Research On Control Strategy For Modular Multilevel Converter Based Power Electronics Transformer Under Unbalanced Circumstances

With the rise of the concept of smart grid and popularization of distributed generation technology, due to its characteristics of high energy density, well flexibility and controllability, power electronic transformer (PET) is playing an increasingly important role in the power system. At present, three-stage PET is the most commonly used topology, consisting of input stage, isolation stage and output stage. Meanwhile, using the modular multilevel converter (MMC) with good scalability as its input stage, is beneficial to broadening applications of the PET. Thus, researches on control strategies of MMC based PET under varieties of operating conditions, are of great significance to the development and stability of the power system.Mathematical models of input stage, isolation stage and output stage are established on the basis of analyzing the work principle of MMC based PET firstly. Meanwhile, different characteristics of dual-modulation carrier phase-shifted pulse width modulation and neareast level modulation are compared to determine the most suitable modulation strategy for MMC based PET. Furthermore, corresponding closed-loop control strategies for each stage are also proposed, considering three phases stay symmetric and asymmetric.On the one hand, an overall control strategy for the input stage is applied while three phases are symmetric, assembling sub-module voltage balance control, current control and negative-sequence 2nd harmonic circulting current supperession control. To satisfy the parallel structure of the isolation stage, a controller which equalizes the input voltages of different DABs is designed to achieve the goal of power balance among different DABs. And a control strategy has been put forward to controll output voltages of the output three-phase inverters. On the other hand, after comparing two current control strategies with different aims which are supperessing negative-sequence AC current and DC voltage fluctuations, a multiplex control strategy is raised under the circumstance where the three phases are asymmetric, including sub-module voltage balance control, current control and circulting current supperession control for all sequences. And two controll targets are realized simultaneously due to this control strategy, one is keeping the three-phase AC currents stay symmetric, and the other is maintaining the DC voltage steady.In the last part, a MMC based PET model is established in the Matlab/Simulink platform. The effectiveness of the proposed controlling methods of three-phase AC inputs and outputs being symmetric or asymmetric is verified through simulation results, covering different operating conditions, whether AC inputs are symmetric or not.