Novel 4-level Submodules Based Modular Multilevel Converter And Its Control Strategy Study

Compared with Voltage Source Converter(VSC),Modular Multilevel Converter,(MMC)has the advantages of high quality of output waveform,low consumption of switching element,low step voltage and good ability in disposing faults.With the benefit of compact topological structure and highly modularized convertor station,by increasing the number of submodules on arm,the output can be extended to any number of voltage level and have high reliability and convenient maintenance.As a result,it strengthens the MMC's competitiveness in the field of high voltage DC transmission(HVDC)and improves the application prospect of renewable power integration and urban power grid intervonnection.In the field of high-voltage transmission,due to the higher power and voltage grade and more voltage levels,based on the traditional submodules half bridge topology of MMC convertor station requires for large number of modules,high switching frequency and a relatively large calculation.Aiming to solve these problems,this essay carried out a new kind of submodule topology which is composed of five IGBTs,three capacitors and a certain number of fly-wheel diodes,which is able to generate four kinds of levels of 0,1,2,3.So that the converter requires for less submodule while in higher level applications.At the same time,compared based on the traditional half bridge of MMC system module,it requires for less IGBT switches under the condition of same output level.Therefore the corresponding control strategy researchof the new submodule has practical meaning.The research's mainly contents are obtained as followed:(1)Analyzed the working principle of traditional half bridge sub-modules half bridge,established the mathematical model of half bridge submodule based modular multilevel converter,expounds its basic operation principle.Proposed a novel four level module topology structure,illustrates its working principle under different working conditions.Gives detailed switch table for subsequent control strategy research.(2)Introduced common modulation strategy and analyzed their advantages and disadvantages for MMC system,according to the characteristics of the new topology which is the ability of generate higher output level,emphatically analyzed the nearest level modulation,the modulation strategy is given in the concrete realization method of MMC system.Combined with nearest level modulation and feature of the new submodule,capacitor voltage balance method suitable for the new submodule based MMC is given,which is able to maintain a steady state for capacitor voltage.(3)The general mathematical model of MMC is established,and the AC circuit controller of MMC system is designed.The AC loop controller is divided into an inner loop current controller and an outer loop controller.The outer loop controller is divided into active power controller and reactive power control.The steady state characteristics of the new submodule based MMC are analyzed by simulation.Compared with the half bridge module MMC,the new sub module MMC is more efficient.(4)Analysed the flexible HVDC power transmission system principle.Established the mathematical model of modular multilevel converter based HVDC transmission system in two-phase rotating coordinate system.Designed control system for HVDC system,set up a new submodule based MMC-HVDC model.Stability and effectiveness of new submodule structure and related modulation,control strategy is verified by simulation in transient conditions.