Development Of Solid State Transformer Rectifier Stage And Inverter Stage System

Solid state transformer is a kind of intelligent transformer which is on thebasis of the combination of transformer technology and power electronictechnology. Voltage transformation and energy transfer can be implemented bypower electronic conversion. Except traditional transformer, but can realize theflexible control for the input current and output voltage can be controlledflexiblly, and the power factor can be regulated freely. In addition, distributedgeneration system can be accessed to grid reliably by the interfaces of AC/DCwhich are provided by the solid state transformer. Strong compatibility, highpower density and intelligent are the characteristics of solid state transformer.With all the advantages, the solid state transformer can be applied forconstruction of the power grid, which not only can improve the controllability ofthe power system, enhance the quality of electric energy, but also can realize thegrid-connection of new energy power generation system, which is the assistancefor the construction and development of smart grid. As a new power device,theoretical and practical researches for solid state transformer are not perfect.The paper takes the rectifier stage and inverter stage system of the solid statetransformer which includes the DC source as the research object. Based on thecharacteristics of solid state transformer, how to ensure the safe and economicoperation of power grid and meet users’ demands for power quality arediscussed.The main research contents are as follows:On the basis of analysis of domestic and foreign research dynamic for solidstate transformer, the characteristics of solid state transformer are introduced,two kinds of schemes of solid state transformer are compared. The topology ofsolid state transformer is selected by the requirement of power grid and users.The equivalent model of solid transformer is established. Solid state transformer is designed in detail on the basis of topologystructure of solid state transformer rectifier stage system. Mathematical modelsin different coordinates are established. A dual loop control strategy with DCvoltage outer loop and current inner loop is developed at a fixed switchingfrequency. Through current loop decoupling, this model is simplified and doubleclosed-loop controller is designed. Load current feedforward control scheme ispresented to improve the ability of disturbance; the principle of three-phasesoftware PLL is briefly introduced, and its control parameters are designed.Simulation and experimental results show that: Grid side current of rectifierstage system has a good sinusoidal properties, and its phase synchronized withgrid voltage;Output side voltage is stable with strong anti disturbanceperformance. These simulation and experiment results demonstrate that thesystem scheme is reasonable and feasible.State space model and small signal model of solid state transformer ininverter system are established, and its equivalent circuit model is obtained;Double closed loop control strategy with voltage outer loop and current innerloop is designed. Through the decoupling of inner current loop and outer voltageloop, circuit model is simplified and transfer function from control variables tooutput is derived. And controller of parameters is designed. Open loop anddouble loop of inverter system is simulated and experimented. Simulation andexperiment results show: in double loop control, the load adjusting ability of theinverter system is good and the output voltage is stable and reliable. The antidisturbance performance is good.The hardware platform is set up with the TMS320F28335DSP as thecontrol core. The control circuit mainly includes signal detection circuit,auxiliary power circuit and fault protection circuit. The parameters of the maincircuit are selected, including rectifier stage input inductor, output side capacitorand LC of inverter output filter circuit. The system debugging results show thatthe circuit designed is reasonable and the system platform is stable.According to the control requirement of the system, the software program isdesigned under the environment of CCS5.5by using the idea of modular design. The main function programs mainly include AD conversion, data processing,PLL, system protection, double closed loop control, generation of SPWM waveand other function program. Debugging results show that the software programis reasonable, its function is normal and it can satisfy the requirements ofsystem.