| In recent years, with the rapid development of the global wind power technology, the penetration ratio of the grid in some areas of China has exceeded 20%. The characteristics of wind power——inherent randomness, intermittent, and volatility——make large-capacity wind farm have significant negative impacts on the grid dynamic stability, frequency, pressure regulation, and other aspects. Doubly fed induction generation (DFIG) is now becoming the mainstream model in the wind power market. However, because the control system of DFIG makes the mechanical and electromagnetic power decoupling, speed and frequency of power system decoupling, DIFG fails to respond rapidly and effectively to the grid frequency, making the "implicit inertia" of rotational kinetic energy hardly have any contributions to the grid. Thereby, the effect of the frequency regulation of the power grid is worsened. When wind power penetration ratio keeps increasing in the power grid, these effects become more obvious, and even threaten the normal operation over the entire system [2]. Therefore, researching the impact that large-scale wind power has on the system’s frequency characteristics is of great important.From the viewpoint of energy conversion, this paper compares DFIG with conventional synchronous generator, and establishes the mathematical model of DFIG with power angle characteristic of synchronous machine in order to respond to load dynamics of power grid. When wind speed is lower than the rated speed, over-speed control method is used; and when the wind speed is higher than the rated speed or the rotational speed of DFIG reaches the maximum, pitch angle control method is utilized to make DFIG deviate from MPPT. With these two ways, DFIG can reserve active power and enhance the ability to respond to grid’s frequency change. Next, the intrinsic mechanism on DFIG providing inertial power support is analyzed. Based on the DFIG model of Matlab/Simulink simulation platform, this paper builds wind turbines with additional virtual inertia to participate in frequency regulation, and then connects these wind turbines to the two areas with four generators model in order to analyze the feasibility of the DFIG participation in frequency regulation.This paper analyzes the actual situation of wind speed, and takes both wake effect and time-delay effect into consideration. Next, using coherency method, this paper divides DFIG (Doubly Fed Induction Generator) in large-scale wind farms into different regions, and then makes the wind turbines within the same region equivalent into one. Furthermore, based on the Matlab/Simulink, the equivalent effect at different wind speed, and fault conditions are studied. The correctness of equivalent method is validated in this paper. Then, taking the Yunnan Power Grid as an example, this paper analyzes the impact of wind power over the frequency of the grid under different penetration ratio at different wind speeds, steady-state operation and sudden load change conditions. With the penetration ratio of wind power in the grid increasing, this paper replaces some traditional wind turbines with DFIG which can participate in frequency regulation and provides the minimum proportion of DFIG participation in frequency regulation under different wind turbines penetration circumstances. Finally, by analyzed frequency modulation capability, utilization of wind energy and wind turbines, this paper concluded that wind farms should try to ensure wind turbines, which working on rated wind speed, participate in frequency regulation with priority, so that frequency regulation capability and economy of wind farm could realize the optimal. |
PDF Full Text Request