The Research On Damping Control For Power System With Doubly-fed Induction Generator Connected

With the gradual increase of wind power permeability,wind turbines bring new challenges to the power system.The problem of low-frequency oscillation in the wind power grid connected system needs to be solved urgently.At present,Doubly-Fed Induction Generator(DFIG)has become the most widely used mainstream model in the wind power industry.However,the back-to-back converter control mode in DFIG makes the turbine speed is decoupled from the grid-side frequency,and the rotational kinetic energy of system is hidden,resulting in the lack of system damping.In addition,wind power is fluctuating,random and intermittent,so that the influence on the stability of power system cannot be ignored.Therefore,we need to study the stability of wind power grid connected system deeply,analyze it qualitatively and quantitatively from the system damping point of view,and improve system damping characteristics by additional damping control strategy.It is of great significance to ensure that the wind power grid connected system can operate safely and stably under the outside disturbance.Aiming at the low-frequency oscillation caused by wind power grid integration,we accord to the basic structure and operation principle of DFIG,the mathematical models of wind turbines,drive shafting systems,generators,back-to-back converters and their controllers,and the grid connected DFIG system are built.At the same time,the simulation model of the system is established in MATLAB/Simulink software,which provides a model foundation for the research on the small disturbance stability in the grid connected DFIG system.Based on the system dissipated energy,DEC is defined to quantitatively describe the system damping.The relationship between the system damping and DEC,which represents the energy dissipation of oscillations,is discussed from the physical level.From the direct method point of view,DEC of the synchronous generator system,the grid connected DFIG two machine system and the grid connected DFIG multi-machine system are constructed.Then,combined with root locus method and damping torque analysis method,the effectiveness of using DEC to quantitatively analyze system damping is verified.In addition,the effects of DFIG structure parameters and control parameters on system damping is discussed based on DEC.And according to the modal analysis results of the two areas & four machines system,the damping between generators under different oscillation modes is analyzed quantitatively.Finally,combined with the time domain simulation results of the two areas & four machines system,the correlation between DEC and system damping is discussed,as well as the low-frequency oscillation characteristics of the grid connected DFIG two areas& four machines system under random wind speed.Based on the negative damping mechanism of the low-frequency oscillation,we proposed an additional damping fuzzy control strategy by combining with DEC.The control strategy improved the system damping by adding control to the reactive power control loop of DFIG rotor-side converter to suppress inter-area oscillation.It is found that the optimal fuzzy control parameters selected by DEC have the best damping effect under inter-area oscillation,while the system frequency response is not greatly affected.Then,the effectiveness of the above control strategy is verified in the New England 10-machine 39-node system,and the effect of the additional damping fuzzy control strategy on the suppression of low-frequency oscillation is discussed under the conditions of constant wind speed and random wind speed.In this paper,we analyzed the system damping characteristics in the grid connected DFIG two areas & four machines system quantitatively by Dissipated Energy Coefficient(DEC)base on the energy function method.Combining with the negative damping mechanism,we proposed an additional damping fuzzy control strategy based on DEC.Finally,we verified the oscillation suppression effect of the control strategy on low frequency oscillation in the New England 10-machine 39-node system.