Research On Reflux Power And Fault Tolerant Control Of Modular Multilevel DC Transformer

Energy Internet can effectively solve the intermittent and uncertain problems caused by new energy power generation,and has been the development trend of future AC and DC distribution network.As an important support link of Energy Internet,the DC distribution network has the advantages of high reliability and low cost,and its key equipment DC transformer has also received extensive attention.In addition,DC transformers can also be used in Electric Energy Router as an intermediate link to provide voltage conversion,energy transfer and electrical isolation functions.Therefore,it is necessary to conduct in-depth research on DC transformers to promote the development of the Energy Internet.Modular multilevel DC transformer(MMDCT)has obvious advantages in the medium and high voltage field,which is easy to expand the voltage level and has the function of fault ride-through.However,MMDCT has the problem of excessive reflux power,which may reduce the power transmission efficiency,cause overcurrent of switching devices,and affect the stable operation of MMDCT;In addition,the sub-module failure will cause the MMDCT to operate asymmetrically and generate a larger circulation,which will also affect the stable operation of the MMDCT.Therefore,this paper has carried out in-depth research on the problem of reflux power optimization control and submodule fault tolerance control of MMDCT.First,the mechanism of the MMDCT based on quasi-square wave modulation is analyzed,and an approximately trapezoidal wave power model calculation method is proposed,which can provide a theoretical and model basis for follow-up research.By comparing with the actual power model,the accuracy and simplicity of the approximate calculation method are explained.A bridge arm voltage balance control method and a power-based load current feedforward control method are proposed.Simulations and experiment verify the correctness of the model and the feasibility of the basic control method.Next,to solve the problem of large reflux power in MMDCT under single phase shift control,a reflux power optimization control based on extended phase shift control is proposed.The optimized working area of MMDCT reflux power under extended phase shift control is obtained by using the approximate power model algorithm.It can be seen from the simulation that when the extended phase shift angle is in the optimized region,the reflux power of MMDCT is zero.Furthermore,for the redundant fault-tolerant control after MMDCT sub-module failure,a quasi-square wave-based sub-module alternate hot standby control strategy is proposed.The cause of the sub-module failure and its impact on the system are analyzed.Using the sequencing voltage equalization control method,the sub-modules with the highest voltage in each cycle are used as hot redundant sub-modules,which has no need to set up additional charging equipment and can reduce the investment time of spare sub-modules.It can be seen from the simulation that when the sub-module fails,the alternate hot-standby redundant fault-tolerant method reduces the charging time by 3.5s compared with the traditional hot-standby method.Finally,for the fault-tolerant control of MMDCT without redundant sub-modules,a control strategy of phase shifting on the high-voltage side is proposed.The asymmetric characteristics of the system after sub-module failure and the energy flow of the bridge arm caused by the phase shift control on the high-voltage side are analyzed.By controlling the phase shift angle on the high-voltage side,the circulating current of the bridge arm caused by asymmetry can be reduced.It can be seen from the simulation that,compared with other nonredundant fault-tolerant control methods,the high-voltage side phase shift control reduces the circulating current and inrush current of the faulty bridge arm,and can improve the utilization rate of the sub-module.