Research On Control Strategy And Fault Protection Technology Of Modular Multilevel DC Transformer

With the rapid development of distributed energy,energy storage technology and DC distribution network,DC transformer plays a very important role in the future power grid,so the research on DC transformer is of great significance.As the core equipment of DC distribution network,the Modular Multilevel DC transformer(MDCT)studied in this thesis can not only connect distributed generation,energy storage and various loads to DC distribution,but also realize power control and different levels of DC voltage conversion,which possesses great research significance.The specific contents of this thesis are as follows.(1)The operation principle of MDCT under phase shift control is analyzed and the model of transmission active power is established.The modulation strategies of medium voltage side and low voltage side are studied,the capacitor voltage balance strategy of half bridge sub module is analyzed,the dynamic equation of MDCT is established.The transmission active power characteristics of MDCT are studied.The accuracy of the dynamic equation and the transmission active power characteristics of MDCT are verified by simulation.(2)The closed-loop control strategy of MDCT is studied to realize the low-voltage side voltage regulation control mode.The large signal and small signal mathematical models of MDCT are established by using Fourier series summation method.The transfer function from the phase shift angle to the output voltage is obtained.The closed-loop controller is designed.The feed-forward controller is used to improve the disturbance rejection capability of the system.The open-loop simulation model is used to verify the accuracy of the large signal model;the disturbance injection method is used to verify the accuracy of the small signal model;the reliability of closed-loop controller is verified by closed-loop simulation,as well as the dynamic characteristics and the disturbance rejection capability of the system.(3)The extended phase shifting control strategy with optimized extended phase shifting angle is introduced in the low-voltage side of MDCT to optimize the effective value of inductor current.The loss characteristics of MDCT are analyzed.The ohmic loss is related to the RMS of inductor current.Aiming at the phenomenon that the effective value of inductor current is larger under the single phase shift control,the control strategy of optimizing the effective value of inductor current is studied,and the expression of the optimal extended phase shift angle is derived,so as to optimize the effective value of inductor current and improve the efficiency of DC transformer.The effectiveness and superiority of the optimal control strategy are verified by simulation.(4)The fault protection technology when the system has bipolar short circuit fault and half bridge sub module fault are studied.The short circuit current characteristics of the medium voltage side with bipolar short circuit fault are studied.The short circuit current limiting method is studied when the low voltage side has bipolar short circuit fault.The bypass and redundancy control technology of half bridge sub module of DC transformer is analyzed.The fault protection technologies are verified by simulation.(5)The hardware in the loop experiment control system based on RTDS is established,and the RTDS hardware in the loop experiment platform of MDCT is built.The correctness and reliability of the control strategy are verified by the experimental platform.