Hierarchical optimization of current-limiting devices with discrete parameters in multi-terminal DC power grids

Optimizing current-limiting devices (CLDs) in multi-terminal DC grids is essential to limit the fault current while reducing the cost of devices. However, current studies mainly focus on optimizing CLDs based on continuous parameters in simple DC grids, which cannot meet the optimal efficiency demand of CLDs in complex multi-terminal DC grids and results in each CLD needing to be customized separately, increasing the production cost of devices. Therefore, to improve the optimal efficiency under complex multi-terminal DC grids, the principle of fault area division based on the coupled degree of converter and DC grid and the corresponding hierarchical optimal implementation schemes of CLDs are proposed first. Then, the effects of CLD parameters on the fault current are analysed, and their sensitive ranges of current-limiting are revealed. In combination with the error range of inductance, the principle of discrete parameter design of current-limiting inductance parameters is put forward. Next, the discrete parameters of CLDs are optimized by an intelligence algorithm. Finally, the hierarchical optimal method of CLDs based on discrete parameters is validated on a six-terminal DC grid in PSCAD/EMTDC. The proposed method can satisfy the demand of current-limiting, which is more in line with the standardization requirements in practice.