Circulating Current Control Strategy For Modular Multilevel Converter

With the development of power electronic technology,multilevel converter attracts more and more attention in the field of high-voltage and large-capacity.As a new multilevel topology,modular multilevel converter(MMC)has a good research prospect in the field of high-voltage and large-capacity,with the following advantages of high degree of modularization and good scalability.Because of its own particular structure,there are many problems to be solved in the aspects of circulating current suppression and the balance of submodule capacitor voltage.In this thesis,research on circulating current and its suppressing strategy of MMC is conducted.The topology and operation principle of MMC are introduced in detail.According to the output voltage characteristic of submodule and arm port,the equivalent mathematical models of the DC-side and AC-side for MMC are derived.According to the output electrical parameters and the modulation function of MMC,the mechanism of MMC circulating current generation is analyzed and mathematically deduced.The components of the circulating current and the coupling relationship between them are introduced by establishing a steady-state mathematical model of circulating current.In order to suppress the circulating current effectively,the suppressing strategy based on proportional-integral control is studied and verified by simulation.In order to achieve better control effects,this thesis presents a suppressing strategy based on sliding mode variable structure control which can improve rapidity and robustness of system.The method of exact feedback linearization is used to solve the problem of coupling phenomenon in the model of MMC circulating current.And introduce the sliding mode variable structure controller to eliminate the circulating component.Theoretical analysis and simulation show that the strategy has a better control effect.When there is a voltage imbalance fault in the grid,a direct power compensation strategy which did not require the separation of positive and negative sequence currents is given to implement two different control targets under unbalanced grid voltage conditions.The second harmonic circulating current can be decomposed into zero,positive and negative sequence components under the faulty condition.The suppressing strategy based on vector proportional integral control is introduced in this thesis.And the method for canceling controller's zero point and controlled object's pole is adopted to adjust parameters.Finally,the effectiveness of this method is verified by simulation.Finally,the software and hardware of RTDS(Real Time Digital Simulator)experimental platform are introduced.Combining the characteristic of its small step environment,build an MMC model based on RTDS platform.The results verify the correctness and effectiveness of the relevant control strategies described in this thesis.