Research On Operation Control Of Doubly-fed Wind Turbine Under Unbalanced Grid Voltage

With the problem of energy security becoming more and more prominent,the current main trend of vigorously developing renewable energy resources is being vigorously developed.Wind power,as a clean and environment-friendly renewable energy source,has received extensive attention.However,after large-scale wind power is incorporated into the power grid,the frequency dynamic characteristics of the power grid are weakened,which is easy to cause the equivalent inertia loss of the high-permeability wind power grid-connected system and the frequency instability of the power grid.In order to solve this problem,many scholars have introduced virtual synchronization technology,while the traditional way of realizing virtual synchronization control based on load shedding operation has caused waste of wind energy.In this thesis,the virtual synchronization control of doubly-fed fans based on rotor kinetic energy is taken as the research objective,and the virtual synchronization control of doubly-fed fans(DFIG-VSG)under unbalanced grid voltage is improved.The main research contents are as followsFirstly,the mathematical models of doubly-fed wind turbine are derived in three-phase coordinate system and two-phase dq rotating coordinate system respectively.the control objectives of the machine-side and grid-side converters of doubly-fed wind turbine are analyzed.the control block diagrams of grid-side converter based on vector control and rotor-side converter based on power decoupling vector control are established,which lay the foundation for the following research on virtual synchronization control of doubly-fed wind turbine.Then,the active power control method of doubly-fed fan is analyzed,and based on this,the current-controlled and voltage-controlled virtual synchronous control strategies of doubly-fed fan based on rotor kinetic energy control are established respectively.The current-controlled DFIG-VSG control strategy combines additional inertia with traditional power decoupling current loop control and rotor current model predictive control to achieve inertial response.The voltage-controlled DFIG-VSG control strategy mainly simulates the excitation characteristics of synchronous generators and the governor model to improve the anti-disturbance capability and stability of the whole system.Finally,the effectiveness of the current-controlled and voltage-controlled virtual synchronization strategies is verified by simulation when the grid voltage drops.Finally,aiming at VSG control strategy of doubly-fed wind turbine under fault,the operation characteristics of doubly-fed wind turbine under unbalanced conditions are analyzed firstly,and the positive sequence and negative sequence of current are separated by using reduced-order generalized integrator.In order to achieve the stability of output current,active power and reactive power of doubly-fed wind turbine during faults,this thesis proposes a positive sequence current compensation control strategy based on VSG control strategy.By referring to different targets and selecting corresponding current control commands,the requirements of multi-target comprehensive compensation can be realized.Finally,the effectiveness of the control strategy is verified by simulation in Matlab/Simulink.