Research On Fault Current Calculation And Coordination Method Of Current Limiting Devices In HVDC Grid

The strong coupling relationships between various power electronic devices present new challenges in the analysis and coordinated limitation of faults in HVDC grids.In existing HVDC grid projects,apart from passive current limit reactors(CLR)for current limiting,there is a lack of effective means for fault current limitation.The measures for short-circuit current limitation are relatively simple,and the effectiveness of current limitation is limited.Especially,there is a lack of research on the coordinated operation of different methods for short-circuit current limitation.To address this obstacle,it is necessary to analyze the characteristics of short-circuit faults in HVDC grids and study the coordination and cooperation of multiple current limitation devices to improve the effectiveness of fault limitation.Regarding the analysis and coordinated limitation of faults in HVDC grids,this paper first establishes a fault equivalent model for the Modular Multilevel Converter(MMC).The discharge characteristics of the internal submodule capacitors of the MMC during faults are studied.Subsequently,an efficient analytical method for short-circuit faults in HVDC grids is proposed,which provides a computational foundation for subsequent research.Then,the fault limitation characteristics of multiple current limitation devices in HVDC grids are studied,and the necessity of coordinated parameter allocation for these devices is analyzed.Finally,for complex HVDC grids,coordination and cooperation methods for multiple current limitation devices are studied from both spatial and temporal dimensions.The main contents of the paper are as follows:(1)Mechanism studies of short-circuit faults on the DC side of HVDC transmission systems.The fault equivalent model of the MMC often uses series equivalent RLC parameters,which neglects the current injected into the AC system.As a result,it fails to reflect the changes in equivalent capacitance caused by submodule capacitor switching during faults,and cannot accurately analyze the fault characteristics inside the MMC.To address this issue,the submodule capacitors of the MMC are equivalent to time-varying capacitors.A fault equivalent model of the MMC that considers submodule capacitor switching and AC current injection is established and solved.For complex HVDC grids,a fault characteristic analysis method based on transient energy flow is proposed to qualitatively analyze the evolution of DC faults based on the damping trends of transient energy flows in different components.According to the distribution patterns of transient energy flow,a transient energy limitation strategy considering the voltage fluctuation range of submodule capacitors is proposed to reduce the energy release of submodule capacitors during faults and thereby decrease the fault current.(2)Efficient analytical methods for short-circuit faults in HVDC grids:The analysis of short-circuit faults in HVDC grids is complex and lacks intuitive interpretation.To address this issue,an efficient calculation method for short-circuit currents on the DC side,suitable for complex HVDC grids,is proposed.This method significantly improves the efficiency of equation formation and solving bysimplifying the fault equivalent model of the HVDC grid while ensuring calculation accuracy.The fault state equation is solved through an approximate matrix exponential function,and the analytic solution is presented as a response of various fault state variables to the initial values of fault state quantities and steady-state current excitations at remote converter stations.The sensitivity of short-circuit currents to different network parameters is quantitatively analyzed.(3)Study on the coordinated cooperation of multiple current limitation devices in complex HVDC grids.The existing configuration scheme of DC reactors in HVDC grid projects does not consider the utilization of current limitation parameters and system dynamic performance.To address this issue,a spatial configuration method for DC reactors is proposed,which reduces the overall reactance value and speeds up fault recovery while keeping the opening capacity of DC circuit breakers constant.A parameter optimization model for the coordinated operation of multiple current limitation devices is established and solved,with the objective of achieving the best overall current limitation effect while minimizing the sum of equivalent impedances of active and passive current limitation devices.The breaking capacity of the DC circuit breaker,MMC blocking threshold,and DC voltage at the converter station exit are considered as constraints.In the existing HVDC grids,DC reactors are uniformly arranged at the head and end of DC lines,without considering the utilization rate of current limiting parameters and the dynamic performance of the system.In this regard,the spatial configuration method of DC reactor is put forward,which can reduce the breaking capacity of DC circuit breaker under the condition that the total reactance of the whole network remains unchanged,and then reduce the investment of HVDC grids.It can also reduce the total reactance in the HVDC grid and speed up the recovery after the fault under the condition that the breaking capacity of the DC circuit breaker remains unchanged.Considering comprehensively that multi-current limiting devices have the best synergistic limination effect on short-circuit current,and the total configuration of current limiting devices is the least,a multi-objective parameter optimization model considering multi-current limiting devices’ collaborative current limiting is established with the interrupting capacity of DC circuit breaker,MMC locking threshold and DC voltage at the outlet of converter station as constraints,and the improved efficiency calculation method is adopted to improve the optimization efficiency.After the fault is cleared,according to the continuation state of key equipment in HVDC grids,according to different areas of HVDC grids.Corresponding recovery strategies are put forward for different areas of HVDC grid.(4)The short-circuit current calculation method and the coordination method of multi-current limiting devices suitable for the digital-physical hybrid simulation platform of 23-terminal HVDC grid are verified by experiments.Based on the digital-physical hybrid simulation platform of 23-terminal HVDC grid,the efficient calculation method of short-circuit fault in HVDC grid is improved,and the capacitance to ground of long DC lines is included in the calculation,which verifies the correctness and flexibility of the calculation method.Aiming at the problem of the coupling coefficient of the physical prototype of the current limiter,the proposed optimal configuration method is revised,and the spatial configuration scheme of multi-current limiter equipment is verified in the digital-physical hybrid simulation platform of 23-terminal HVDC grid for the first time.Compared with the scheme of installing CLR evenly on DC lines,the proposed spatial cooperation scheme of multi-current limiting equipment has a current limiting effect of 36.41%in the 6-terminal physical dynamic model platform.In the 17-terminal digital simulation platform,the current limiting effect reaches 48.74%.The experimental results verify the correctness and effectiveness of the proposed space allocation method,and the related space allocation scheme is of reference significance to engineering practice.