Study Of Unified Modeling And Systematic Control Of Modular Multilevel Cascaded Converter

The multilevel converter family named Modular Multilevel Cascaded Converter(MMCC)has become an attractive multilevel solution for medium and high voltage applications.Compared with the conventional multilevel converters,MMCC offers a large amount of voltage levels with linear expanding hardware complexity,less harmonic injection,a modular construction for redundancy,flexible scalability for different voltage requirements and a reduced switching frequency as well as power losses.A comprehensive classification based on graph analysis is presented.The graph-based classification extracts the graph information of each MMCC topology and uses it as a reference to term all the MMCCs.In the framework of graph theory,each branch and each terminal are uniquely termed and symbolized,which forms the basis for the following structural modeling and analysis.The proposed generalized modeling method divides MMCC modeling into four decoupled parts,which are external current modeling,external power modeling,internal circulating current modeling and internal branch energy balancing.By strictly following this method,a standard state-space model of an MMCC system can be obtained that embodies the unique model features of multi-input and multi-output,coupling effects of state variables,nonlinearity and time-variance.A layered internal branch energy balancing strategy is developed to balance energy distribution among the MMCC branches.The generally-defined circulating currents play an essential role on realizing energy exchange among the branches and finally reaching the balanced goal.A structured balancing model is formulated based on branch grouping results from graph analysis.To demonstrate the suitableness of the developed framework for the control purpose,a generalized control design combining multivariable control theory and optimal principles is proposed.It is especially suitable for MMCC system based on multivariable models,which systematically handles the control issues of MMCC external functionality and internal energy balancing in an optimized way.Three case studies are discussed by following the unified modeling framework and multivariable optimal control,which are medium voltage direct current transmission system based on back-to-back MMCs,single-delta-configured MMCC for reactive power compensation and modular multilevel matrix converter for interconnecting two grids with different frequencies.In the end,the implemented three-phase MMC system in laboratory is described and the corresponding experimental results under a cascaded multivariable controller are presented.