Transient Stability Analysis For Voltage Controlled Doubly-fed Induction Generators Using Virtual Inertia Method

Wind power grid-connection is an important direction for the development and utilization of new energy.Recently,the most popularly used doubly-fed induction generator(DFIG)is usually controlled to be directly connected to the grid with the adoption of control methods based on the maximum power tracking(MPPT)control.With the development of power electronics technology and the actual requirement of the engineering application,the installed capacity of DFIG is constantly increasing,which leads to the problem of decreasing inertia and weakening damping of power grid and increases the occurrence possibility of power grid faults.However,due to the MPPT control,the active power output of DFIG is decoupled from the power grid frequency,and DFIG does not respond to the fluctuation of the power grid frequency.In this case,DFIG is obviously unable to support the power grid.In this paper,a control strategy for DFIG based on voltage control type is proposed,which realizes the introduction of virtual inertia and the compensation of equivalent inertia,remodels the external characteristics of DFIG to reflect the characteristics of the voltage source,and realizes the frequency and power supports to power grid.The designed control strategies make it possible for DFIG to realize independent island operation and synchronous grid-connected operation both.Considering multiple time scales,this paper conducts transient modeling and stability analysis according to the proposed control methods for grid-connected DFIG.On the one hand,through theoretical analysis,simulation implementation and experimental verification,it proves the effectiveness of the designed control strategies.On the other hand,the relationship,rule summary and stability analysis of the transient characteristics of DFIG under grid faults during the transient grid-connected operation periods are carried out,and the influence of the control parameters,transient power controls and grid conditions on the transient stability of DFIG is studied,so as to realize the integrity of the designed control strategy analysis.