Improved active current limiting control for flexible DC power grid to consider the effects of selective and higher fault resistance

The current limiting control strategy of modular multilevel converters in flexible DC power grids can effectively reduce the requirements for line protection speed and the breaking capacity of DC circuit breakers (DCCB). However, it can easily lead to the interruption of power transmission at non-fault terminals and expanding the scope of the fault impact. Secondly, it did not consider the severity of matching faults, which may lead to excessive suppression of fault currents and reduce the sensitivity of the protection system. Therefore, this paper proposes an active current limiting control (ACLC) strategy with fault identification capability to further improve its selectivity and adaptability. Firstly, the starting element in the ACLC selectively identify fault areas based on the difference in the slope of the first backward traveling wave (TW) of the current. Secondly, a current limiting coefficient is constructed based on the slope of the first backward TW of current to achieve fault current limiting. On this basis, setting amplitude control to improve the adaptability of ACLC to fault resistance. Finally, a complete fault handling scheme was proposed, including ACLC, fault identification and fault isolation. The simulation results show that the proposed ACLC strategy is selective while reducing the technical requirements of DCCB. In addition, the adaptability of ACLC to fault resistance has also been improved.