Nonlinear Analysis of PLL Damping Characteristics in Weak-Grid-Tied Inverters

Under weak grid scenarios, the widely adopted synchronous reference frame phase-locked loop (SRF-PLL) exhibits complex behavior due to the possible grid phase or frequency disturbances. In this brief, the nonlinear damping characteristics of the PLL is identified. The damping ratio of the PLL under various grid strength is explored thoroughly. The root cause of the PLL instability is unveiled. It is shown that, with the low short-circuit ratio (SCR) of very weak grid, the damping ratio will drop sharply due to the nonlinearity of the PLL phase detector. A practical parameter design method is developed accordingly to avoid instability caused by insufficient damping. Simulations and laboratory experiments verify the validity of analysis results.