| Flexible DC power grid has the advantages of low transmission line loss,easy access to distributed energy and interconnection of asynchronous AC power grid.It is one of the key technologies to build the future power grid.However,due to the characteristics of low inertia and weak damping of flexible DC grid based on modular multilevel converter,the DC fault current develops rapidly and spreads widely.Moreover,MMC with half bridge sub-module lacks dc fault current blocking capability.Therefore,it is of great practical significance to study the fault characteristics of flexible DC power grid and put forward appropriate DC fault treatment technology for promoting the application of flexible DC power transmission system.This paper focuses on the DC fault handling technology of flexible DC power grid.Firstly,the research background of flexible DC transmission technology is introduced,and the research status of DC fault handling technology of flexible DC grid at home and abroad is summarized.Based on the introduction of the structure and working principle of MMC,the pole-to-ground fault characteristics and bipolar short-circuit fault characteristics of half bridge MMC are analyzed in detail,and the DC fault current expression is derived.In addition,the bipolar short-circuit fault characteristics of MMC based on full bridge sub-module with blocking current limiting scheme and the number of full bridge sub-modules are analyzed.Moreover,the DC fault characteristics of full bridge MMC with non-blocking fault current limiting scheme are analyzed,and the DC fault current expressions under different step-down degrees are derived.A simulation model is built to verify the correctness of the theoretical analysis.The results show that the derived DC fault current expression can accurately reflect the development trend of DC fault current,which provides a reference for the capacity configuration of DC circuit breaker.Secondly,an enhanced MMC topology with flexible blocking capability of DC fault is proposed,which can quickly block DC fault current while considering voltage stress.Based on the detailed description of the composition structure and control strategy of DC fault blocking,the process and sequence of DC fault blocking of the proposed topology are analyzed,and the expression of reactor voltage stress during DC fault blocking is deduced.Based on this,the parameters of key equipment are designed,and the proposed topology is compared with the typical MMC topology scheme with DC fault blocking capability.Finally,the simulation model is built by MATLAB/SIMULINK platform to verify the effectiveness of the proposed scheme.Finally,considering the requirement of continuous operation for MMC during dc fault,the composition and shortcomings of typical DC fault ride through control strategy of hybrid MMC are analyzed based on the topology and working principle of hybrid MMC.Based on this,a hybrid MMC transient-state and steady-state cooperative control strategy based on AC-DC decomposition mapping is proposed.The proposed control strategy suppresses dc voltage fluctuation caused by power mutation during normal operation of MMC through ac control block,and utilizes DC control block to suppress dc fault current.The AC control block,DC control block and cooperative control method of the proposed control strategy are introduced in detail,and the stability of DC control loop and the influence of its parameters on DC fault current are analyzed.On this basis,two optimization schemes,parameter switching control and parameter adaptive control,are proposed to further optimize the DC control loop of the proposed control strategy to enhance its DC fault current suppression capability.The simulation model is established by PSCAD/EMTDC to verify the effectiveness of the proposed control strategy in DC fault ride through capability and DC voltage fluctuation suppression capability. |
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