Research On Control Strategy Of MMC-HVDC In Wind Power System

As a new type of converter,Modular Multilevel Converter(MMC)has the advantages of low switching loss,less harmonic content,convenient design and control,so it gradually replaces the traditional Voltage Source Converter(VSC),and is widely used in high voltage direct current(HVDC)or flexible direct current transmission,namely MMC-HVDC.This topology has the advantages of large transmission capacity,high reliability,decoupling control of active and reactive power,and is very suitable for wind power system applications.However,due to its unique topological structure and operation mechanism,MMC generates inter-phase circulating current,and makes the sub-module voltage and bridge arm current fluctuate.For the voltage balance control of sub-module,the current decentralized voltage control system has complex structure,coupling and low reliability.Therefore,this thesis adopts the balance control principle of overall input.For the modulation strategy of MMC,Nearest Level Modulation(NLM)based on step wave modulation has the advantages of fast dynamic response and low switching frequency,but its switching frequency is uncertain.For the control strategy designed in Chapter 3 of this thesis,carrier phase shifted PWM modulation strategy will be adopted.MMC also has the problem of how to start and control the system.In order to solve these problems,the following research work has been carried out:The operation mechanism and modulation strategy of three-phase MMC are studied.The mathematical model of MMC and coordinate transformation from threephase stationary to two-phase rotation are deduced.The control strategies of inner DQ axis current decoupling,outer fixed active power,reactive power and fixed AC and DC voltage of MMC are designed.In order to improve the quality of MMC output waveform,the generation mechanism of circulating current is studied and the circulating current suppression controller is designed.In order to improve the utilization of DC voltage and reduce the fluctuation of bridge arm current,the method of injecting third harmonic into voltage modulation wave is designed.In order to reduce the voltage fluctuation,the voltage balance strategy of sub-module is designed,and the steady-state simulation analysis of the system is carried out.The validity of waveform optimization is proved,which provides a basis for the later transient process analysis.For the MMC-HVDC system with AC network connected at both ends and the MMC-HVDC system applied in wind power system,the start-up strategy is designed,and the transient process of the two systems,i.e.start-up process and AC/DC fault,issimulated.The waveforms of DC line voltage,active and reactive power of two converter stations,AC side current and capacitor voltage of sub-modules are analyzed and verified.The validity of the system start-up strategy and control strategy is discussed,and the working condition can be restored after the fault occurs.