Zero-sequence longitudinal differential protection of transmission lines

This paper explains a new, earth fault-sensitive algorithm for the longitudinal differential protection of transmission lines. The sensitivity of longitudinal differential protection in radial networks was examined. In particular, problems which can influence the protection’s speed and reliability were considered, such as great line length and the presence of resistance at fault position. In addition, the algorithm was tested for isolated and low-impedance earthed networks. The paper’s thesis is that rather than the stabilization current, the direction of the zero-sequence currents brought to the relay should be used. The zero-sequence currents’ direction is identified without using any voltage inputs. The suggested algorithm was compared to algorithms used by renowned relay manufacturers. Various types of external and internal faults were simulated. The results prove that the described algorithm is prompt and highly reliable under all the aforementioned conditions which can cause unwanted irregularities in the operation of the longitudinal differential protection.