Research On The Key Control Technology Of Power Composite Modular Multilevel Solid-state Transformers

With continuous and deepening development of China's AC-DC hybrid distribution network,traditional power transformers can no longer meet the requirements of future power grids.As a new type of electrical energy conversion device,SST can not only achieve the same function of voltage conversion and electrical isolation as the traditional transformer,but also has the advantageous feature of connecting the AC-DC hybrid power grid,independent active and reactive power adjustment,etc.Therefore,SST is one of the key equipment of the future intelligent distribution network.At present,SST is mainly limited by the problems of multiple power conversion stages and thus low power density.Therefore,this thesis proposes a new type PI-MMSST topology with hybrid frequency modulation in light of the current research hotspots of SST and this topology can effectively reduce the number of power conversion stages and improve the power density.Compared with the existing hybrid frequency SST topology,PI-MMSST does not need to use additional filters,which has high practical significance.This thesis studies the working principle,modulation method and control method of this topology,and the main work is organized as follows:1)After review of the currently proposed hybrid frequency modulation SST topologies,and summarization of their characteristics,a new PI-MMSST topology is proposed in Chapter 2.Then,the structure,working principle,and the modulation signal superposition of the topology are analyzed in detail.2)Based on the PI-MMSST topology,a hybrid frequency modulation method is proposed in Chapter 3,namely the common carrier modulation method.In Chapter 3,the principle and modulation process of the method are explained in detail,and a double-layer sub-module capacitor voltage equalization control strategy is proposed for the hybrid frequency modulation method,and the simulation verification analysis is performed.The simulation results prove that the topology can effectively implement the hybrid frequency modulation function under the common carrier modulation method,and the capacitor voltage of the sub-module can be effectively balanced.3)To improve the common carrier modulation method,a carrier-phase-shift / square wave hybrid modulation method based on the PI-MMSST topology is proposed in Chapter 4,where the principle and modulation process of this method are explained in detail.Then this chapter analyzes the cause of the capacitor voltage offset of the sub-module under this method,proposes a four-layer sub-module capacitor voltage equalization control method,and elaborates the control process of the method and its basic principle,which is verified by simulation.The simulation results prove that the PI-MMSST topology can perfectly achieve the designed hybrid frequency modulation function under the carrier-phase-shift / square wave hybrid modulation,and the capacitor voltage of the sub-modules can be effectively balanced with the four-layer control being effective.