DC Fault Analysis And Parameter Configuration Of Flexible HVDC Transmission System

Flexible DC power grid based on MMC has advantages of high modularization,low line loss and new energy access.Flexible DC power grid is one of key technologies for the rapid development of power grid in the future.However,the flexible DC power grid contains many power electronic devices.The electromagnetic interaction between AC and DC power grids is formed through MMC,which easily produces overvoltage problem and puts forward higher requirements on the insulation.In addition,the low inertia and weak damping characteristics of the flexible DC power grid lead to the rapid growth of fault current and serious overcurrent after the DC side fault.Basing the above elements,on the one hand,it is important to analyze the fault characteristics of the DC side of the flexible HVDC transmission system based on MMC.On the other hand,it is necessary to propose effective schemes for the configuration of voltage limiting devices and current limiting devices.Therefore,this dissertation focuses on the dc fault characteristics analysis and configuration of the protection devices in the flexible DC power grid.Firstly,the background of flexible DC power grid is described,the current research status of fault characteristics analysis and configuration of protection devices in flexible DC power grid are summarized.On the basis of introducing the working principle and simplified model of MMC,the fault characteristics of polar-to-ground and polar-to-polar are studyed,we can find that polar-to-ground fault main brings overvoltage problems while polar-to-polar brings overcurrent problems,which provides a basis for the following study of overvoltage and overcurrent.Then,the effect of current limiting reactor and fault current limiter on fault current under polar-to-polar fault are analyzed,and a fault current calculation method that considers the act of inductive fault current limiter is proposed.Six-terminal dc grid is set up in PSCAD/EMTDC software,the comparison of calculated value and simulation proves the validity of the proposed fault current calculation method,which provides support for the optimal configuration of current limiting devices.Secondly,in view of the dc side short circuit fault with serious overvoltage:polar-to-ground fault,the mechanism of dc side polar-to-ground overvoltage for typical grounding mode is deeply studied,and then the conclusion is extended to other grounding modes.Later,the correctness of the overvoltage mechanism is verified through a two-terminal dc grid in PSCAD/EMTDC.On this basis,the influence of fault resistance,grounding modes and pole line distance on overvoltage is analyzed emphatically.In addition,in order to suppress fault overvoltage reliably and reduce the configuration pressure of system insulation devices,combining with the principle of lightning arrester space configuration and parameter selection,a lightning arrester space configuration scheme is proposed.The effectiveness of the lightning arrester space configuration scheme is verified through two-terminal dc grid.Finally,in case of polar-to-polar fault,the spatial configuration optimization problem of multiple current limiting devices in complex multi-terminal power grid is studied.Based on the coupling degree between MMC and dc grid,the area division principle suitable for complex multi-terminal dc grid is proposed.Then,according to the requirements of current limiting in each region,combining with the current limiting mechanism and configuration principles of critical source and grid devices,the typical configuration schemes of current limiting in each region are proposed.Based on this,the parameters of current limiting devices in different regions are optimized hierarchically and the overall optimal efficiency is improved consequently.In addition,according to the influence law of device parameters on fault current and the error range of inductance parameters,the selection principle of discrete parameters of current limiting devices is proposed.In order to normalize and standardize the parameter of devices,a parameter optimization method of current limiting devices based on discrete parameters is proposed.After that,the effectiveness of the proposed configuration scheme of current limiting devices,hierarchical optimization method of current limiting devices in source and grid side and discrete parameters optimization scheme of current limiting devices are verified through a six-terminal dc grid.