H.V. and E.H.V. overhead transmission lines and the associa-ted substations have been subjected to surge overvoltages dueto atmospheric lightning storms. This problem causes danger-ous damage to H.V. and E.H.V. apparatus and lines apartfrom the economic losses due to power networks outages. Over-voltages produced on lines caused flashover of lines inslation and may reach substations, in the form of travelling waves, and damage its equipments, modern surge absorbers(lightning arresters) for E.H.V. levels are designed to offer protection to equipments and lines for lightning overvoltages and currents as well as switching surge overvoltages where the energy involved is much higher, it classified according to their internal structure: Gap type arrester without current limiting functions; (material is Sic), Gap type arrester with current limiting capability; (material is Sic) and Gapless metal oxide varistors(material is ZnO). In this research studies and measures were taken to protect H.V. and E.H.V lines and substations. Since complete prevention of lightning and hence interruptions of power circuits is not absolutely possible. It was the objective of this work is tolimit to a reasonable rate the number of lightning and switching interruptions and hence reduce the degree of damage. A safe, reliable and economical lightning protective system(using ZnO arresters) was developed and tried in the Egyptian Unified Network for different system voltages. This new technique was developed using two computer programmes, namely Matlab and Alternative Transient program (ATP).
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