Tower Structure Effect on a Back-Flashover Phase
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07-01-2011, 12:29 PM

Akihiro AMETANI, Naoto NAGAOKA, Toshihisa FUNABASHI, Nagahiro INOUE

The paper has investigated a difference of flashover phases between extra-high voltage (EHV) and highvoltage transmission lines based on EMTP simulations. The number of ground wires and a tower footing resistance cause a significant effect on the three-phase archorn voltages. A higher probability of a back-flashover across the upper-phase archorn is very noticeable in the case of a tower with two ground wires and a lower footing resistance, i. e. transmission lines of 275kV and above. A uniform probability of flashovers on each phase is characteristic to a transmission line with one ground wire and a high footing resistance corresponding to low-voltage transmission.

It is essential to investigate a lightning surge for a reliable operation of a power system, because the lightning surge overvoltage is a dominant factor for the insulation design of the power system and the protection of equipments in power stations and substations. When lightning strikes the top of a transmission tower, a lightning current flows down to the bottom of the tower and causes a tower voltage rise which results in a back-flashover across an archorn.
A multistory tower model is well-known and is widely used for a lightning surge analysis in Japan . The tower model shows in general high probability of upper-phase flashovers [4], while a measured result of flashover phases on a 77kV transmission line shows a nearly the same probability of flashovers on each phase . The discrepancy seems to be caused by a difference of the tower configuration. This paper investigates the effect of the tower configuration on the archorn voltages including the effect of the conductor type and the number of phase and ground wires based on EMTP simulations of a lightning surge by using the multistory tower model.

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