Dynamic performance analysis, stability margin improvement and transfer power capability enhancement in DFIG based wind turbines at weak ac grid conditions

Wind farms are usually located in remote areas far away from utility grid and load centers. Hence, long transmission lines, used between wind farms and utility grid, result in a low value for the grid strength at connection point. This paper first studies stability of the grid connected DFIG wind turbine taking the PLL dynamics into account, and then presents an efficient control approach to stabilize the system and to enhance transfer power capability at weak grid conditions. Hence, unified dynamic modeling of the whole DFIG, including dynamics of the PLL, stator flux, rotor current, drive train, dc-link, rotor side converter (RSC) and grid side converter (GSC) current controllers, dc-link voltage and generator speed controllers, and grid is presented. Then, small signal stability of the full system is carried out and impact of the grid short circuit ratio (SCR) on stability of the system is examined. Next, rotor control structure is modified to increase the stability margin and transfer power capability under weak grid conditions. By the modified control structure, the grid SCR is virtually increased to enhance the system performance at weak grid connections. Finally, results of theoretical analyses are verified by time domain simulations conducted in MATLAB-Simulink environment.