Adaptive fault nature identification and soft restart criterion for hybrid multiterminal UHVDCs

• Principles outlined for safe 'soft restart', ensuring security through adjustable sine wave injection. • SRC distinguishes between permanent and non-permanent faults, unaffected by measurement errors or noise. • SRC tolerates high fault resistances, covers entire DC lines, and reduces short-circuit currents. • SRC's speed remains well within the LCC-MMC-UHVDC deionization time, minimally impacting real-time restart performance. The hybrid multiterminal LCC-MMC UHVDC, despite its efficiency in bulk power transmission, is challenged by difficulties in prompt restarting after DC faults, while the existing method poses security risks to inverter-side converters. In this paper, shortfalls of existing techniques are analyzed, emphasizing the need for a fault nature identification method independent of reflected waves. The dynamics of MMC short-circuit currents during signal injection are examined, paving the way for a secure restart. Based on these findings, a novel soft-restart criterion (SRC) is proposed, achieving a balance between sensitivity, reach, and security. The Fréchet distance (FD) is utilized for graphic characteristic comparison, where calculated results are compared with actual measurements to ensure accurate fault nature identification. Furthermore, countermeasures are implemented to mitigate disruptions arising from synchronization errors and noise contamination. Case studies demonstrate that the proposed method exhibits sensitive and secure fault nature identification, robust performance, high resistive tolerance, and cost-effective timing. It eliminates dead zones and effectively mitigates short-circuit currents.