Research On The Modulation Technique And Control Strategy For MMC-HVDC

With the development of power electronics technology in modern power transmission field,the application of VSC-HVDC system based on voltage source converter in power engineering has been widely concerned by people.However the two-level and three-level VSC-HVDC system have low voltage level,big loss,huge volume filter and other problems,in a certain extent limit the further application in the DC transmission project.Modular multilevel converter uses the modular design,has strong expansibility,good flexibility,high number of output levels,can meet the requirements of different voltage and power levels.That eliminates the need for high power AC filter,reduces the switch frequency of sub module,is suitable for high voltage and high power VSC-HVDC occasions.The VSC-HVDC technology based on modular multilevel converter has broad application prospects in the aspects of passive network power supply,renewable energy integration,isolated load power supply,city center power supply and so on.Therefore,it has important practical significance to study its modulation technology and control strategy and other related technologies.This paper analyzes the topological structure and working principle for modular multilevel converter,then its mathematical model is established and the main circuit parameters of bridge arm sub module capacitance value,bridge arm inductance value,bridge arm module number and DC side voltage are studied.Describe in detail the several modulation techniques for modular multilevel converter,respectively to compare their advantages and disadvantages and applicable occasions.The sorting and modeling method combined in the capacitor voltage control method and modulation technique are used together.Choose the carrier phase shift modulation and the nearest level modulation that are suitable to VSC-HVDC occasions for mathematical analysis.Then use MATLAB software to analyze the converter output voltage and current quality,sub module capacitor voltage fluctuation and circulation inhibition characteristics under the two kinds of modulation techniques.The results show that the harmonic characteristics of nearest level modulation are superior to carrier phase shift modulation.Finally,the mathematical model of modular multilevel converter HVDC system based on two-phase rotating coordinate system is established,simplifies the model.Analyze the converter station double closed-loop vector control strategy to achieve the effectivecontrol of modular multilevel converter VSC-HVDC system.Set up the 11 levels simulation model of double and single active terminal for modular multilevel converter HVDC system in MATLAB/SIMULINK,through the simulation analysis of DC voltage,AC voltage,active power,reactive power and other control variables about the two cases on above,proved that the both control strategies have good dynamic response characteristics and control performance when the control variables change,improve the stability of system operation.