Modeling,Analysis And Control Of Frequency Response Of DFIG Based Wind Power System

Renewable generation installation is now of global range and continues to draw investment interests and research attentions.Electric power system is currently experiencing a grand transformation since the increasing installation of devices with power electronic converters.Increasing integration of renewable generation would bring multiple challenges to power system stability since the reduction of system equivalent inertia.System frequency regulation,especially primary frequency regulation,is now under enormous pressure when severe power shortage occurs.Compared with distributed power generation system,wind power generation participating in system frequency regulation represents more practical concerns since its large-scale centralized grid connecting characteristics.The highly controllable output dynamics of wind turbine generators lead to potential contributions of device inertia response to system stability.Considering the existing control strategies of wind turbine participating in frequency regulation,the inertial control based on rotor kinetic energy control(RKC)of doubly fed induction generator(DFIG)type wind turbine(WT)has more technical advantages and application prospects.Based on the power system with highly-injected wind generation and focused on DFIG type WT,this paper studies the modelling,control,and analysis of DFIG type WTs participating in power system frequency regulation.The concrete content is in following:(1)The practical variable of induction generator is redefined and the amplitude-phase model of DFIG is deduced.The new practical variable will not be affected by the system frequency fluctuations.The variation of system frequency can be accurately calculated in the new generator model,thus benefitting the analysis and control research of DFIG type WT under system frequency fluctuations.(2)The equivalent inertia and equivalent damping of DFIG type WT are derived,which provides the basis for analysing the frequency response of wind generation system.The influence factors of DFIG type WT frequency response are studies through frequency domain analysis and time domain simulations.The results indicate that DFIG type WT has a better support for system frequency when it works in the constant power area,while it may even worsen the system frequency behaviour when it works in other areas.(3)An enhanced frequency control strategy is proposed.When DFIG type WT operates in the constant speed area or in the maximum power point tracking(MPPT)area,the regular control system of DFIG type WT will have significant impact on the efficiency of common inertial control system.Based on identifying the relevant parameters of frequency control,the enhanced frequency control strategy is designed to solve the above-mentioned issue.When the virtual inertia control is activated,this control strategy will send the virtual speed measurement signal to the control loop of DFIG type WT,and then the weakening effect of MPPT control and speed control to virtual inertial control loop will be blocked.This enhanced control strategy can improve the frequency support ability of DFIG type WT effectively under low wind speed.Then,aiming at the problem of secondary frequency drop caused by enhanced frequency control,a soft exit strategy is designed.By optimizing the control parameters,the first frequency drop amplitude is minimized.Meanwhile,the secondary frequency drop amplitude will not exceed the primary frequency drop amplitude.(4)A general system frequency response(G-SFR)model is introduced since the classic system frequency response(SFR)model is not suitable for power system with large-scale hydro-turbine installation or renewable energy integration.By using standard transfer function to describe the equivalent dynamic model of hybrid prime motors and the speed governor,the G-SFR model solve the adaptability problem of classic SFR model.The structure of G-SFR model is simple and the parameters can be exclusively identified.The validity and practicability of G-SFR model can be verified by simulation results and measured data.The above study results provide a theoretical basis for modelling,analysis and control of DFIG type WT frequency response and will benefit the frequency dynamic security of power system with DFIG type WT.