Stability analysis of doubly-fed wind power generation system based on phase-locked loop

Phase-locked loop (PLL) has been widely used in the doubly-fed wind power generation system (DFWG) to obtain the phase angle of the utility voltage accurately. But in weak power grid, the dynamic of PLL is coupled with DFWG controller and this may cause wind turbine system instability. This paper presents the impact of PLL on the DFWG system stability. In order to investigate the system stability, detailed and simplified system model are given. The stability condition is obtained from the simplified model and verified by detailed model simulation. It is shown that the stability of DFWG system based on PLL is related to the parameters of PLL regulator and the short-circuit capacity of wind turbine grid access point. The instability reason can be explained by a so-called psilaequivalent line reactancepsila concept. Same stability problems will also be found in other system which uses a voltage oriented vector control strategy in which the oriented voltage phase angle is observed by a PLL in a weak gird.