| Wind power is a promising renewable power generation technology with potential for large-scale development.Nevertheless,soaring wind power penetration level brings about many problems such as reduced system inertia,severer active power disturbance,and deficiency in primary reserve,which challenges the frequency stability of modern power systems.Such circumstances pose an urgent need for wind turbine generators that conventionally implement the generation-efficiency-oriented maximum power point tracking control to possess primary frequency control strategies that can support system active power balance at contingency.For wind turbine generators implementing the type of primary frequency control that utilizes the limited kinetic energy stored in wind rotor,turbine stability is fundamental for effective participation in primary frequency regulation.State-of-the-art primary frequency control strategies of wind turbine generators are usually designed with consideration of basic wind scenarios.Besides,these strategies utilize predetermined electric power curve to realize frequency support,which are generalized as a type of non-closed-loop rotor speed control in this thesis.However,actual turbulent wind that varies at the time scale of the turbines’ electromechanical dynamics significantly affects turbine stability and frequency support performance.To tackle this problem,this thesis at first performs stability analysis on wind turbines with non-closed-loop rotor speed feedback primary frequency control strategies under turbulent wind,and then proposes turbine-stability-constrained improvement strategies for the temporary frequency support and the rotor speed recovery phases of wind turbine primary frequency control,respectively.Main contributions of this thesis are delineated as follows:(1)Interpretation of turbine instability mechanisms under turbulent wind.Firstly,this thesis investigates the stability of wind turbine generators with non-closed-loop rotor speed feedback primary frequency control strategies(including the constant power method,the linear power method,and the additional power method)on the rotor speed-wind speed plane.Then,mechanisms of turbine instability under turbulent wind are interpreted on the basis of stability analysis and experimental results.Inclined by wind lulls and the release of kinetic energy,the turbine may decelerate to a critical speed and then will operate in deep stall which means that the aerodynamic power is always smaller than the electric power regardless of wind speed variations.In this case,the turbine cannot revert to the stability domain even if the wind speed thrives and will keep decelerating until it shuts down.(2)Improved wind turbine primary frequency control strategy with supplementary closed-loop rotor speed control.Considering the advantages of closed-loop feedback control in terms of maintaining system stability,this thesis proposes an improvement strategy in order to enhance the stability of wind turbine generators participating primary frequency regulation.The proposed method exerts a closed-loop rotor speed control signal on the predetermined electric power references so that the rotor speed will track the predesigned reference profile despite wind speed variations.The proposed method overcomes the turbine instability problem of existing non-closed-loop rotor speed feedback primary frequency control strategies,which lays the foundation for reliable frequency support performance of wind turbine generators under turbulent wind.(3)Improved wind turbine primary frequency control strategy with optimized rotor speed reference.On the basis of contribution(2),this thesis further explores the advantages of closed-loop rotor speed regulation in terms of adaptive electric power output to varying wind speed.Hence,this thesis proposes a closed-loop rotor speed feedback wind turbine primary frequency control with rotor speed reference optimized by the particle swarm algorithm,which substitutes the predetermined electric power reference function by closed-loop rotor speed control.With the adaptability of electric power to wind speed variations and optimized rotor speed reference,the proposed method can further improve system frequency response while ensuring turbine stability under turbulent wind.(4)A novel adaptive rotor speed recovery strategy.Under turbulent wind,existing rotor speed recovery strategies adopting consistent power reference functions may fail to restore the rotor speed due to turbine instability,or induce secondary frequency drop due to fluctuating electric power output.In this regard,this thesis proposes an adaptive rotor speed recovery strategy where different power reference functions suitable for wind gusts and lulls are switched adaptively.With the proposed strategy,the turbine can accelerate at wind gusts and stabilize at wind lulls by operating along suboptimal power curve,thus reliable and grid-friendly rotor speed restoration under turbulent wind can be achieved. |
