Gas-insulated substations (GIS) have different specifications in proportion to air-insulated substations. Due to space limitation in the field of the gas-insulated substation (GIS), it is difficult to install extra arresters near the power transformers. Voltage magnification is due to reflections of transient waves in various junctions, low surge impedance and decrease in the length of conductors in the GIS. These problems cause to increase the propagation of transmitted and reflected waves within the conductors in proportion to air-insulated substation; thereby overvoltages in GIS are more important than air-insulated substation. In air-insulated substations, probability of failures across insulator strings or bushings is acceptable since air insulation is self restoring. In GIS, the entire gas insulated assembly must be protected because the gas insulated system must be considered as non-self-restoring. According to aforementioned reasons, it follows that insulation coordination design of the GIS has been critically important. This paper presents practicable and beneficial approaches to the industry to look for the optimum approaches for lightning incoming surge mitigation. These approaches are included effects of number of surge arrester in the each feeder, system configuration and decreasing of number of surge arrester and as costs, location of surge arrester and cable, and terminal components. Lightning overvoltages due to direct lightning stroke with varying intensity current have been investigated. For accurate calculations, ATP/EMTP software has been used.
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