Studies On Three-Phase Solid State Transformer Based On SiC MOSFETs

Under the background of the energy internet,Solid state transformer(SST),as one of the most important equipment for smart grid application,will play a more important role in the future power system.It can realize complicated requirements of power system,environmentally friendly,change/control the customer voltage levels,achieve a unity power factor operation,monitor energy usage and record information,provide DC power bus,etc.Some issues existed in the three-phase cascaded modular SST,such as the complexity of the topology,difficulty in controlling,and it is needed to solve voltage and power unbalance of each H-bridge module.A simple three-phase SST topology based on high-voltage silicon carbide(SiC)power semiconductor device is adopted in this paper.The simple topology revitalizes the traditional topology and has a great practical prospect,the topology and control are relatively simple.Firstly,this paper introduces the advantages of SST in contrast to the traditional transformer,then summarizes the research status and the working principle of SST from three aspects,including single-phase SST,three-phase SST and SST based on SiC devices.This paper also introduces the advantages of SiC-SST with the development of silicon carbide power electronic devices.Secondly,the three stage structure of SiC-SST system is analyzed in this paper.Its circuit is analyzed and the mathematical model is established,including the front-end rectifier and DAB converter.The circuit of front-end rectifier is analyzed,then mathematical models of front-end rectifier are established in three-phase stationary abc coordinate and the synchronous rotating dq coordinate,this two kinds of model transformation matrix is also given.The working process of the DAB converter is analyzed under six time interval.Then the expression of equivalent leakage inductance of the current in each interval is given and the process of energy transfer is analyzed.Next,the control strategy of every stage of SiC-SST is analyzed in this paper.Double closed-loop control is adopted in the front-end rectifier stage,including inner current-loop and outer voltage-loop.The fast current response of the direct current control strategy is used in inner current-loop.Single-phase-shift(SPS)control is used in DAB converter,its first-order small-signal equivalent model and the transfer function expression are derived.Then voltage following closed-loop control of DAB converter is accomplished.And then a control strategy under unbalanced grid voltages is analyzed.Simulation results in PSIM software have verified the effectiveness of the two control strategies.The drive circuit of SiC MOSFET is different from Si MOSFET.It is bad for SiC devices without effective or rapid protection.The characteristics and circuit model of SiC MOSFET are analyzed in this paper.The research status of drive and overcurrent protection circuit for SiC MOSFET is introduced.Then this paper presents a new type of high voltage SiC MOSFET drive and overcurrent protection circuit on the basis of the the existing circuit.The circuit meets the special requirements of the driving voltage of the SiC MOSFET and the SiC MOSFET will be protected quickly under the over current situation.Simulation and experiment results have verified the effectiveness of the circuit.Finally,the experiment prototype based on SiC devices(C2M0080120D)was built on the basis of the theory of SiC-SST system and the simulation results,including hardware and software design of the system.The front-end rectifier was verified by experiment results.