DC Fault Detection in MTDC Systems Based on Transient High Frequency of Current

Isolation of the fault dc lines during the initial short period after fault is a major challenge in the multi-terminal dc (MTDC) system based on a voltage-source converter (VSC). Existing protection schemes are mainly designed based on the numerical simulations, which lacks the theoretical analysis. In this paper, a high-frequency equivalent model of a VSC-based MTDC system is first proposed for the fault current calculation. Based on the dc fault analysis, it is validated that the high-frequency fault current components flow in the fault line and then dramatically decay in the healthy line. Accordingly, a novel dc fault detection method for the VSC-based MTDC system is proposed. In the proposed approach, the primary protection utilizes the transient high-frequency energy of line current and the fault line can be identified quickly without communication. In addition, the influence of the transient traveling-wave appearing on the long-transmission line on the proposed protection scheme is also investigated. Numerous simulation studies carried out in PSCAD/EMTDC have demonstrated that the proposed high-frequency equivalent model can be utilized for the initial dc fault analysis of the VSC-based MTDC system and the proposed protection scheme is effective in different time windows, different fault locations, and high fault resistances. Compared with the existing rate of change of current-based protection scheme, the proposed primary protection requires relatively low sampling frequency, is insensitive to the parameter changes, and has high robustness with respect to the outside noises and data missing.