Design Of Multi-terminal High Voltage Converter With DC Fault Self-cleaning Capability

A multi-port converter refers to a converter that can connect multiple ports and realize power conversion and energy transfer among the multiple ports.The multi-port converter reduces the number of devices used by the converter through the multiplexing of the devices to achieve the purpose of reducing the size and cost of the converter.Multi-port converters have broad application prospects in the fields of new energy power generation grid connection,microgrid,smart grid and flexible DC transmission technology.At present,the research on multi-port converters is mainly focused on low-voltage and small-capacity multi-port converters.The design of multi-port converters that can be applied to medium-high voltage and high-power occasions is still small,and the technical details are not perfect.The high-voltage high-power multi-port converter has less damping on the DC side,and the fault expansion speed is faster.When a short-circuit fault occurs on the DC side of the high-voltage multi-port converter,it will cause great damage to the system,so it is necessary to study the DC side.Multi-port high-voltage converter with fault self-clearing capability.This article takes the nineswitch group converter as the research object,and the specific work is as follows:(1)First introduced four basic sub-modules,including Half Bridge Sub-module(HBSM)without the ability to provide reverse voltage and Full Bridge Sub-module(Full Bridge Sub-module,FBSM),Clamp Double Sub-module(CDSM)and Improved Hybrid Sub-module(IHSM).Then,by analyzing the equivalent circuit of the DC side bipolar short circuit fault of the nine-switch group converter,the electrical quantity that affects the short-circuit current when the nine-switch group converter is short-circuited is obtained.Then based on the characteristics of the submodules,a hybrid nine-switch group converter based on HBSM and FBSM and an improved hybrid nine-switch group converter based on FBSM and CDSM(IHSM)are proposed.(2)The basic topology of the hybrid nine-switch group converter is introduced,the control principle and modulation method of the hybrid nine-switch group converter are proposed,the equivalent circuit of the bridge arm at the time of blocking is established,and the hybrid nineswitch group converter is studied The potential circulation path of the short-circuit current in the converter during blocking is analyzed,and the ratio of the number of mixed submodules in each bridge arm of the hybrid nine-switch group converter is analyzed.In addition,considering the economy,a unipolar hybrid nine-switch group converter is obtained by replacing the FBSM in the converter.The simulation verifies the feasibility of the topology of the hybrid nine-switch group converter and the ability of the converter to clear faults on the DC side.(3)The basic topology of the improved hybrid nine-switch group converter is introduced.Aiming at the characteristics of the improved hybrid nine-switch group converter topology,the matching ratio of the bridge arm neutron modules is obtained.The feasibility of the topology and the self-clearing ability of the DC side faults are verified through simulation.Finally,the nine-switch converter proposed in this paper is compared with the traditional two-port converter to illustrate the superiority of the hybrid nine-switch converter and the improved hybrid nineswitch converter.