Dynamic Characteristics Analysis And Improved Control Strategies Research Of Doubly-fed Induction Generators Connected To Grid

With the exploration of wind energy resources and the development of wind power technology,the proportion of wind power in power system is increasing.The dynamic stability problems of grid-connected wind generators have been attracting wide attention.With different kinds of wind turbines connected to the grid,its effects on the power system stability have been widely analysed and researched both in domestic and overseas,and then a series of optimization control measures are used to improve the dynamic stability of wind generators and power system.This paper takes doubly-fed induction generator(DFIG)which has wide prospect of application as research model,matlab/simulink soft as experimental platform,IEEE classic system as simulation examples,mitigating the adverse impacts on grid with DFIG connected and making wind generators and power system run more stable as research purposes.Related theory analysis is carried on and appropriate improved control strategies are also proposed.Differential and algebraic per unit equations of wind turbine aerodynamic model,two quality shafting model,pitch control system model,induction generator model,double converters control system model in DFIG system are established under the given base values,and the linear state equations for small signal stability analysis are obtained by Taylor series expansion.The small signal stability and damping characteristics with DFIG operating on all wind speed in the one machine infinite bus(OMIB)system are studied and some meaningful results are got:if the system is disturbed when working steady below rated wind speed,DFIG shafting will introduce a weak damping oscillation mode.If the system is disturbed when working steady above rated wind speed,DFIG pitch control system will also introduce a weak damping oscillation mode.Both two kinds of oscillation modes are related to mechanical system dynamics with low oscillation frequency,while the oscillation modes with high oscillation frequency associated with the stator and rotor flux variation have good damping characteristics.Both oscillation modes and damping characteristics are almost the same when DFIG is controlled as PV or PQ node connected to the power system.Based on the analysis of small signal stability,the effect of impaction from DFIG electromagnetic torque under closed-loop system on shafting damping is studied.Considering the coupling between mechanical part and electrical part,DFIG shafting can be suffered a certain impact under grid fault inevitablely.In order to mitigate the shafting oscillation intensity under the external disturbance,extend the lifetimes of mechanical components and improve the system dynamic stability as well,a kind of power system stabilizer(PSS)module based on rotor side converter control system is designed.Through rapid controlling of DFIG electromagnetic power,the damping power with the same direction of rotor speed variation is used to weaken the shafting oscillation.Detail solution of PSS design parameters is provided in this paper.DFIG shafting damping can be improved greatly,which is validated by eigenvalue analysis and time domain simulations.Wind power output can't be kept constant as conventional thermal power and hydropower.There likely exist sustained cyclic small disturbances with different frequencies because of the strong stochastic fluctuations in wind speed.These small disturbances may cause forced power oscillation in wind power system and the resonance can be induced when disturbance frequency is close to an inherent oscillation frequency so that it leads to forced power oscillation with higher amplitude.In order to study the inner relation between force disturbance source and the forced power oscillation,this paper provides a frequency domain analysis method of DFIG forced power oscillation in OMIB system.Based on the linear differential and algebra equations,the target transfer function between wind speed and DFIG power output is created.Amplitude frequency characteristic curves from transfer function can show the relation between amplitude ratio of DFIG output power oscillation to wind perturbation and disturbance frequency intuitively.The oscillation modes reflect resonant frequency and the resonant strength can be reflected by the damping.Simulation experiments are carried out and the results show that the analysis method is correct and applicable,which also provides new ideas about how to make effective measures to weaken forced power oscillations.Meanwhile,sustained cyclic small disturbance in wind speed may also cause forced power oscillation produced by other synchronous generators in power system.In the power system with DFIG,this paper offers the process of machine-network interface equations including DFIG,synchronous generators and loads through the transform relationship between different coordinates.Whole system characteristic equation is finally obtained,and the target transfer functions between wind speed and each generator's power output are created,which extends the applied range of the proposed method.Taking IEEE three machines nine nodes system as an example,disturbance frequency which may cause resonance with large oscillation amplitude,the intensity of resonance and the reason of producing resonance are obtained.Then some effective measures are proposed like installing power system stabilizer and optimizing parameters of DFIG pitch control system to reduce forced power oscillations in this paper.Along with large-scale wind farm integrated into the power system,the rapid changes of wind power output due to large fluctuations of wind speed in a short time will have negative effects on system stability.Combined with speed control and pitch control,a novel power smoothing control strategy of DFIG based on constraint factor extent-limit control is proposed,which can realize peak clipping and valley filling of DFIG output power to a certain extent,smooth the wind power injected into grid as well as be helpful to reduce the reserve capacity supporting for wind power in power system.The proposed control strategy is applied to IEEE three machines nine nodes system.The moving range of overall load static characteristics curves narrow.By setting different power extent-limit values and comparing with the conditional control strategy of Maximum Power Point Tracking(MPPT),the variation of synchronous generators speed decrease and grid frequency can be improved to different extent.