| Modular multilevel converter is considered to be the most typical topology for medium and high voltage applications in the future with the advantages of high modularity,low switching frequency,and easy expansion.At present,MMC has achieved great development in HVDC,but there are still many issues deserved to further be researched in the application of medium and high voltage motor driving.For example,when MMC is used in the field of motor driving,the modulation strategy must have good characteristic of frequency conversion and be easy to implement digitally.Space vector pulse-width modulation(SVM)is suitable for frequency conversion application,because it directly tracks magnetic chain.But the complexity of SVM increases sharply with the increase of the number of MMC submodules.Therefore,this paper focuses on the research of SVM used in MMC.This paper proposes a simplified space vector modulation method applied to MMC.Firstly,the paper puts forward the single-submodule grouping method,which combines three SMs respectively from three phase legs to form a new object,named sub-unit.By this method,the sub-unit is structurally similar to the three-phase two-level voltage source converter.Therefore,it is possible that the two-level SVM can be used in each sub-unit.In order to achieve better output voltage,the sampling time of each sub-unit is staggered evenly during a sampling cycle and each sub-unit is modulated by its own sampling value,which makes the compositing vectors of sub-units in the same bridge are similar to multi-level SVM and improves the equivalent switching frequency and DC voltage utilization of MMC.By controlling each sub-unit with two-level SVM,the complexity of SVM is decoupled from the number of submodules,and this method can be used in MMC with any number of submodules.Finally,the paper presents a capacitor voltage balancing strategy based on rotating the sampling time of sub-units,ensuring the charging and discharging time of each sub-unit is consistent.Then the capacitor voltage get balanced without complicated closed-loop control.Furthermore,this paper analyzes the selection principle of the rotation period,and gives the optimal rotation period.In order to increase the number of switching vectors of each sub-unit and improve the control flexibility of sub-units,this paper expands the SVM of MMC based on multi-submodule-grouping method.Firstly,this paper combines two submodules in the same phase into one sub-units,increasing the redundant vector of each sub-unit.But the capacitor voltages of submodules in the same phase within a sub-unit will be unbalanced,because of uneven output.This paper furtherly presents a rotation strategy for each sub-unit,which not only includes the rotation of sampling time between sub-units,but also involves the gate signals rotation within a sub-unit.The principles of the rotation period for each layer are discussed in this paper.In the end,this paper deeply studies the influence of the sampling time interval between the corresponding sub-units in the upper and lower bridge arms on the number of MMC output voltage levels.By using different sampling scheme,MMC can work in either N+1 level mode or 2N+1 level mode without changing the topology.Finally,based on the control hardware architecture of the MMC experimental platform,the algorithm process of software is designed.The experimental results verify the effective performance of SVM for different submodule grouping methods and the capacitor voltage balancing strategies of MMC. |
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