Two-Stage Differential Flatness Control For Solid State Transformer Considering Electric Vehicle And Renewable Energy Access

Due to the shortage of global fossil energy and the worsening environmental problem,wind and solar energy as the representative of renewable energy,are favored by countries because of their green,energy-saving and friendly advantages.On the one hand,countries are actively exploring the development and utilization of renewable energy.On the other hand,the electric vehicles,with new energy power as their main power source,have been vigorously developed by countries due to their advantages of low carbon emission and environmental friendliness.However,wind power and photovoltaic power generation are random and intermittent,direct grid connection of them may cause the power quality of the distribution network to decline,and the electric vehicles are randomly connected to the grid for charging,so the reliability of the power supply can not be guaranteed.The DC micro-grid provides an effective way for intermittent renewable energy and electric vehicles access.Renewable energy generation devices and electric vehicles are connected by DC form,which saves a lot of converter link and can improve energy efficiency and economy.The function of solid state transformer is outstanding,can not only transform voltage,electrical isolation,energy transform,can also isolate fault,compensate reactive power,improve power quality and provide AC/DC port and so on.It can be used as power router to connect the DC micro-mesh and grid organically,realizing energy coordination management.This dissertation studied the control of solid state transformer when electric vehicles and renewable energy generation devices were connected to the DC micro-grid,the details were as follows:(1)Analyze the working principle of solid state transformer,introduce three kinds of commonly used medium and high voltage solid state transformer topologies,and compare their advantages and disadvantages.On the base of the development of DC micro-grid,a novel DC micro-grid architecture based on solid state transformer was proposed.And a 3-stage solid state transformer topology was designed in the thesis.(2)The mathematical models of three stages of solid state transformer were established,and the control strategy of each stage was given.The voltage and current double closed-loop decoupling control was used in the input stage to realize the stability of the primary DC bus voltage.The isolation stage was controlled by the open loop control.The output stage used double-loop control to realize the stable output of AC voltage.Based on the particle swarm optimization algorithm,the charging strategy of electric vehicles was optimized for DC bus voltage stability.The system model was set up on PSCAD/EMTDC platform,and the simulation was carried out,which showed the proposed novel DC micro-grid architecture based on solid state transformer had strong load adaptability.(3)Double active bridge topology was adopted in the isolation stage for making full use of solid state transformer’s characteristic of the two-way flow of energy.The mathematical models of voltage source converter and double active bridge were established.It was proved theoretically that the primary DC bus voltage of solid state transformer was not only affected by the disturbance of voltage and current on the network side,but also related to phase shift of isolation stage and the secondary DC bus voltage.And then,a two-stage differential flatness based control for solid state transformer was proposed.Finally,simulation was carried out on PSCAD/EMTDC platform to verify the effectiveness of the proposed control strategy under five working conditions,such as orderly access of electric vehicles,access of electric vehicles of different scales,access of renewable energy of different scales,short circuit on the network side and load asymmetry,which showed the proposed two-stage control strategy was superior to d-q decoupling control,and could achieve better primary DC bus voltage stability.