Analysis of ANSI 67N backup protection scheme for MV grids

This paper analyses the possibilities and impacts of a backup system for ANSI 67N protection function, which is a directional earth fault protection that can be used as the main protection for lines in non-effectively grounded MV distribution grids. The paper compares two backup non-directional protection functions – ANSI 50N and ANSI 46. The analysis is carried out both theoretically and practically, using symmetrical component systems for a theoretical perspective and a single-line-to-ground fault (SLG) in the MV grid for a practical perspective to confirm the theoretical analysis. Fault records from an open phase fault are also provided, confirming the advantages of ANSI 46 over ANSI 50N. The problem and disadvantage of ANSI 50N is that due to the gradual increase in capacitive currents (IC) of lines (cabling of MV networks) and the fact that IC changes during operation (changes of line connections), this protection function may cause an undesirable false trip.