Small Signal Stability Analysis of DFIG Integrated Power System Considering PLL Dynamics Under Different Grid Strengths

The power from wind farm of DFIG technology interacts with power system through dynamics of phase-locked loop (PLL). Therefore, it is imperative to understand the participation of PLL dynamics on the power system's small signal stability (SSS). This paper focuses in investigating the interaction of PLL of a grid-connected wind farm with a multi machines power system at different grid strengths. Modal analysis is conducted on modified IEEE 39 benchmark system to study the behavior of low frequency electromechanical oscillations (LFEOs) under the influence of various conditions of grid strengths. It is revealed that, the decreasing grid strength has a remarkable impact on LFEOs, and the modes associated with PLL in such a way that they approach in the vicinity of right half of the complex plane which significantly lessens system's small signal stability. Another interesting behavior is observed that the super-synchronous operation is the riskiest state in which, even with a small active power output of wind system at weak grid condition, small signal stability reduces drastically.