| With the increasing penetration ratio of the renewable energy into the power system and the increasing capacity and voltage level of the power electronic converters,the modular multilevel converters(MMCs)play an important role in the power systems,including the power generation,power transmission,power distribution,and power consumption.Compared with the traditional two-level or three-level converters,MMC has significant advantages in high-voltage and high-power applications,including low device voltage stress,high power quality on the AC side,high redundancy,high scalability,and high reliability,etc.In the classic half-bridge submodule(HBSM)based MMC,the AC and DC voltage levels are strictly clamped and coupled,i.e.,the modulation index(the ratio of the AC output phase voltage amplitude to half of the DC bus voltage)cannot be higher than one.Mixed by the HBSMs and half-bridge submodules(FBSMs),the hybrid MMC is able to achieve a wider modulation index range.Due to the negative output of the FBSMs,the hybrid MMC can achieve the over-modulation operation,meaning that the modulation index can be easily increased higher than one without affecting the quality of the AC output quality.The over-modulation operation characteristics of hybrid MMC provide greater potential for the promotion and application of MMC in various medium and high voltage scenarios.On the one hand,the over-modulation operation of the hybrid MMC,as a steady-state operation mode,can provide more design freedom for the design of the grid-connected system based on MMC,and improve the optimization space of the grid-connected system.At the same time,within a certain range,the over-modulation operation can reduce the total capacitance of the MMC and increase the power density of the MMC.On the other hand,the over-modulation operation capability can improve the flexibility of the hybrid MMC,which helps the hybrid MMC to ride through a variety of emergent operating conditions,including fault-tolerant operation under the sub-module failures,and continuous power transmission when the DC bus voltage of the hybrid MMC requires to be reduced,etc.As the special operation mode of the hybrid MMC,the over-modulation operation is quite different with the operation of the HBSM MMC or FBSM MMC,which means the conventional approaches in the existing research might lose effectiveness.The over-modulation operation of the hybrid MMC has generated many new problems and has attracted much attention in the following aspects,including the modulation approach,the parameter design,the capacitor energy balancing,and the overall performance evaluation.By solving the above problems,this paper lays a solid foundation for the further development and application of the hybrid MMC with over-modulation operation.More specifically,the research of this paper includes the following aspects:1.The improved carrier phase-shift modulation approach is proposed.Considering the wide application of the hybrid MMC in the medium-voltage occasions,the limitations of existing modulation strategies are clarified for the over-modulation operations.An improved carrier phase-shift modulation is proposed,where the modulation waves and carrier waves are re-designed by introducing the concept of critical DC voltage and the DC and AC modulation indexes.The improved modulation strategy can be adapted to the steady state and dynamic over-modulation operations.Also,it can be generalized for any modulation index.The simulations and experiments are carried out to verify the effectiveness of the above theory and approach.2.The compact capacitance design approach is proposed to further reduce the overall capacitance for the hybrid MMC under the steady-state over-modulation operations.Firstly,the hybridization ratio ranges of the hybrid MMC are compared for different operating requirements,which provides a guidance to select the hybridization ratio based on the practical requirements.Secondly,the influence of the hybridization ratio,modulation index,and operating range are considered to acquire the energy fluctuation range of the HBSM and FBSM capacitors,where an improved piecewise approach is proposed for the over-modulation operation.The difference between the capacitor energy fluctuations is revealed for the HBSM and FBSM capacitors,and a more compact capacitance design principle is put forward and verified by the simulations and experiments.3.The energy balancing control approach is put forward.To avoid the energy/voltage imbalance between the HBSM and FBSM capacitors under the emergent occasions with increasing modulation index,the method of the second order circulating current injection is adopted to rebuild the arm current of the hybrid MMC.By figuring out the characteristics of the critical voltage balancing state,the voltage feedback control is employed to acquire the reference amplitude of the second order circulating current.The acquired reference is the optimal value to realize the minimum power losses compared with the existing approaches.4.The hybrid MMC topology is improved based on the self-balancing branch.Two self-balancing branch structures are proposed aiming at the energy balancing realization and more compact capacitance design.These two self-balancing branches operate with the similar mechanism,which are equipped between the HBSM and FBSM capacitors to build an additional energy transferring channel and then reduce the energy deviations in the over-modulation operations.Besides,the overall capacitance of the hybrid MMC is reduced since the voltage ripple is suppressed by the self-balancing branches.According to different hybridization ratios,the self-balancing branches are arranged in different ways to establish self-balancing cells,and the component parameters of the self-balancing branches are given for different self-balancing cells.5.The performance of the hybrid MMC is evaluated on both converter level and system level.At the converter level,the different MMCs with the over-modulation capabilities are compared in aspects of the power density,power losses,operating range,and costs.At the equipment or system level,the grid-connected power electronic transformer for large-scale wind turbines is taken as an example to be evaluated,where the MMC acts as the interface to be directly connected to the medium voltage grid.The power electronic transformers based on different MMCs are compared to clarify that the over-modulation operation of the hybrid MMC is beneficial to enhance the power density and to reduce the cost of the power electronic transformer. |
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