Research On Optimized Protection And Clearing Strategy Under Overhead Line Faults In DC Grids

With the development of power electronics technology,VSC-based DC grids have become an important direction for the development of future energy internet.The VSC-based DC grids not only have the advantages of high redundancy,flexible control and high reliability,but also can realize multi-power supply and multi-drop power receiving.The VSC-based DC grids provide an effective solution for solving the problem of large-scale renewable energy consumption.However,in the same time,the fault current of the DC grids develops faster compared to the AC system,so the fault has a wider range of influence,and the phenomenon of "local fault,network-out" is more likely to occur.In addition,as the mainstream form of power transmission in the future,the use of overhead lines will inevitably lead to an increase in the probability of transient fault of DC grids.Therefore,in order to ensure the safe and stable operation of the high-voltage and large-capacity DC grids in the future,DC line fault isolation and system recovery are one of the key issues to be solved urgently.In view of the above problems,the paper studies the optimized protection and clearing strategy under overhead line faults in DC grids.It analyzes the fault characteristics of DC grid lines,and establishes the equivalent circuit network model of bipolar VSC-based DC grids at fault time.A quantitative analysis method of short-circuit current transient characteristics under different short-circuit types and arbitrary fault locations for bipolar VSC-based DC grids is proposed,thus determining the breaking performance requirements of DC breakers.In addition,based on the protection technology route of DC breakers,the paper analyzes action sequence of DC line protection and DC breakers,and proposes two optimized coordination strategies of line protection and DC circuit breakers,so that the protection and DC circuit breakers can work in parallel mode,thereby reducing the total protection action time,reducing DC circuit breakers' fault current breaking stress and the risks of proximal converter station blocking.Finally,a novel reclosing strategy based on the step-by-step operation of transfer branch is proposed.Compared with the reclosing strategy based on the synchronous input of transfer branch,the adverse effects such as protection misoperation,insulation damage and damage of power electronic components can be eliminated during the reclosing process of DC circuit breakers.The communication equipment between DC circuit breakers can be omitted,so the proposed reclosing strategy has good economics.The research work in the paper provides a reference for fault line isolation and system recovery of overhead line DC grids,accelerating the isolation speed of DC line faults,reducing the breaking current stress of DC circuit breakers,and guaranting the reliable and continuous operation of DC grids,which is conducive to promoting the development and engineering application of DC grids.