Research On Low Voltage Ride Through Control Of Double-fed Wind Turbines In Flexible DC Transmission System

The penetration of wind energy in the power grid is getting higher and higher.Wind energy becomes a major player in grid operations.As a result,wind power systems must now participate in grid support and provide ancillary services.The voltage drop caused by grid faults,cable faults,etc.is a problem that wind farms must consider to solve.Wind farms must support the grid during grid instability and transition.Based on the topology of doubly-fed wind turbines with flexible DC transmission,this paper studies the operation control strategy of doubly-fed wind turbines under DC bus voltage drop.In this paper,the rotor-side converter,DC-side converter and energy storage-side converter are controlled accordingly,and the simulation model and experimental platform are built for verification.The main contents are as follows:First,this paper analyzes the development status of wind power technology,and explains the impact of voltage dips on wind turbine systems and the importance of low voltage ride through technology in wind power generation.Then,the current status of low voltage ride through researches were summarized.Second,a mathematical model of the DFIG wind power generation system based on VSCHVDC is established,as well as the mathematical model of the doubly-fed wind turbine(DFIG),the stator-side converter(SSC),rotor-side converter(RSC)and DC-side converter.This paper proposes the control strategy to achieve maximum power point tracking,decoupling control of active power and reactive power,and the goal of stabilizing DC bus voltage.Furthermore,a series of simulation analysis of the DFIG wind power generation system based on VSC-HVDC is carried out by using Matlab/Simulink.Third,aiming at the topology of the DFIG wind power generation system based on VSCHVDC,a control strategy for DC bus voltage drop is proposed,which is studied from the control of RSC and DC-side converter and the introduction of energy storage device.The control strategy can effectively suppress the over-current phenomenon of the stator and rotor side and the large fluctuation of the DC bus voltage of the doubly-fed wind power generator caused by the low voltage drop of the DC bus.According to the above mathematical model and control strategy,the simulation platform is built to verify its feasibility.Finally,from hardware selection to software design,the experimental platform of flexible DC transmission doubly-fed wind power generation system was designed and constructed.The experimental platform uses the frequency converter and the motor to simulate the wind turbine together,and drives the DFIG to work.The DSP28335 and dSPACE are selected as the core controllers to control converters to ensure stable operation of the system.The proposed control strategy is verified by a series of experiments.