Analysis Of Multi-Device Current-Limiting Contribution And Adaptive Fault Current Suppression In Flexible DC Grids

Flexible DC grid owns fast decoupling power control ability.It can realize multipower supply,multi regional power grid connection and complementary optimal energy resources.Flexible DC grid is an effective technical means to solve the problem of grid connection and consumption of high percentage of renewable energy.The flexible DC grid is fundamentally different from the traditional AC grid in terms of fault occurrence,fault evolution,and fault clearance.The fault current of flexible DC grid has no over-zero point and rises rapidly,and the over-current capability of electronic equipment is poor.Without effective fault control,the fault current will seriously threaten the safe and stable flexible DC grid and even AC system.So,an effective DC fault current suppression and clearance strategy in DC grid is especially urgent.This paper,with the support of the National Key Research and Development Program(2018YFB0904600),focus on the analysis of fault current limiting contribution,parameter coordination and action sequence coordination of flexible DC grids with multi-type current limiting devices.This paper establishes the analysis model of multi-device fault current limit contribution of flexible DC converter stations and multi-terminal flexible DC grids,analyzes the influence of the parameters and action sequence of the fault limiting impedance on the contribution of current limiting,and proposes a parameter optimization scheme and an action sequence coordination method.The main work of the paper is as follows.(1)It is difficult to quantify the current limiting effect of different current limiting devices,this paper proposes the definition and calculation of the contribution of each current-limiting device to the fault current amplitude and energy suppression in flexible DC grids.Firstly,the flux-current equation of a flexible DC converter station with currentlimiting devices is established based on the action sequence of the current-limiting reactor(CLR),fault current limiter(FCL),DC circuit breaker(DCCB)and their fault-equivalent model.Analyze the coupling relationship and current limiting mechanism of multi-type devices to suppress fault currents,and define the cfault current amplitude limiting ontribution to quantify the suppression effect of different current limiting devices.Then derive the MMC transfer energy function under DC fault,and the definition of the fault current energy suppression contribution is proposed.The coupling relationship of each device in the DC grid is analyzed,and the method that decomposing the DC grid into the superposition of multi single-terminal DC systems is proposed by using λ-i magnetic flux equation,and the fault current amplitude and energy limiting contribution analysis of the DC grid is realized.(2)It’s difficult to coordinate parameters and action timing between multi-fault current-limiting devices.To analysis of the impact of changes in the parameters and timing sequence of current-limiting devices,the fault current limiting contribution is used as an indicator.Firstly,the fault current-limiting contribution analysis is combined with the state-space equations to propose a simplified calculation method for fault currents in flexible DC grids by only updating the current-limiting contribution matrix.The method can accommodate different current-limiting device topologies and dynamic switching within the device without changing the state-space equations of the grid.On this basis,the maximum contribution of the current-limiting device is taken as a constraint to analyze the sensitivity of the parameters of the current-limiting device,and the optimal combination of impedance parameters of various current-limiting devices is obtained.Then,analyze the influence of the action sequence of the current limiting devices on the contribution of current limiting,and obtain the range of action timing and change law under different parameter combinations.(3)To realize the optimal configuration of multi-current-limiting equipment parameters,the optimization method of multi-current-limiting equipment parameters for flexible DC grids based on the cost sharing of current-limiting contribution is proposed.Through the fault current limiting contribution analysis,the cooperation requirements of FCL and CLR to limit the fault current amplitude and energy are studied;the grid equivalence method based on the most severe fault point of DC grid is proposed,which able to consider the corresponding most severe operating conditions,the FCL equivalence method based on parameter sensitivity analysis is proposed too;the costs of FCL,CLR and DCCB are analyzed,and a multi-type fault current limiting device parameter optimization method is proposed to optimize the overall economy of fault current suppression and clearance devices;Four scenarios of DC grid that equivalent arrangement CLR,non-equivalent arrangement CLR,CLR and high cost FCL coordination,CLR and low cost coordination FCL are optimized and analyzed.(4)To address the problem that the existing current limiting strategy fails to dynamically operate according to the fault conditions,resulting in low equipment utilization efficiency,an adaptive current limiting strategy for flexible DC grids based on fault severity assessment is proposed.Based on the DC grid fault current limiting mechanism,the relationship between transition resistance and fault current is analyzed;on this basis,a fault severity assessment method for DC grid based on single-end transient information is proposed;based on the fault current limiting contribution analysis,a fast calculation method for FCL action sequence under the coupling between multi-terminal fault current limiting devices is proposed,which not relying on inter-station communication;an adaptive suppression strategy for DC grid fault current,which integrated grid main protection,is proposed.The strategy adapts the fault current limiting action sequence to the fault severity,improves the fault current limiting effect and extends the service life of FCL.In summary,this paper obtains a method to coordinate multiple current-limiting devices of flexible DC networks in terms of parameters and action sequence.Based on the simulation verification of the Zhangbei flexible DC grid,it is known that the multidevice parameter optimization method can reduce the fault current amplitude by 41%compared to the equivalent CLR scheme with the same economy;The fault adaptive current limiting strategy can further reduce the fault current amplitude by 10%.