Control Strategies Of VSC-HVDC Based On Modular Multilevel Converter

The development of modern power electronic technology make the flexible HVDC technology and have developed further and the structure of voltage source converter also have a new breakthrough,hence creating the third generation of HVDC flexible technology which on this basis.In this paper,it first made a simple narrative to the electric power transmission technique based on the voltage-sourced converter and a brief introduction to the application situation of practical engineering.However,this paper mainly focus on the working principles,modulation and control methods of the modular multilevel converter in HVDC field.The main content is as follows:(1)MMC remedies a lot of technical defects of VSC,MMC-HVDC system has many unique technical advantages,all these are attributed to the topology of MMC.From the topology of the converter,this paper studied theinternal topology structure and the operating principles of sub-module.Paper through analysising the three-phase structure of MMC,putting MMC each phase into the number of sub module at any time,and combining with the balance bridge with some electric potential theory of the circuit principle,is establishing a simplified equivalent model of MMC-HVDC to facilitate the back analysis.(2)Modulation strategy which used in VSC and based on PWM is described in detail.After that,according to the modular feature and the need for multi-level output in the actual operation,Nearest Level Modulation(NLM)algorithm has been presented and applied to the simulation model at the end of the text.(3)Aiming at the problem of unbalance voltage in the charging and discharging process of sub module capacitor voltage,proceed from sub module capacitor voltage stability control and balance control to achieve the control of the sub module capacitor voltage.In terms of stability control,established the correct mathematical model.In capacitor voltage balance control,putting forward a new method of voltage-sharing for the complexity and switching frequency of sub module capacitor voltage sequencing,final finished the control of the sub module capacitor voltage.(4)MMC-HVDC and VSC-HVDC still use voltage source converter,reference tradition control strategy of traditional VSC-HVDC transmission system,so it is introduced into the control of MMC-HVDC.At the same time,the mathematical model of MMC-HVDC is analyzed and lead into voltage difference.At last,I complete the design of converter station-level control strategy in MMC-HVDC.(5)One 9-level MMC model has been built in PSCAD/EMTDC environment.Based on the simulation results,we firstly vertified whether the pair complete charging sub module,as well aswhether the designed converter station-level control can be realized.The 21 level model is obtained though extend the level number and the capacitor voltage balance control is verified by the results.