University of Mohaghegh Ardabili Journal of Operation and Automation in Power Engineering 2322-4576 2026 07 19 A Novel Simplified Protection Algorithm for Multi-Terminal HVDC Systems Using Travelling Wave-Based Modified Frequency-Spectrum Hilbert Transform 4760 10.22098/joape.2025.16764.2298 EN Ehsan Akbari Department of Electrical Engineering, Mazandaran University of Science and Technology, Babol, Iran. Journal Article 2025 02 09 High-voltage direct current (HVDC) transmission systems in multi-terminal configurations are prone to faults due to their structural complexity and multiple power-carrying branches. A failure in such systems may propagate fault waveforms, causing damage to other branches and interconnected networks. To prevent this, accurate and fast protection schemes are required for fault detection, classification, section identification, and location estimation. Traveling wave (TW) theory has been widely applied as a rapid protection method due to its simplicity in implementing complex structures. However, its accuracy decreases under noise and frequency disturbances. This paper proposes a modified frequency spectrum Hilbert transform (MFSHT)-based TW protection scheme that compensates for frequency disturbance errors in conventional Hilbert spectrum analysis. Voltage signals are measured at multiple points in the HVDC grid and processed by the protection algorithm. Simulation results demonstrate that the proposed scheme achieves 100% accuracy in fault detection, classification, and section identification, with an average fault-location error of less than 0.2%. Sensitivity analyses and comparative studies further confirm its effectiveness and superiority. https://joape.uma.ac.ir/article_4760_533e9431ad6de74836d0180e63e0415b.pdf