Reduced-scale experiment of transmission-line tower impulse grounding impedance
The lightning withstand level of the transmission-line is directly affected by the tower impulse grounding impedance. Reducing the tower impulse grounding impedance is an effective measure to increase the lightning withstand level of the transmission-line. In the analysis and calculation of transmission-line lightning protection, the tower impulse grounding is mostly applied as a specific resistance. However, the impulse grounding impedance shows nonlinear spark effect and inductance effect under the lightning current. In this paper, reduced-scale experiment was utilized to study the tower impulse grounding impedance characteristics. Compared with full-scale experiment, reduced-scale experiment can be much low-cost and convenient. Reduced-scale experiment means the grounding electrode is a model reduced from the real grounding electrode with a certain scale n. According to the reduced-scale theory, not only the spatial scale, but also the time scale reduces, which means the rise time of the applied impulse current should be fast enough. Three-electrode field-distortion gas switch and low inductance circuit was used to realize fast current rise time and low jitter. After the platform of reduced-scale experiment completed, experiments were carried on to study the effect of impulse current amplitude, grounding electrode size and soil resistivity on the impulse grounding impedance. It can be concluded that the impulse grounding impedance decreases with the increasing amplitude of current but it shows saturation effect; with the increasing grounding size, the impedance also decreases with saturation effect; the impulse grounding impedance has approximate linear relationship with soil resistivity when the soil resistivity is less than 580 O·m. With these conclusions, this paper proposed an improved type grounding electrode by adding needle shaped conductors on typical grounding electrode to reduce the grounding impulse impedance. Then the effect of the quantity and length of the needle shaped conductors on impulse grounding impedance was studied. The result indicates that adding needle shaped conductors with suitable quantity and length can reduce the grounding impedance effectively.
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