A Pilot Protection Scheme for Transmission Lines of Half-Bridge MMC-HVDC Grids Using Cosine Distance Criterion

One of the major challenges in half-bridge modular multilevel converter-based high-voltage dc (MMC-HVDC) grids is the rapid detection and isolation of short-circuit faults before converter blocking and the entire grid shutdown. With the advancement of the hybrid dc circuit breakers (HDCCBs) in the ultra-fast isolation of faults, a considerable part of the problem has been solved. In this paper, relying on the performance of these circuit breakers and using the cosine distance criterion, a new pilot protection scheme is designed and presented for detecting short-circuit faults in transmission lines of half-bridge MMC-HVDC grids. While this scheme requires a comparatively low sampling frequency, its fault detection speed is quite independent of fault resistance. Moreover, this protection has the correct performance for any fault location along the line under protection and is completely stable against all external faults. The results of the evaluations on a 4-bus test grid under different fault conditions confirm the correct, ultra-fast, stable, selective, and reliable performance of the proposed line protection scheme.