Approximate Calculation Method of Short-Circuit Current of Multi-Terminal Hybrid DC Transmission System Considering Control Strategy

The hybrid DC transmission system, which gives full play to the advantages of conventional DC and flexible DC, has become a research hotspot in the fields of long-distance transmission and renewable-energy grid connection. However, HVDC transmission lines are long and the working environment is complex, resulting in high fault probability. Once a short-circuit fault occurs on the DC line, it will generate fault current several times the rated current within 10ms, exposing the whole system to a serious overcurrent risk and threatening the sTab.operation of the system. In addition, the highly nonlinear characteristics of the converter station make the existing short-circuit current solutions ignore the response process of the control system and rely on the modeling and simulation results. It is impossible to obtain the specific analytical formula of the short-circuit current to realize the quantitative analysis and calculation of the fault characteristics, which brings difficulties to the protection and design of DC control systems and restricts the development of hybrid DC transmission technologies. Therefore, considering control response, an approximate calculation method for short-circuit current of multi-terminal hybrid DC transmission systems is proposed. First, three system model simplification measures are proposed, which take into account both accuracy and computational complexity, including port processing, non-fault-pole discharging-loop processing and AC system processing. Second, the system topology model and control model are modeled. Based on the fault-component network, the short-circuit current is obtained regardless of the action of the control system, and then the corrected current under the control action is obtained by using this calculation result. The two are superimposed to obtain the final complex frequency domain analytical formula for the short-circuit current, and the inverse Laplace transform is used to obtain the time-domain solution for the short-circuit current. The proposed method corrects the trigger angle of the LCC side, using Pade approximation and piecewise linearization method to reasonably reduce the order of short-circuit current model, which takes into account the calculation accuracy and efficiency, to effectively avoid the problem that the inverse Laplace transform cannot be solved due to the high order of complex frequency domain expression. Finally, based on PSCAD/EMTDC, the simulation analysis on different fault distances, transition resistance and fault types is carried out. It can be concluded that, the proposed method can accurately describe the transient characteristics and trend of the fault current at the outlet of each converter station before the blocking of the MMC station, obtaining the analytical expression of the fault current which provides a quantitative calculation method of fault current for power grid planning, equipment parameter selection and protection setting. In addition, the transient current increment considering the control characteristics of the converter station is less than that without considering the control characteristics. Therefore, the fault transient characteristics can be quickly analyzed and judged by using the fault current calculation results without considering the control. Finally, it can be concluded that, in terms of calculation efficiency, the proposed method does not have the problem of excessive computational complexity due to the order reduction. The calculation error is mainly related to the factors neglected or simplified in the calculation process, including the approximate equivalent processing of the model, the approximate fitting process of the waveform and the accuracy reserved when the intermediate variables are obtained, but the overall relative error is still controlled within 10% to meet the accuracy requirements of fault current calculation. © 2024 China Machine Press. All rights reserved.