Coordinated Control Strategy Of Weakly-synchronized Grid Containing Large Wind Farms

With the depletion of fossil fuels and environmental issues becoming more and more critical, wind power has developed rapidly as a kind of renewable and clean energy. Wind power becomes one of the most widely used renewable energy in the world. Areas of owning good wind are mostly located at remote places, far away from load centers. Typically, most of the wind turbines are located at remote places where the power grid is usually very weak characterized by middle or low voltage level. The wind farm is facing a weak grid, and the impedance is very large and the short-circuit capacity is very small. Frequency and voltage in weak grid are prone to fluctuation and imbalance. The large grid inpenance and small short-circuit capacity of weak grid can have negative impacts on the stability of wind power grid-connected system. In the case of operating in a weak grid system, when wind power becomes a significant portion of the power system or even the sole energy source, the wind power genetator and its control systems are expected to maintain the grid voltage and frequency stability. Therefore, focused on grid frequency and voltage stability, this paper studies the wind power system in weak grid and proposes a coordinated control strategy.First, this paper analyzes the model and the characteristics of weak grid, and analyzes the effects on the frequency and voltage stability considering the different wind turbine typies, wind power penetration and grid strength. Wind turbines can not effectively respond to frequency changes, bad for frequency stability of weak grid because of the increasing penetration of wind power. Reactive power compensation of wind turbine capacity is limited, and can not maintain the voltage stability of weak grid. The intensity of the grid with wind farms weaker, the ability to resist voltage fluctuations weaker.Second, in order to utilize the frequency and voltage modulation potential of variable speed wind turbines, this paper linearizes the WECS of DFIG, and analyzes the regulation capacity of active and reactive power. A method of multiple model predictive control based on the model predictive control for the frequency regulation in weak grids with variable-speed-constant-frequency wind farms integrating into is proposed. Depending on wind speeds, the control strategy is divided into several prediction model banks. Pitch angle and generator torque are controlled simultaneously to regulate active power output to improve the frequency regulation capability.Third, by introducing static synchronous compensator (STATCOM), this paper presents a coordinated control strategy of active power and reactive power to deal with fault and load disturbance in weak grids. Then, a central control system is built up. The central control system achieves the active and reactive power balance objectives through real-time coordinated control of the wind farm, the compensation unit and grid system.Finally, simulation results show that the proposed coordinated control strategy can effectively improve the frequency and voltage stability of the grids.