Reliability evaluation with extended mast terminal (EMT) to protect Pt. Vale Indonesia 150 kV transmission line against back flash over and shielding failure of lightning impulse

150 kV overhead transmission lines that span from Balambano, Larona and Karebbe Hydro Power Plants to nickel processing plant switchyard are located in a high lightning density area. Before EMTs installed in 2015, there were many complaints of damage to the tower insulators due to lightning strikes that caused total black out. The EMT installation was performed on selected towers at a location based on the mapping in an environment with high lightning density and based on historical location data of reported insulation damage. The selected towers are equipped with isolated lightning rod (early streamer), double shielded down conductor and magnetic tape current measurement system while other towers that have not yet been decided to be installed with EMT will be further evaluated. The engineering modification is to avoid the effect of back flash over and shielding failure of lightning strike that potentially damages the insulator and triggers ground fault. The transmission lines condition was observed due to their good performance after improvement of lightning protection system (LPS). The significant reduction of transmission line outages is reported. Analysis was performed on the existence of EMT that contributed to the failure reduction. Its presence provides protection against damage to high voltage tower insulators as indicated by the increasing value of MTTF. The failure rate is calculated to determine the reliability level of the EMT system which is then compared with the level of reliability without using the EMT system. This lightning protection equipment is reliable enough to avoid direct lightning strikes to the tower and wire lines and to prevent back flash over and shielding failure. The improvement reduces outages and increases the reliability significantly.