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Generation of a long laser-plasma channel capable of triggering and guiding an electrical discharge is a crucial issue for laser-triggering protection system. We make a long plasma channel to increase the probability of triggered lightning by laser. To produce a long laser plasma channel, we proposed a new technique called hybrid plasma channel method which combines weakly (<1O' cm3) and strongly (>>lO' cm3) ionized plasma channels to maximize laser-energy efficiency of discharge guiding. We investigate the characteristics of the hybrid plasma channel method through several laboratory experiments.The weakly ionized channel was generated by ultra violet laser pulses (248nm) in air. As the number density of electrons in weakly ionized channel is proportional to 1 .1 power of laser intensity (5.5 eV), nitrogen and oxygen molecules can not attributed to the source of ionized plasma. It is suggested that dissociation process of impurities in air whose density is 10' '-1012 cm3 plays an important role in plasma formation and leader triggering effect.The 50 % flashover voltage (V50) using the hybrid plasma channel method is lower than that without the weakly ionized plasma channel. It was also found that higher repetition rate of the plasma generation on lowers the V50 furthermore.

Abstract:Grounding resistances of transmission towers in Japan are commonly required to be less than 10 ohm to avoid lightning back flashover accident. The surrounding soil, therefore, must be improved by chemical or construction ways if the resistance are much higher than the recommended value. In order to reduce the cost of these works, the authors have investigated on flashover simulation with various grounding conditions. In this paper, we propose the particular model for grounding resistance concerning the impulse characteristics of an actual 500kV-transmission tower footing base. We also present the results of the EMTP simulations with the proposed model, where the results suggest that it be not always necessary to limit the grounding resistance less than 10 ohm and it may be possible to reduce construction cost for transmission towers.

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