamic interactions. Hence, the analyzed stability conclusions and designed improved control strategies for the system with single-sequence PLL structure during asymmetrical faults are not applicable to grid-connected systems using the dual-sequence PLLs. This article investigates the fundamental cause of small-signal instability for VSC during asymmetrical LVRT caused by the dual-sequence PLLs. In addition, the influence of current control loops (CCLs) control parameters on the dynamic performance of dual-sequence PLLs is analyzed. Finally, an improved control scheme is designed to improve the dynamic stability of the system adopted dual-sequence PLLs during asymmetrical LVRT. The validations are carried out in simulations and experiments to prove the proposed schemes’ effectiveness.