Nonunit Distance Protection Algorithm for Multiterminal MMC-HVdc Systems Using DC Capacitor Resonance Frequency

High-voltage dc (HVdc) transmission has been largely used to interconnect asynchronous systems and integrate renewable energy resources into electrical grids. However, the high short-circuit currents and low-tolerance of power electronics equipment impose new challenges for these systems’ protection. To address these challenges, a new distance protection algorithm for HVdc grids with modular multilevel converters (MMCs) is proposed in this article. The proposed algorithm identifies the faulty line/cable using the resonance frequency of a dc capacitor installed in each terminal. The technique was tested in a four-terminal HVdc grid with five cables and ten circuit breakers. The algorithm was highly selective for faults in the lines and provided fast identification, in less than 1 ms, without communication amongst terminals. The algorithm was tested in hardware under high-noise conditions and provided reliable results.