Modeling,Analysis And Control Of MMC-Based HVDC Grids

At present,China is in the initial stage of the energy revolution.The goal of the energy revolution is to build a modern energy system that is low-carbon,safe,efficient and sustainable.The modular multilevel converter(MMC)based DC grid has lots of advantages,which is very suitable for large-scale renewable power integration.It is also considered as an important direction for the future development of power system.However,compared with the AC grid,the MMC-based DC grid is a new type of power system,and engineers still lack sufficient experience on its planning,design,operation and scheduling.Therefore,in this thesis specific research is carried out on the modeling,analysis and control of the MMC-based DC grid.The thesis is organized as follows:(1)Complete mathematical model and steady-state characteristic of MMC.The mathematic model of the MMC is one of the key issues for the DC grid as it is the basis for all relevant research work.Firstly,the input and output structure of the MMC mathematical model is defined.Secondly,based on the average switching function,the differential equations of the MMC are obtained.Then,through the proposed successive approximation approach,the analytical expressions of the electrical quantities are derived.The proposed successive approximation approach contains three main steps.In the first step,the preliminary analytical expressions of the MMC's electrical quantities based on the ideal step-wave of the arm voltage with constant submodule(SM)capacitor voltage is deduced;the second step is to derive a new expression of the arm voltage considering the time-varying SM capacitor voltage expression which is derived in the first step;and the final analytical expressions of the MMC's electrical quantities are derived in the third step.Finally,the simulation results verify the accuracy of the complete mathematical model.(2)Analysis and selection of main circuit parameters for MMC-based DC grid.With reasonable main circuit parameters,the dynamic and steady state performance of the system can be effectively improved and the investment cost can be greatly reduced.Firstly,based on the fundamental equivalent circuit of the MMC,the design method for the rated valve side no-load voltage of the link transformer is proposed.Secondly,by introducing the concept of the equivalent capacity discharging time constant H,the nearly constant relationship between the fluctuation ratio of the SM capacitor voltage and the H-value for different projects is discovered,which means the H-value is independent of particular projects.Thirdly,the voltage and current stress of the SM power electronic devices is investigated for the 4 extreme operating conditions by the analytical formulae of the MMC;and it is discovered that the current stress of the SM power electronic devices changes greatly with the operating condition.Besides,by introducing the concepts of the phase unit series resonance frequency and the circulating current resonance frequency,the principle for designing the parameter of the arm inductor is established.Finally,based on the factors of suppressing fault current rising rate and avoiding converter blocking,the selection method of the DC smoothing reactor is given.(3)Start-up control strategy for MMC-based DC grid.There are hundreds of SM capacitors in one MMC.Before the MMC can be put into operation,these SM capacitors should be charged to their rated voltage.The whole start-up process can be divided into two stages,including uncontrollable charging stage and controllable charging stage.At the uncontrollable charging stage,the charging characteristics of MMC connected to active system and passive system are analyzed.The maximum charging rate of the MMC connected to active system is between 71%and 74%;while of the MMC connected to passive system is between 35%and 37%.At the controllable charging stage,two methods can be used to continue to charge the SM capacitor,including DC voltage slope control and sequentially unblock control.Through the proposed start-up control strategy,the MMC-based DC grid can smoothly transition from an uncharged state to a steady state.(4)Steady-state control strategy for MMC-based DC grid.The steady-state control strategy directly determines the response of the MMC-based DC grid.Therefore,it is necessary to carry out research on the steady-state control strategy that is suitable for the MMC-based DC grid.Firstly,three control strategies are introduced,including master-slave control,DC voltage droop control and master-slave with DC voltage droop control.Besides,in order to maintain the stability of the DC voltage,a secondary DC voltage regulation control is proposed.With this secondary DC voltage regulation control,the DC voltage of the reference station can be maintained at the preset value,and the power fluctuation of the whole network can be shared by multiple converters.(5)Fault characteristic analysis of MMC-based DC grid.Analyzing the fault characteristic of the MMC-based DC grid is very important.It will help us to understand the fault mechanism,and can provide theoretical basis for the design of DC fault handling strategy that is suitable for the MMC-based DC grid.Firstly,the basic equivalent circuits of the MMC before and after blocking are respectively obtained.Secondly,the analytical DC fault current expression of MMC before blocking is derived based on the operation circuit in complex frequency domain.Thirdly,after the MMC is blocked,the DC current expression is also derived.Besides,three DC fault clearance solutions and the corresponding characteristics are discussed in details.(6)DC fault handling strategy for MMC-based DC gridDC fault isolation technology is the most important technical obstacle that restricts the development of the DC grid.Firstly,the two basic ways of breaking DC current are discussed,namely the way by the series inserted infinite resistor and the way by the series inserted capacitor.Secondly,the realization methods based on the way by the series inserted infinite resistor are analyzed.Thirdly,according to the way by the series inserted capacitor,three DC circuit breaker structures are proposed.Besides,the economy of four DC circuit breakers is compared.Finally,in order to solve the problem of power supply for HVDC circuit breakers,a self-powered control strategy is proposed.