E-REDES test “zero sequence admittance protection” vs “directional earth fault protection”

E-REDES is the Distribution System Operator for mainland Portugal. For the medium-voltage grid, E-REDES uses an impedance grounding system, primarily using a neutral. One of the advantages of this grounding system over other types of neutral connection options is the higher amplitude. Amplitude of fault current, which makes it much easier to detect faults and ensure selectivity using conventional earth fault functions like non-directional/directional zero sequence overcurrent protection. Nonetheless, in recent years, there has been an increase of non-operations of directional earth fault protection, resulting in longer fault clearing time. The primary cause identified for these non-operations is an increase in the quality factor (Q) of the neutral reactance used, leading to a discrimination problem. As an alternative, E-REDES team decided to study the zero-sequence admittance-based protection. To gain more confidence in this protection function before a real-case application, admittance measurements were collected during real earth-fault occurrences in five different networks over a one-year observation period. In this paper, the authors intent to present the considerations taken on the application of this function, a possible setting as well as the results obtained based on admittance measurements during the observation period and the lessons learned during the implementation in a real case.