Research On Fault Isolation Strategy Of MMC-HVDC System Based On Hybrid Topology

China is in a critical period of diversification of energy types and innovation and upgrading of energy industry structure.The flexible DC transmission technology based on modular multilevel converter(MMC)is widely used in the construction of modern new smart grid with green new energy as the main body,with broad development prospects.However,the demand for system operation with reduced DC voltage under special operating conditions cannot be satisfied by traditional MMC converters based on half bridge submodules(HB-MMC).In addition,the ability to block short circuit current paths is not possessed by HB-MMC.The stability of system power supply is reduced due to the energy impact generated by alternating current and DC transient electrical quantities in case of DC fault.Therefore,the research on the half-bridge and full-bridge sub-module hybrid MMC fault isolation technology with DC fault traversing and voltage reduction operation capability,is regarded as the key to improve the fault self-healing capability and operation flexibility of the flexible and direct power grid.Focusing on the MMC-HVDC flexible DC transmission system with half-bridge and full-bridge hybrid topology,the research is carried out from the aspects of topology ratio design,fault current suppression,AC feed electrical isolation,etc.The ±500k V double ended half-full bridge hybrid MMC simulation model is built to verify the correctness of the scheme.The main research work is as follows:(1)Half-bridge and full-bridge hybrid modular multilevel converter is taken as the research object.Based on the short circuit fault mechanism of HVDC transmission line,the equivalent circuit mathematical model and the step-down operation principle of hybrid MMC topology are analyzed.Aiming at the influence of AC feed current on two-side system during DC fault,the model parameter calculation method considering the interaction of AC and DC fault transient electrical quantities is studied.The above research provides theoretical support for the corresponding full-bridge sub-module proportion design scheme and fault isolation and suppression strategy.(2)Taking DC fault self-clearing and half-bridge sub-module capacitor voltage balance as constraints,combined with the operation characteristics of hybrid MMC with high AC modulation ratio.The proportion range of the whole bridge sub-module meeting the constraint is obtained,and the proportion design scheme of the whole bridge sub-module is proposed.The clearing speed of DC fault current and the fluctuation range of sub-module capacitor voltage are optimized,and the influence of high AC modulation ratio on the design scheme is analyzed.The lightweight design requirements of hybrid MMC are further met.(3)In order to realize controllable suppression and adaptive control of DC short-circuit current,the influence of transient characteristics of AC feed electrical quantity is fully analyzed in this study.The current limiting coefficient,which is combined with DC control terminal transmission line current is introduced.Short-circuit current can be inhibited by adjusting the number of sub-modules.The AC/DC fault interconnection channel can be blocked through the application of AC isolation module.The harm of AC fault power feed to MMC converter and DC system is weakened.(4)By introducing the capacitance-resistance current transfer branch and the line resistance energy-dissipation branch at the DC terminal,the fault transient electrical quantity under the isolation and current-limiting dual-mode control is accelerated to be consumed.Based on the fault current transfer path and component energy consumption characteristics,the design scheme of branch topology and component parameters is proposed.The influence of double-branch switching on system fault transient characteristics is fully studied to accelerate the internal consumption of DC fault energy.The isolation effect of AC and DC electrical quantities in dual mode is further consolidated.The operation stability of hybrid MMC flexible DC transmission system and the ability of DC fault self-processing are improved through the research.