Rotor Speed Protection And Coordinated Control Of DFIG Wind Turbines

Wind power plays an important role in the process of modern-day energy substitution.In recent years,wind generating technology has gained speedy development in China.However,the doubly fed induction generator(DFIG)-based wind farm(WF)is connected to power grid through electronic devices,so the wind turbine's(WT's)rotor speed is decoupled from system frequency and the WF does not have the similar frequency control capability as the synchronous generators.Therefore,it is bound to bring about new challenges for the frequency stability and security with the large-scale wind power connected into the power systems.Thus,research on the technology and capability of frequency regulation with wind power is an inevitable requirement for the development of power grid.Inertia control based on rotor kinetic energy can make the DFIG-based wind turbine participate in the process of frequency regulation in some ways.However,due to the finite kinetic energy of the rotor,the WT must exit the process of frequency regulation when the rotor speed hits its lower limit,which is likely to cause a secondary frequency drop.It will weaken the effect of inertia control of DFIG.Therefore,it is necessary to study the rotor speed protection strategy of the WT to reduce the secondary drop of the system frequency.In addition,the characteristics and forms of frequency control with wind power are different from the traditional synchronous generators because of the use of power electronic instruments.It will have a new impact on the original thermal power with the participation of the WT during frequency regulation of the power system.With the increase of installed wind capacity,this impact is certainly more obvious.Thus,it is necessary to conduct further research on coordinated control of frequency regulation between wind power and thermal power.In order to achieve better overall effect of frequency response,this paper studies the rotor speed protection strategy of the WT and coordinated control strategy based on the principles and characteristics of frequency regulation of DFIGs.Firstly,this paper summarizes the domestic and international research status of frequency control and coordinated control strategies with wind power.In this paper,the WT's frequency control strategies without reserve capacity is collectively referred to as inertia control,and the WT's frequency control strategies with reserve capacity is collectively referred to as primary frequency regulation.In the next step,this paper introduces the basic principle of inertia control and primary frequency regulation of DFIGs,and deduces the dynamic expression of system frequency in step disturbance and ramp disturbance scenarios based on the simplified frequency response model.The dynamic expression establishes the foundation for analyzing the characteristics of frequency regulation with DFIGs and the calculation of frequency response index.Then,this paper selects five typical inertia control forms to study and simulate the influence on frequency response index with the change of system parameters.It is concluded that the inertia control of DFIG has the characteristics of fast response speed and diverse control forms.Through comparative analysis,this paper summarizes the characteristics of the appropriate inertia control form that the DFIGs should have.In addition,the primary frequency regulation characteristics of DFIG are simulated and analyzed in the over-speed control and variable-pitch control mode.Due to the reserve capacity,the DFIG can perform long-term frequency support like a synchronous generator.But the mode of load shedding control will reduce the generation benefits of the WT.Furthermore,inertia control of the DFIG is likely to cause a secondary frequency drop.Concerning this issue,this paper proposes an improved design algorithm of rotor speed protection and recovery.The improved design of rotor speed protection enables the DFIG to automatically run to a certain point on the mechanical power curve during frequency regulation,which will avoid sudden drop of the active power by exiting frequency control directly.And the improved design of rotor speed recovery enables the DFIG to slowly return to the original maximum power point tracking(MPPT)mode to ensure the generation benefits of the WT.Lastly,this paper analyzes the influence of inertial control of wind power on primary frequency regulation of thermal power,and proposes two objectives of coordinated control of frequency regulation:speed up the initial reaction speed of synchronous generator's primary frequency control,and avoid the secondary frequency drop.A coordinated control strategy between wind power and thermal power is proposed both in the communication scenarios and without communication equipment.The simulation results in various scenarios show that the strategy can make full use of the respective frequency control advantages of wind power and thermal power,and significantly improve the overall effect of frequency response.