Research On Control Strategy Of Doubly-fed Wind Turbine Based On Virtual Synchronous Generator

With the increasing penetration of wind power generation in the grid,the installed capacity of existing wind turbines has already accounted for 10.5% of that of the system,and the trend of power system dominated by power electronics has risen to a prominent spot.On the one hand,the widespread adoption of phase-locked loops in power electronic devices causes a significant reduction in the power system inertia and therefore disturbs frequency stability;on the other hand,the randomness and volatility of new energy sources leads to extra burden being put on the power system.Currently,a heated topic draws wide attentions on how to ensure the safe and stable operation of the system preconditioned on large-scale connection of renewable energy.In view of that,this thesis explores the improvement in the control strategy of the doubly-fed wind turbines connected to the grid via power electronic devices,and the main work is as follows:First,a detailed doubly-fed wind turbine model was built.By the principle of the energy transfer process,the author established the model of the wind turbine power system and energy conversion system in turn,and thoroughly studied the energy conversion mechanism of the wind turbine.Meanwhile,the author also investigated the existing AC excitation principle of the wound motor and the vector control strategy of power electronic converter,thereby laying a solid foundation for further improvement in the controlling method of doubly-fed wind turbines.Secondly,the model of virtual synchronous generator was built.By adopting the mathematical model of the second-order synchronous generator,the electromagnetic and electromechanical transient model of the synchronous generator are obtained.Besides,the control parameters of the virtual synchronous generator were theoretically analyzed and discussed.On that base,a self-synchronized-control virtual synchronous generator model was built,with the phase-locked loop being completely removed from the control system,so as to simplify the control structure of the virtual synchronous generator.Moreover,the virtual synchronous generator control strategy has been verified with simulations.Finally,an improved control strategy for doubly-fed wind turbine based on virtual synchronous generator was proposed.Taking both the existing vector control and the electromagnetic coupling relationship of the doubly-fed wind turbine into account,the rotor motion equation of the synchronous generator set is introduced based on the traditional stator voltage orientation control.As a result,the doubly-fed wind turbine gets rid of phase-locked loops.In the meantime,the rotational inertia contained in the impeller and generator rotor in the doubly-fed wind turbine was explored,and the coupling relationships of active-frequency and reactive-voltage were reconstructed inside the doubly-fed wind turbine,adopting no extra physical devices nor increasing control complexity.The improved control strategy enables the doubly-fed wind turbine to achieve the same external characteristics as the synchronous generator while the output power meets the maximum power tracking.The correctness and effectiveness of the proposed control strategy are verified under different operation conditions.