Research On The Fault Current Limiting Technology And Reclosing Strategy For The Flexible DC Grid

The flexible dc grid has outstanding advantages on the integration of renewable energy generation,power supply of passive network,interconnection of asynchronous grids,and so on,thus becoming one of the main trends in the future power system.However,different from the ac power system and conventional high voltage direct current(HVDC)transmission system,the propagation speed of the fault in flexible dc grid is much faster,with much larger overcurrent to the system,which will seriously damage the whole system.Therefore,how to improve the system fault survival and self-healing ability becomes one of the most important scientific problems for the flexible dc grid.In this paper,the dc fault current limiting technique,restarting and reclosing strategy,as well as the accurate fault location method are researched,to improve the power supply reliability and operation security of the flexible dc grid.Firstly,the quantitative index of dc fault current limiting requirement in flexible dc grid is analyzed,based on the dc fault transient characteristic.On this basis,the current-commutation-based dc fault current limiter(FCL)and self-adapt dc fault current limiting method are proposed respectively,for different voltage-level application.On the premise of enough fault current limiting capability,the proposed methods avoid the negative influence of fault current limiting reactor on flexible dc grid operation stability,as well as the negative influence on fault current clearing speed of the dc circuit breaker(DCCB).Further,for multi-port application,the improved topology of the dc FCL is designed,to significantly reduce the required number of the power electronic switches,thus to reduce the investment.For the proposed topologies,the experiment prototypes and test platform are built to verify the working principle and advantages of the proposed FCLs.Secondly,with increasing of the voltage level and system capacity,there is no doubt that the overhead line will be widely used in the future flexible HVDC grid,because of the economic advantage.Different from the dc cable,the fault probability,especially the nonpermanent fault probability,of the overhead line is much higher.Therefore,effective reclosing or restarting strategies must be configured to improve the power supply reliability.For the flexible HVDC transmission system based on the self-eliminating MMC,a novel restarting control strategy based on active injection concept is proposed.While for the flexible dc grid based on DCCB,a novel reclosing strategy which uses residual voltage characteristic to identify fault property(permanent or nonpermanent)is proposed.Compared with the traditional restarting/reclosing strategy,the proposed methods pre-indentify the fault property before restarting the system,thus can completely avoid secondary overcurrent damage to the system.In addition,for fault recovery of the dc grid,the accurate fault location is another important technology(especially for the permanent fault condition),because it can guarantee the quick investigation and removing of the fault point.In this paper,the applicability of the traditional natural-frequency based fault location method in flexible dc grid is analyzed.Then the improved method is proposed.In the proposed method,the second natural frequency is discriminated and extracted to calculate the fault distance,because the first natural frequency is very low and used data window is very short in flexible dc grid.In addition,the scheme to avoid influence of multiple signal classification(MUSIC)false frequency phenomenon is also designed.Compared with traditional method,the proposed one calculates fault distance based on a shorter data window,thus being more suitable for flexible dc grid.And the influence of the MUSIC false frequency phenomenon is avoided,further guaranteeing the fault location accuracy.