The Research Of Submodule Capacitor Voltage Detection Method For Modular Multilevel Converter Based On Submodule Grouping

Modular multilevel converter(MMC)is a novel topology of multilevel converter,being composed of many basic power modules.With several attractive features such as the numerous level number,the easy realization of redundant structure and high voltage grade and reliability,the excellent quality of the output voltage waveform,it has been used in flexible direct current transmission.Because of the extensive number of submodules and the complicated mechanism of internal energy conversion,different running status and detecting method will have influence on the working status of modules.How to ensure the economy and stability of MMC in running is the vital direction in research of MMC by many scholars from different nations.This paper studies on the MMC through the theoretical analysis,software simulation and experiment.The research is focused on the optimization of the traditional detecting method of submodule capacitor voltage.The detail content is as follows:Firstly,this thesis presents a structure and operation mechanism of MMC,building a simplified model for MMC.On the basis of the former studies,two normal modulation strategies are introduced in this paper: carrier phase-shifted PWM(CPSPWM)and nearest level modulation(NLM).Also,the paper analyses the balancing control strategy,stable control strategy and output characteristics under two modulation tragedy respectively.The reasons why CPSPWM is applied to the case of less level and why NLM is applied to the case of more level are explained,which lay the foundation of the latter research on voltage detection based on dividing the submodules into groups.Secondly,in order to obtain the higher voltage grade,the converter station based on MMC needs lots of cascaded submodules due to the voltage limit of semiconductors.For example,the Trans Bay Project has more than 200 submodules.Obtaining the voltage of submodule in real time is the precondition to perform the control of stable running.The traditional submodule capacitor voltage measuring method requires a sensor for each submodule to obtain the voltage in real time,which increases the system costs and burden of data acquisition and communication system when the number of submodule is extremely large.To solve this problem,a new submodule capacitor voltage measuring method based on submodule grouping is proposed.In this method,the submodules are divided into groups,equipped with a sensor for each group to obtain the submodule voltage based on NLM.Through two voltage value before and after or voltage observer,we can acquire the voltage correctly by calculation.Then,because of the submodule grouping,the submodules not be measured directly may overcharge,which will cause the over-voltage of submodules.To solve this problem,an over-voltage prevention based on forced-engaging strategy has been presented.In this strategy,before the loads working,all submodules will be pre-measured to obtain the real voltages of submodules.The observes will be adjusted by using the real voltages in case of the overcharge,thus protect the over-voltage from happening.Simulation results verify the feasibility and efficiency of the new submodule capacitor voltage measuring method.Finally,the experimental platform of single-phase nine-level MMC is introduced.The novel submodule capacitor voltage measuring is carried out on the experimental platform.Experimental results verify the feasibility and efficiency of the new measuring method based on submodule grouping.