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.
In the development of wind turbine (WT) technology, several types of WTs such as fixed speed type, limited variable speed WT, doubly-fed induction generator (DFIG), and full-scale variable speed WT have been considered [1,2]. Among the different alternatives to obtain variable speed, the system based on DFIG-WT has become the most popular . As shown in Fig. 1, the DFIG stator winding is directly connected to the grid, and the rotor winding is supplied from the back-to-back voltage source converters (VSCs), known as rotor side converter (RSC) and grid side converter (GSC).
In this paper, the RSC is used for the generator speed control, and reactive power support, and the GSC for the dc-link voltage control and reactive power regulation . The grid synchronization is a main step for the control of the RSC and GSC that is normally done by using the phase locked loop (PLL). This paper first examines stability of the grid connected DFIG wind turbine system taking the PLL dynamics into account, and then proposes a new control approach to stabilize the system under weak ac grid conditions.
This paper deals with the stability analysis and dynamic stability enhancement of the grid connected DFIG based WTs under weak ac grid conditions. Hence, unified dynamic modeling of the whole DFIG system, including the dynamics of the PLL, stator flux, rotor current, drive train, dc-link, RSC and 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 SCR on the stability of the system is examined. It is shown that for the SCR values lower than 3, the modes corresponding to the dc-link voltage and thus the whole system become unstable. Next, the RSC control structure is modified, in which a negative virtual inductance is introduced at the stator side, and thus the grid SCR is virtually increased that stabilizes the system at weak grid connections. Simulation results verify the theoretical analyses and depict that at SCR = 3 and in the case without the virtual impedance emulation, the responses of the electromagnetic torque, output active power and dc-link voltage are unstable and reach very low values. On the other hand, in the case with the negative virtual inductance emulation, the system responses become stable. This is because the modified control approach virtually increases the grid strength and consequently enhances the DFIG stability margin and power transfer capability