Research On Modular Multilevel Cascaded Converter And Its Applicability In Urban Grid

Urban grid is the important infrastructure of the construction of the modern city.The improvement of power quality and transfer capability are the development goal for urban grid.Modular multilevel cascaded converter(MMCC)is a family name of one kind of full-controlled power electronics devices based high-voltage high-power power converters with the cascaded configuration of power modules.The topological structure of MMCCs varies between different applications.MMCCs can be connected to the medium/high power bus without transformers.MMCCs have the advantages of small area occupied and low loss,also it can work in four quadrants.This thesis focuses on the internal characteristic of MMCCs and its applicability in urban grid as follows:This thesis summarizes the common problems of the internal characteristic of MMCCs,namely internal power fluctuation(PF)and circulating current(CC).In this thesis,the MMCCS are equivalent to connected graph,and the formation mechanism of common problem of MMCCs are analyzed by graph theory.Then,the generalized analysis method of MMCCs is proposed.The accurate amplitude frequency characteristic of PF and CC can be obtained by this generalized analysis method,and it can be appropriate for all MMCCs with different topological structure,input/output and power module.On this basis,a generalized suppression stratedgy of CC is proposed for MMCCs.This stratedgy has the clear physical meanings and is easy to realize.What’s more,the energy balancing metod of MMCCs can be combined with the proposed suppression stratedgy of CC.The study of MMCCs common problems provides the theoretical basis for its applicability in urban grid.It is very economical to mitigate the power quality problems for sensitive and nonlinear load in intensive and comprehensive approach.Unified Power Quality Conditioner(UPQC)is an ideal equipment to mitigate the power quality issues of voltage and current comprehensively.In this thesis,a hybrid configuration of MMCC-UPQC is proposed which requires less power modules and IGBT switches.The proposed UPQC can utilize the compensation voltage to support the generation of compensation current.The proposed MMCC-UPQC can be connected to the medium/high power bus without transformers,and it has some merits of high power density and low cost.Further more,a compensation strategy based on simultaneous reactive power injection for UPQC(namely UPQC-SRI)was proposed to address the issue of voltage sag.The proposed UPQC-SRI determined the injection angle of compensation voltage with consideration of optimal configuration of UPQC current-carrying.Moreover,the compensation strategy also considered the current-carrying limit of UPQC,and then the zero active power injection region of UPQC-SRI(also called UPQC-SRI region)could be obtained.For the case of exceeding the UPQC-SRI region,the limit value of injection current of shunt converter could be determined by this proposed strategy.Finally,the proposed MMCC-UPQC and voltage sag strategy are verified on the PSCAD/EMTDC platform.Based on the existing power transmission hallway,the improvement of transmission line capacity with advanced power supply technology is the prospective approach to improve the capability of power supply in urban grid.This thesis presents a TSBC-MMCC based low frequency ac transmission(LFAC)system for power cable in urban system.The LFAC system is realized by installing two TSBC-MMCCs at each end of the power cable respectively.The proposed LFAC system decreases the charging power of power cable by reducing its work frequency.It is obvious that the LFAC system increase the transmission capacity of power cable and is compatible with exsiting ac system.In this thesis,PF and CC of TSBC-MMCC were analyzed in low frequency working condition.The frequency and magnitude of characterized frequency of CC were derived,and the relevant parameters of TSBC-MMCC were also analyzed in detail.Then the mathematic model of TSBC-MMCC is established,and the control mode of TSBC-MMCC based LFAC is proposed,which achieve the flexible power flow control.Finally,the TSBC-MMCC based LFAC is verified by simulation.To depth study of all MMCCs and its applicability,this thesis develops a MMCC general experiment platform.The fundamental unit of the main circuit of the experiment platform is the MMCC power branch.The parameter design,devices selection and branch controller are described in detail.The scheme of testing experiment is presented to assure the reliability of assembled MMCCs.The system controller of experiment platform is developed with the principles of unified design and hierarchical operation.This controller is extendable that meets the demand of the large number of power modules for MMCCs.For the general software,this thesis proposes a hybird modulation for MMCCs which realizes the precise and efficient voltage output.In the end,the prototype of TSBC-MMCC is built using the MMCC general experiment platform.