Coordinated Control Of Voltage Fault Ride Through Of Doubly-fed Induction Generator

With the increasing proportion of wind turbines in the modern power system,disconnecting large scale wind farms from the grid will seriously threaten the safety and stability of the power system.Research on safety and reliability of the grid-connected wind farms is receiving more and more attention from experts and scholars worldwide.The doublyfed induction generator(DFIG)is the most widely deployed model in wind farms,but due to its unique structure,voltage fluctuation of the system will directly impact the stator side of the DFIG,and the reactive power regulation capability of the wind turbine is small because of small capacity of the converter,thus the fault ride-through capability of the doubly-fed wind turbine is limited.This paper analyzes the characteristics of the DFIG and proposes coordinated control strategies for low voltage ride through(LVRT)and high voltage ride through(HVRT)for DFIGs respectively based on existing methods.First of all,this paper introduces the structure of the DFIG,and analyzes the control strategies of the rotor-side converter(RSC)and grid-side converter(GSC).Based on the PSCAD/EMTDC simulation platform,a single grid-connected DFIG model is built to validate the model,and the electromagnetic transient characteristics of the DFIG based wind turbine during voltage dips and voltage swells are analyzed.Next,based on its topology and operating principle,this paper analyzes the control strategy of the DC chopper circuit,and points out that the repeated switch of DC chopper will cause DC voltage and electromagnetic torque oscillation under severe voltage dips.A dynamic chopper resistance is designed to limit DC voltage and mitigate the oscillation,and the its effectiveness is verified by simulations under different fault severity.Furthermore,an improved DC chopper based on adaptive dynamic resistance is proposed,and a comparative simulation with the traditional DC chopper is carried out to verify its control effect.Then,this paper investigates the HVRT problem of wind farms connected to HVDC transmission systems.During the fault period,the reactive power adjustment capability of the DFIG itself is fully utilized,and based on this,a new type of large-capacity synchronous condenser(SC)is deployed at the point of common coupling(PCC)to the wind farm.The reactive power control of both DFIGs and SCs is cooperated according to their response speed to improve the HVRT capability of the wind farm during the fault period and speed up the fault recovery.Simulations under DC blocking,reactive power disturbance and continuous commutation failure verify the effectiveness of the method.