Research On Control,Operation And Frequency Regulation Issues Of Wind Energy Conversion Systems

The exploitation of renewable energy has been considered in the developing strategies among many countries.These countries declared aggressive renewable energy policies,and large-scale of renewable power sources are integrated into the power systems.The wind power is regarded as the most mature renewable technologies,whose penetration is hug in the moderrn grids.However,the operation and control manners of wind power devices are different from that of the conventional machines.This is mainly decided by the intermittency and randomness of wind inflows,that drive a turbine.This dissertation focuses on the modelling,control and integration of wind energy conversion systems(WECS).The two aspects discussed are:(1)we study the advanced controls applied in blade pitch control systems;(1)the inertial response of wind power systems is investigated for frequency supports.1.WECS hybrid modeling considering electromechanical transients.The WECS is a complex system with electromechanical and electromagnetic interactions.Conventional modeling methods cannot address such electromechanical transients within a wind turbine.In this dissertation,detailed Type 3 and Type 4 WECS models are developed based on the high-fidelity turbine simulator FAST(Fatigue,Aerodynamic,Structure,Turbulence),with systematic designs for the controls of power converters.The model simulation is based on a real 600kW wind turbine CART3,and we adopt aerodynamic and geometric parameters of the real turbine system.The proposed model can be used in studying complex electromechanical transients in a turbine system.The simulation results are consistent with the variable-speed operation features of a WECS.These simulations and analysis can be served as an important basics for the following field tests.2.Advanced controls applied in the turbine pitch control system.The performance of the pitch controller is deterministic to the structural loads of a wind turbine.As the capacity and flexibility are increased in modern wind turbines,advanced controls based on high degree-of-freedom(DOF)turbine model draw more attentions nowadays.In this dissertation,we analyze and improve a disturbance accommodating control(DAC)first.The DAC is based on the internal model principle,which can provide disturbance resistance capability for the control system,so the performance of active structure controls can be improved in a wind turbine.However,the typical DAC cannot suppress the disturbance completely.In this regard,we propose to invoke the Hamilton-Jacobi-Bellman(HJB)equation with the linearized turbine model considering the disturbance term.The feedforward control law,which can reduce the disturbance effect significantly,is derived through the optimal control theory.The simulation results indicate that the improved DAC can significantly reduce the steady-state errors in the rotor speed under highly turbulent wind speeds.Furthermore,considering the aerodynamics-related nonlinearity in wind turbine system,linear controllers designed based on the system linear state-space model cannot yield desired performance when the wind speed varies in a wide range.The closed-loop system might be instable when the operating point deviates significantly from the selected one.To solve the varying system parameters induced by the nonlinearity,we propose to design the pitch angle controller using the model reference adaptive control(MRAC)scheme,which combines the model reference control(MRC)algorithm and the adaptive law,making the output of the closed-loop system identical to the reference model all the time.The dead-zone characteristics is incorporated in the controller parameter adaptions,so the robustness of the system is enhanced for stable operations.3.Wind turbine inertial control strategies considering the generator torque limit.Modern WECS is decoupled from the power grids through the back-to-back power converters.As a result,compared to synchronous machines,a similar inertial response is not available in WECSs when sudden change happens in the grid frequency.A turbine inertial control strategy considering the generator torque limit is demonstrated in this dissertation.This method can maximize the turbine's capability in grid frequency regulations,and at the same time maintain the secure and reliable operations of the generator and power converters.The simulations are carried out through a comparative analysis between the proposed method and the commonly used inertial controls.The results verify the enhanced inertial response of the proposed method.The mechanical loading analysis indicate the inertial control can reduce the out-of-plane turbine loadings,while the loadings on the shaft and tower(side-to-side direction)are increased.4.Enhanced inertial response based on the coordinated control of the wind power and energy storage systems.The turbine needs to accelerate to restore the kinetic energy after the frequency support.Considering the potential secondary frequency dip(SFD)happening during the turbine deloading operation,we propose a coordinated control scheme of the WECS and the energy storage system(ESS)to enhance the frequency support capability.The supercapacitor ESS is deployed at the DC link of the power converters,so the grid-side converter is responsible for the charging/discharging control of the ESS.We also discuss the sizing of the ESS to ensure the stable PWM operations.A damping control is embedded into the torque control loop.The oscillations in turbine shaft and tower are mitigated during the inertial response.5.Real-time simulation platform based on the HIL technology.HIL is a prevalent hybrid simulation method,which can test the response of physical devices against the virtual system.In the proposed HIL-based testbed,the dynamics of the power system are emulated in a digital real-time simulator(DRTS).Based on the communication layer between the turbine System Control and Data Acquisition(SC AD A)and the DRTS,the turbine will respond the virtual frequency decline in the closed-loop simulations.This is the first time that the inertial response is investigated in the real industrial-scale turbine platform.The simulated inertial control algorithm is realized in the CART3 real-time control system using the rapid prototyping technology.The proposed simulation method is significant to study wind turbine generator(WTG)participating in grid frequency regulations,and other types of turbine auxiliary controls can be also demonstrated through the platform.