Research On General Steady-state Analysis Model And Its Application For Modular Multilevel Converter

Compared with the traditional two-level and three-level converters,the modular multilevel converter(MMC)has many advantages,such as:1)without direct connection of semiconductor devices;2)high quality and low harmonics of output voltages;3)low switching frequency and low switching losses;4)the modular structure is easy to assemble;5)strong sub-module(SM)fault-handling ability.Due to these advantages,it have been attracting great interests of industry and researchers in recent years.The MMC has broad prospects in many application fields,such as VSC based high voltage direct current(VSC-HVDC)transmission,offshore wind farm integration,medium-voltage motor drive,asynchronous grid interconnection,and island power transmission.Therefore,the research on MMC has important theoretical and practical significance,which will have a profound impact on the development of future power grids.In this dissertation,the MMC is taken as the research object.On the basis of its operation and control principles,a general MMC model is proposed for the steady-state analysis,which is not only high-accurate but also applicable for multiple control methods.Based on the proposed model,the research is carried out on the SM capacitance selection and steady-state performance optimization.The main research contents are summarized as follows.(1)A general MMC model for the steady-state analysis:the steady-state analysis plays important roles in the main-circuit parameters design,operating performance evaluation,protection setting,and so on.Thus,this dissertation presents a general MMC model for the steady-state analysis.In the proposed model,various steady-state performance optimization methods are considered,and the mutual interactions of the electrical quantities in the MMC are comprehensively taken into account.In the derivation of the model,firstly,the coupled relationships among the electrical quantities are analyzed.Then,an equilibrium equation is established based on the interaction mechanism between the internal and external electrical quantities of the converter;this equilibrium equation can reflect the inherent steady-state characteristics of the MMC.After that,the expressions of electrical quantities are analyzed,and simultaneous equations are derived from the equilibrium equation for solving the unknown parameters in the expressions.Finally,the time-varying steady-state values of electrical quantities can be directly calculated when all parameters have been known.In addition,taking the optimization method of circulating current injection as an example,the proposed general model is compared with the traditional models.The comparison results show that the proposed model can reflect the impact of one quantity on others more accurately.(2)A simplified capacitor voltage calculation method,and analyzing the steady-state characteristics of capacitor voltage:the SM capacitor voltage is an important index in the capacitance selection.The accurate method of calculating the steady-state capacitor voltage is the prerequisite of selecting an appropriate SM capacitance for the MMC.Since the general MMC model requires a complex calculation,a simplified method for calculating the capacitor voltage is proposed,which is applicable for the MMC with closed-loop power and circulating current controllers.In this calculation method,the capacitor voltage can be calculated in one stroke by only using one expression;hence it significantly reduces the calculation burden.In addition,the steady-state characteristics of the capacitor voltage are analyzed;and some problems in the existing capacitance selection methods are found from the analysis.(3)A new SM capacitance selection method:the SM capacitor is a key component in the MMC;it has a great impact on the cost,volume,weight,and safety of the converter.A step-by-step capacitance selection method is proposed based on the proposed general MMC model.In searching the minimum required capacitance,the Secant Method is used to reduce the iteration times.Both of the maximum capacitor voltage and the capacitor voltage fluctuation are considered in the proposed selection method,and it resolves the problems in the existing methods.(4)Analyzing the effects of DC signal injection:compared with the optimization methods of circulating current injection and third-order harmonics injection,the current research on the DC signal injection is insufficient.In this dissertation,an expression is derived for accurate describing the relationship between the average capacitor voltage and the DC component of the modulation signal.Then,the mechanism of using DC signal injection to reduce capacitor voltage is analyzed.Finally,the research is conducted on the positive and negative effects of the DC signal injection in the steady-state performance optimization.The research can provide the theoretical basis for optimizing the steady-state performance of the MMC with DC signal injection.(5)A capacitor voltage reduction method based on the DC signal injection:the SM capacitor should be large enough to diminish the negative impact of capacitor voltage fluctuation.If the capacitor voltage could be reduced,the SM capacitor with smaller capacitance can be adopted.Therefore,on the basis of the inherent characteristics of the MMC,a control strategy of capacitor voltage reduction is proposed by adding a specific DC component to the original modulation signal.The dissertation presents the calculation procedure of the injected DC signal,and analyzes the area required of dc signal injection.By adopting the proposed method,the constraint of maximum capacitor voltage can be eliminated from the capacitance selection;and hence the safety requirement can be met with smaller SM capacitance.The above research are verified by both simulation and experiment.The results prove that the conclusions are correct and the proposed methods are effective.