Research On DC/DC Converter And Its Fault Current Limiting Ability For Flexible DC Network

Compared with the AC grid,the flexible DC grid does not have problems such as reactive power compensation and frequency fluctuation.It can easily connect large-scale long-distance new energy sources and realize interconnection with traditional energy sources,which is the key direction for future power grid.However,the DC bus need to be connected by DC/DC converters.The DC/DC converter is the key equipment for constructing the DC grid.This paper focuses on the key technologies of DC/DC converter for flexible DC grids and its fault current limiting ability.Firstly,the background of flexible DC grid is expounded,and the purpose and significance of DC/DC converter for DC grid are derived.The research status of existing DC/DC converter and its fault current limiting capability is summarized.Describe the shortcomings of existing DC/DC converters and the importance and urgency of studying DC/DC converters for DC grids.The basic principle of modular multilevel converter(MMC)is introduced.The working principle of MMC DC/DC converter is expounded.Two control strategies of phase shift control and DC transformer control are analyzed in detail.Secondly,aiming at the MMC-H bridge DC/DC converter topology,the voltage and current expressions in the AC chain are derived,and the expressions and algorithms of transmission power and backflow power are given.Summarize the influencing factors of backflow power,consolidate the boundary of zero backflow power.Aiming at the problem of backflow power of MMC-H bridge type DC/DC converter based on single phase shift(SPS)control,a variable frequency optimization control strategy is proposed to suppress the backflow power.For the MMC-H bridge DC/DC converter based on extended phase shift(EPS)control,the primary and secondary backflow power of the AC chain are analyzed in detail,and the global backflow power minimum curve is proposed to achieve the minimum backflow power.The backflow power suppression strategy was verified by simulation and experiment.After that,the DC/DC converter based on virtual DC motor(VDCM)control is studied.The MMC DC/DC equivalent DC motor model is established,and the equivalent mechanical rotation equation and armature loop equation are obtained.The transfer function and control strategy of VDCM are constructed.It lays the foundation for the design of parameters of the MMC DC/DC converter.A small-signal model of DC/DC converter based on VDCM control is established.The stability of closed-loop control under different parameters is analyzed by Bode diagram.Using simulation to compare traditional PI control strategy with VDCM control strategy,simulation results verify the superiority of VDCM control strategy.Finally,the fault current limiting ability of the DC/DC converter is studied.The mechanism of the bipolar short circuit fault and the current limiting mode of the H-bridge are summarized.The fault current development mechanism of the MMC topology with bipolar short-circuit fault is analyzed in detail,and the fault current expression is obtained.For the MMC DC/DC converter,the control strategy under different modes of hybrid full-bridge-half-bridge submodule MMC(hybrid FHMMC)topology are studied.The current limiting strategies of hybrid FHMMC topologies when remote and proximal faults occur are summarized.The three-port DC/DC converter is built by simulation to verify the current limiting effect of the hybrid FHMMC topology when the remote and proximal bipolar short circuit fault occur respectively.Besides,a novel current source MMC topology is proposed.The fault current limiting function of short circuit fault and steady state normal operation are realized by switching the working mode.The fault current limiting capability of the new current source MMC topology is verified by simulation.