Emergency Control Strategy Study On Reactive Power And Voltage Of Grid-connected DFIG

In recent years, wind power generation technology has been developed rapidly.However, because the wind speed has some characteristics such as random, intermittent and so on, original distribution of power flow, node voltage and the grid losses are changing, which have brought new challenges to reactive power optimization of power grid. Especially when it comes to power grid failure, it is necessary to realize fault ride through at the same time meeting the needs of the grid reactive power support to ensure the safe and stable operation of the power grid. Under this background, this paper takes doubly fed induction generator(DFIG) as the main research object. On the basis of the deep understanding of its power characteristics, it is focused on the voltage stability problem of the wind farm in steady state and transient operating condition, and makes the most use of the DFIG economic operation. The main work can be summarized as follows:When the wind farm is under stable operation circumstance, this paper proposes that DFIG’s reactive power regulation ability is applied to participate in reactive voltage control by regarding the DFIG as a continuously adjustable reactive power source in the range of the reactive power limit. The Immune Genetic Algorithm(IGA) is adopted to solve the problem that DFIG reactive power output is unknown. In addition, according to the demand of wind farm construction, the influence of the wind farm with different installed capacities and locations where wind farms are connected to grid on the reactive power optimization is discussed. The objective function that minimize the sum of active power losses and voltage deviations of the system is established to analyze and calculate the reactive power optimization by using the improved IEEE-30 nodes system, verifying the feasibility of the algorithm.Aiming at the low voltage faults in grid, in order to improve the Voltage Ride Through Low(LVRT) and meet the demand of reactive power support for grid, this paper improves the traditional vector control of DFIG and designs a reactive power coordinated control module. Secondly, it is discussed that the influence of Crowbar resistance and switching control strategy on reactive power supporting capability of wind turbine. In the end, propose a reactive voltage emergency control strategy on the coordination between STATCOM and DFIG. The grid-connection simulation model of DFIG with a capacityof 1.5MW is built in the PSCAD/EMTDC platform. The simulation analysis shows that the proposed control strategy can improve the DFIG reactive power better and reduce the required capacity of STATCOM.Low voltage faults will cause that wind power generating units incapable of the LVRT capacity are separated with grid. As a result, the output power of wind farm reduces and voltage elevates, leading to the expansion of the faults area of the wind turbines. To solve this problem, this paper analyzes the reason of voltage swell and DFIG transient process when the grid voltage increases, and a HVRT control strategy is proposed. Finally, the effectiveness and feasibility of the proposed control strategy is verified by simulating the model built in the third chapter.