Frequency Droop Control Strategy Of Doubly Fed Induction Wind Turbine Under Deloaded Operation

In order to save fossil energy and protect the environment,wind power,as one of the renewable energy,which can replace the traditional fossil energy,is developing rapidly with the advantages of abundant reserves and massive utilization.However,with the increase of wind power penetration,wind power deepen influence on power systems.Under the influence of randomness and intermittence of wind power,the power balance of power systems is difficult to be maintained.In addition,the doubly fed induction generator wind turbine(DFIG-WT),which utilizes power electronic grid interface,is unable to make effective response to the system frequency change.Therefore,because of the strong power volatility and the lack of frequency regulation capability,large-scale wind power will bring huge pressure to the active reserve of power systems.In order to ensure the security and stability of the system and facilitate the development of wind power,it is of great significance to study the technology for wind power participating into frequency regulation.The main contents of this paper are as follows:1)A simulation platform for DFIG-WT to participate in frequency regulation is established.Firstly,the wind speed model,wind turbine model,pitch angle control model,generator model and converter control model of DFIG-WT are introduced.Based on those models,the generic model of DFIG-WT for frequency regulation simulation is established.Secondly,the principle of frequency regulation is described.The mathematical model of interconnected power grid is given,including generator-load model,prime mover model,governor model,and tie line model.Finally,combining the generic model of DFIG-WT with the interconnected power grid model,a simulation platform for DFIG-WT to participate in frequency regulation is established,which provides a platform for the verification of the proposed strategies.2)A wind ramp control strategy considering droop control is proposed in this paper.Based on energy storage system,this paper proposes a wind ramp control strategy.The control framework of wind farm operation assisted by energy storage system is constructed.The energy storage system executes the charging and discharging instructions,which come from control center,to regulate the system frequency while avoiding the occurrence of wind power ramp.Finally,the simulation is carried out on the interconnected two-area power system.The simulation results show that the proposed control strategy can effectively restrain the wind power ramp and improve the frequency stability of the system.3)An adaptive strategy considering droop control and wind power fluctuation suppression is introduced.In order to improve the frequency stability of the power systems and suppress the fluctuation of wind power output,this paper proposes an adaptive strategy considering droop control and wind power fluctuation suppression.The strategy is composed of droop control and wind power fluctuation suppression control.Membership function is used to implement the smooth transition of the controls.When frequency deviation is large,wind turbine responds to the system frequency deviation signal through droop control to maintain the system frequency stability;when frequency deviation is within the allowable range,wind turbine will suppress the fluctuation of wind power to reduce its impact on power systems.Finally,the simulation results on the interconnected power system show that the proposed adaptive strategy is effective in frequency regulation and wind power fluctuation suppression.