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Modification in Topology and Control Method for Improving Performance of Modular Multilevel Converter in 2N+1 and N+1 Modulation

Razani, Ramin | 2017

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 50536 (05)
  4. University: Sharif University of Technology
  5. Department: Electrical Engineering
  6. Advisor(s): Parniani, Mostafa
  7. Abstract:
  8. In Modular Multilevel converter, it is necessary to attain different objectives simultaneously. One of the most effective control methods which can cope with this kind of systems is the Model Predictive Control (MPC). This method predicts the values of control objectives based on different feasible states and determines the optimal one using cost functions. The number of feasible states can increase when the number of switches increases in the converter. This makes the MPC unable to be applied in the MMCs with hundereds of submodules (SMs) in each arm because of its heavy computational burden. In this thesis, a modified model predictive control is proposed to reduce the computational burden. The proposed strategy controls each objective independently and also eliminates unnecessary use of cost functions. The SMs capacitor voltages are controled by a modified sorting algorithm that is capable to reduce the switching frequency. Superiority of the proposed strategy is verified by simulation results. The MMC can operate in N+1 and 2N+1 modulations. In the N+1 modulation, the number of output voltage levels is N+1 and in the 2N+1 modulation is 2N+1. When the MMC operates in the 2N+1 modulation, the circulating current will increase dramatically. This increase will prevent the converter to work properly. In this thesis, a Hybrid Modular Multilevel Converter is proposed containing half-bridge SM and at least one full-bridge SM with half of the half-bridge SM voltage rating in each arm. The proposed topology is capable of minimizing the circulating current when the converter operates in the 2N+1 modulation. Furthermore, the total converter loss is declined as compared to the usual MMC, and the proposed topology can proceed its operation even if various fault conditions occur in the SMs. Performance of the proposed Hybrid MMC is verified by several simulation and experimental case studies
  9. Keywords:
  10. Modular Multilevel Converters (MMC) ; Model Predictive Control ; Circulating Current Control ; Controlling Goals ; Computation Overhead ; 2N+1 Modulation

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