Research On Control Strategy Of A New Power Electronic Transformer Based On MMC

Power electronic transformer is a new type of multi-functional intelligent transformer.It can not only realize voltage conversion and isolation,but also realize intelligent functions such as AC/DC grid interconnection,power flow control and power quality regulation.It will play an increasingly important role in distribution network.Among them,power electronic transformer based on modular multilevel converter(MMC)has high voltage DC and low voltage DC ports,which can effectively access renewable new energy,and also has the ability to regulate the power quality and control the power flow distribution of power grid.Therefore,the power electronic transformer based on MMC is studied in this paper.Firstly,a new topology of power electronic transformer based on MMC is studied in this thesis.The working principle of the topology is analyzed,the mathematical model of MMC and multiple active bridge converter(MAB)is established,and the characteristics that MAB has balanced input voltage and reduced output ripple is obtained in this paper.Secondly,the inner and outer control loop of MMC are designed in synchronous rotating coordinate system.PI control and passive control are used respectively in the current inner control.The control mode of MAB open loop and closed loop is given.The simulation results show that the MMC inner current loop with passive control has better performance,and the MAB has good characteristics of balancing capacitor voltage of sub-modules.Then,the mathematical model of the new power electronic transformer under unbalanced power grid is analyzed,and the relationship between double frequency fluctuating power and negative sequence modulation wave is established in positive sequence synchronous rotating coordinate system.Based on this,a power resonance compensation control strategy is proposed,and simulation is carried out for different control targets.Finally,hardware-in-the-loop simulation platform based on real time digital simulator(RTDS)is built,including controller design and software design.The feasibility of passive control for inner current loop under balanced grid conditions is verified on the test platform.