Modeling And Stability Analysis Of DFIG In Multile Time-scale Based On Weak Grid

With large-scale wind power generation connected to the power system,it will give rise to a serious of problems such as grid oscillation,frequency instability and alike.The key technology to solve the abnormal interaction between wind turbine and power system is modeling in multi-time scale.And it is significantly important to select a right time-scale to analysis the intrinsic characteristic and external characteristic of wind turbine.Therefore this paper focus on multi-time scale modeling and stability analysis of doubly fed induction generator(DFIG)connected to weak grid and the main works and contributions are as follows:1.According to the mult-time scale characteristics of control system of DFIG,the mathematical model in current control time-scale is built including current inner control loop,but ignoring the slow outer power loop.Based on this model,the stability of the voltage at the point of common coupling(PCC)with small disturbance in current control time-scale is analyzed under different strength of AC weak grid grids.The results of analysis indicate that the PCC voltage stability under small disturbance will be more likely to lose when a DFIG adopts current control strategy and connected into a weak grid.2.The system mathematical model in DC voltage control time scale including the outer power control loop and phase-locked loop is established,ignoring the rapid dynamic process of the inner current control loop of a DFIG,Based on this model,the effects of grid strength and controls parameter of PLL and outer power control loop on the stability of dc-link voltage in DFIG are analyzed.The results of analysis indicate that the dc-link voltage will lose stability and the controller for the rotor will have the effect on the stability of DC link voltage with decline of grid strength.But,when DFIG is connected into a strong grid,the controls parameters of PLL would have main influence on dc-link voltage.3.According to the analysis above,the PLL and current control is the key factor that induced unnormal interaction between DFIG and weak grid.Therefore a virtual synchronous control strategy without PLL and current control loop for DFIG is presented.By the proposed method,the interaction between DFIG and weak grid is analyzed.The results of analysis indicate that the real power stability of system can be enhanced when damping coefficient is changed within the range of 40-120.The bandwidth of real power control loop will decrease with strength of grid increase when adopted virtual synchronous control strategy which means the real power stability will be enhanced.4.The simulation models of the DFIG are established in PSCAD/EMTDC to validate the theoretical analysis and control strategy.The simulation results are consistent with the theoretical analysis results.