| Compared with traditional high voltage direct current transmission(HVDC),modular multi-level converter(MMC-HVDC)can achieve four quadrant independent power control without commutation failure.Compared with flexible direct current transmission(VSC-HVDC),the cascade mode of sub-modules avoids the problems of trigger consistency and short-circuit failure caused by voltage-connected series connection of power devices.MMC-HVDC is very suitable for interconnection between regional power grids and new energy grid-connected applications.It has broad DC application prospects and has great significance for the development of China's power grid.The MMC-HVDC with overhead lines faces the problem of fast and reliable removal of DC fault current.Converter reliability analysis is another important research point.In view of the above problems,this paper will analyze the hybrid MMC system from the point of view of fault control method and reliability analysis respectively.Firstly,based on the steady state operation principle of the hybrid MMC,the initial basic ratio of the sub-modules is analyzed to ensure sufficient DC voltage control capability of the converter.The constant current control ability and realization method of the hybrid converter station during the fault period are analyzed,and the source side fault current control ability is realized by reasonably switching the whole bridge sub-module.Aimed at the four-terminal flexible DC network composed of semi-fully hybrid converter stations,the rules and characteristics of post-fault current discharge are analyzed.The conclusion is that the near-end converter station is the main source of fault line current.It is feasible to clear the fault line current through the control cooperation of the converter station at the near end of the faulted line.Based on the analysis of fault characteristics of DC grid based on hybrid MMC,a control strategy for fault current clearance of hybrid converter station is proposed.The principle of controlling the fault line current by using the fault line near converter station is to output negative level.So that the fault current feeding from the far terminal end is attracted to the converter station and the fault line current is dynamically controlled to zero.The scheme can clear DC side faults without dependence on DC circuit breakers.In order to improve the reliability of the scheme,the coordination scheme considering the failure of mechanical switch breaking is also studied,which effectively protects the converter station safe operation.Then from classic probability's view,construct reliability analysis model given that the reliability of module keeps constant and independent.Firstly,considering the direct coupling of submodules,the reliability function of hybrid MMC is established,and the influence of submodule redundancy on the reliability of hybrid MMC is analyzed.Secondly,the correlation of submodules is decoupled by coordinate transformation to analyze the reliability of hybrid MMC.Thirdly,the configuration scheme of redundant submodules under the constraints of IGBT number and reliability degree is discussed by using the reliability calculation results of hybrid MMC decoupled by coordinate transformation.Finally,redundant submodules are allocated according to the principle of equal reliability increment rate to achieve the constraints of the highest reliability and the lowest cost. |
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