Abstract: Lightning stroke on a tower structure is one of the major reasons which results in high induced voltages at tower arms due to the excessive lightning current flowing through the tower to earth. If this induced Lightning stroke on a tower structure is one of the major reasons which results in high induced voltages at tower arms due to the excessive lightning current flowing through the tower to earth. The worst case is to have this induced voltage on the first tower close to a substation. If it is higher than the withstand level of insulator string, the insulation of substation' equipment will be exposed to transient overvoltage (fast front back-flashover (FFBF)). The peak of this transient overvoltage is affected by the value of system earthing resistance. This paper studies the effect of surge arrestor (SA) earthing resistance and the footing resistance of 1st tower adjacent to a 25 kV traction feeder station on FFBF. Three case studies using PSCAD/EMTDC software are presented to simulate the back-flashover with variable earthing resistances at the 1st tower footing and SA. The paper also addresses the critical earthing resistance for designing an economic protection system for solving the problem of transient overvoltage produced due to fast front back-flashover.   This study proves that the proper design of a grounding system for the first tower and SA not only enhances the safety of the system but it also minimizes the construction cost of the grounding system

Keywords: Back-flashover; Insulation Coordination; Fast Front Transient Study; 25 kV Traction Feeder; Footing Resistance; Lightning Impulse Withstand Voltage; PSCAD/EMTDC

| DOI: 10.17148/IARJSET.2020.7515