Virtual Multi Shaft Coupling And Power Oscillation Suppression Technology For Doubly Fed Wind Farms

With the rapid increase of wind power in new energy sources system,it gradually replaces synchronous generators with regulatory capabilities,causing a serious threat to the frequency stability and power support of the power system.The traditional virtual synchronous generator does not fully unleash the potential of power electronic converters for grid connection support of wind turbines.Under the constraint of single degree of freedom synchronization,there is a design contradiction between virtual inertia and damping,and the vibration suppression bandwidth is narrow,resulting in increased resonance risk.To achieve the "Carbon Peak" and "Carbon Neutral" planning requirements of new power systems,adopting virtual shaft coupling,increasing the vibration freedom of wind turbines,and achieving efficient suppression of power oscillations are key issues that need to be urgently studied to ensure the safe grid connection of high proportion wind power.Based on the vibration suppression theory of mechanical dynamics,the paper establishes a multi degree of freedom motion model for regional power grids by introducing virtual shaft,and then analyzes the anti resonance energy vibration suppression mechanism of virtual shaft,virtual inertia,and additional damping in linear design,achieving an integrated design of inertia and damping functions.On this basis,the paper is conducted on anti resonance frequency tracking,bandwidth expansion function design,and resonance risk reduction methods under multiple degrees of freedom.The linear and nonlinear virtual shaft control strategies are proposed.Combined with the consistency characteristics of multiple wind turbines,multiple virtual shafts design method is discussed to stimulate the overall vibration suppression effect of the station.The specific content includes::(1)Aiming at the problem that it is difficult to reconcile the parameter design contradictions between inertia and damping in the rigid connection of a virtual synchronous generator,the power oscillation characteristics and transfer mechanism of the decoupled,rigid,and flexible coupling relationship between the wind turbine and the synchronous generator shaft system are analyzed.Based on the flexible connection characteristics of the generator shaft system,the virtual shaft stiffness between generator sets is defined.By adding a virtual shaft to the wind turbine and establishing a shafting coupling between rotor angles of wind turbine and synchronous generator,a new degree of freedom of vibration is introduced for the grid connected system,and a two degree of freedom rotational dynamic model is built together with the synchronous generator unit to break away from synchronous operation.Using the anti resonance frequency tracking theory,a flexible virtual shaft based power oscillation suppression method for wind turbines is proposed,and the controller parameters are optimized based on the optimal tuning and fixed point theory.Accorfding to build RT-LAB hardware-in-the-loop simulation platform with high penetration of the wind power,it is verified that the wind turbine can significantly improve the active support performance of the power grid by effectively suppressing power oscillations under the proposed control strategy.(2)Aiming at the low efficiency of transient energy transfer between generators during oscillation,based on the virtual synchronous generator model,the rotor angle coupling characteristics of wind turbine and synchronous generator units before and after assembling virtual shaft are analyzed.The system state space model with virtual shafting is established to evaluate the impact of virtual coupling parameters on system stability using root locus.On this basis,the Hamilton energy equation before and after the introduction of virtual shaft is constructed using the Hamilton principle.The oscillation energy transfer process of wind turbines is analyzed,and the necessary conditions for complete energy transfer between wind turbines and synchronous generators are obtained.The virtual shaft control strategy suitable for doubly fed wind turbines is proposed from an energy perspective.After incorporating the proposed virtual shaft into the system,wind turbines can more efficiently capture oscillation energy and significantly improve their ability to damp power oscillations.(3)To reduce the resonance risk introduced by the grid connection support control of linear shaft wind turbines,nonlinear cubic coupling is selected to eliminate resonance points and expand the frequency band width of the wind turbine for suppressing system power oscillations.Utilizing the nonlinear power coupling between units and introducing the cubic stiffness of the shaft system,a virtual synchronous nonlinear shaft motion model for doubly fed wind turbines is established.It can coordinate and solve the design contradiction between inertia and damping commonly existing in virtual synchronous machines.Based on the optimal bandwidth theory,the bandwidth characteristics of power oscillation suppression with linear and nonlinear shaft are compared and analyzed.A nonlinear virtual shaft control method for doubly fed wind turbines is proposed,and the controller parameters are designed.The proposed control strategy can effectively expand the fan vibration suppression bandwidth,avoid resonance risks,and improve the support performance of friendly grid connection.(4)To solve the problem of consistent response within a wind turbine cluster and collaborative optimization among wind turbines,a dynamic model of power system with virtual multi shaft coupling is established,and the mechanism of the influence of electrical distance of multiple virtual synchronous generators on the power oscillation characteristics of the system is analyzed.By analyzing the rotor angle amplitude frequency oscillation characteristics between virtual synchronous generator and synchronous generator under multi shaft coupling,the consistent response conditions of virtual synchronous generators in doubly fed wind farms are obtained.Compared with a single generator model,introducing multiple degrees of freedom into a virtual multi shaft system can significantly improve the oscillation suppression performance of a virtual synchronous cluster.On the basis of achieving consistent response within the cluster,a virtual multi shaft control method for wind farms is proposed,and the multi shaft evaluation function is used to optimize the design and excitation of the overall vibration suppression performance of multiple machines.The proposed control strategy can form the overall effect of wind farm vibration suppression through consistency optimization and multi generators coordination,and improve the support ability to suppress system power oscillations and frequencies.