| Compared with traditional fixed-speed and recently full-scale power-converted induction generators,more and more large scale wind farms have employed the wind turbines which are based on doubly fed induction generators(DFIGs).The DFIG can offer some technical and economic advantages including independent control of active and reactive power,optimal speed tracking for the maximum wind power capture,reduced cost of power converter etc.The high voltage alternative current(HVAC)connection used to be the first option of wind farm integration for its proven characteristics and reliability.Nowadays the Grid Code is becoming more and more strict,the HVDC scheme inevitably encouters the problems like large amount of capacitive reactance current when transmitted through long distance ac cable.The HVDC scheme provides a better solution for the integration of wind farms.Moreover,the isolation by HVDC link is quite beneficial to improve the low voltage ride through capability of wind turbines.Traditional thyristor-based HVDC technology is based on the commutated voltage provided by the ac grid and needs large amount of reactive power compensation.Newly IGBT-based fexible HVDC technology dose not need the commutated volvage and has the capability of regulating active and reactive power respectively,thus is very suitable for the wind farm integration.On the other side,the modular multilevel voltage source converter based HVDC(MMC-HVDC)technology is the developing area in HVDC application.Therefore,this thesis will focus on the control algorithm design of MMC-HVDC transmission system used to interconnect DFIG-based wind farm to ac grid.This thesis analyzes the operation principle of the DFIG,including active and reactive power relationship,maximum wind power capture,operation range of DFIG and field oriented vector control technique,which establish the theoretical foundation for succeeding study.Based on the operation principle discussion of submodule/converter,a control algorithm for the start-up of MMC-HVDC transmission used in wind farm integration is proposed in this thesis.The the relation between current-limiting resistor and the maximum ac current of grid side conveter is given.To avoid the overcurrents when IGBTs unlock,selection range for the slope parameter of DC voltage control loop is proposed.For the integration of doubly fed induction generator-based wind farms through a voltage source converter-based HVDC transmission system,a detailed algorithm for start-up control of DFIG-based wind generators,based on stator flux orientation and closed-loop control of rotor current is analyzed.A passive network voltage control algorithm of wind farm side modular multilevel converter is proposed to act as an infinite voltage source.Furthermore,a damping control signal is introduced to suppress the potential active power fluctuation of the wind farm during stable operation.A dynamic chopper controlled breaking resistor(DCCBR)based modular multilevel VSC-HVDC transmission system is proposed in this thesis to improve the low voltage ride through(LVRT)capability of wind farm composed of doubly fed induction generators.When a voltage dip caused by ac network fault is happened,the detailed operation principle,protection criterion and maximum turn-on duration time of the DCCBR are discussed for the implementation of low voltage ride through of wind farm.After fault clearance,the coordinate control of DCCBR and grid side modular multilevel converter(GSMMC)is described to improve the power recovery capability.Simulation results confirm the effectiveness of the proposed DCCBR scheme to realize the LVRT of the wind farm.Furthermore,after fault clearance the power recovery speed of wind farm is much higher than the requirement of domestic Grid Code.The Nanhui modular multilevel VSC-HVDC transmission system,the first VSC-HVDC project in Asia,also the world's first demonstration project built with MMC topology applied in wind farm integration had been put into operation in June,2011.The main devices and single line diagram of this project are introduced in this thesis,followed by the field test results including DFIG cut-in and power step experiments.The results validate the possibility and stability of the proposed control schemes of MMC-HVDC transmission system for wind farm integration. |
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