Experimental evaluation of shielding angles' effects on lightning performance in a 400 kV AC double-circuit composite pylon

摘要

A 400 kV AC double-circuit composite transmission pylon is proposed in a Danish project titled 'Power Pylons of the Future' [1]. The new pylon is featured by a 'Y' configuration without insulator strings, thus the geometry of overhead lines (OHLs) in the new pylon is totally different from that in traditional towers, shown in Fig. 1. It is shown that two shielding wires are located at tips of the pylon and grounded through a single bare metallic conductor outside the pylon. With the unique geometry, the pylon designer is faced with a challenge regarding the determination of the shielding angle, i.e. the complementary angle of β in the figure, which is of great importance to the lightning performance of a transmission tower. Two shielding angles with values of -60° and -70° are under consideration, since in both cases the pylon has a good mechanical performance and acceptable visual impact to the environment. Electrical-geometric model (EGM) is used to analyze the effectiveness of different shielding angles, with the conclusion that the shielding angle of -60° is preferable, since the maximum lightning current that can lead to shielding failure is smaller than that with shielding angle of -70°. To verify the feasibility of EGM method in the evaluation of unusual negative shielding angles in the composite pylon, a scale model test is conducted, with the concept that lightning attachments to OHLs is equivalent to long-gap discharges to equivalent conductors installed in reduced-scale pylon models. The scale model tests demonstrate that a shielding failure region exists in the center of the pylon, which is consistent with the conclusion of EGM method. Also, the spacial shielding failure probability around the pylon with shielding angles of -60° and -70° are obtained and the maximum lightning currents in both cases that can lead to shielding failure are calculated. Combining results derived from EGM method and the scale model test, the conclusion can be drawn that OHLs in the pylon with a shielding angle of -60° has better lightning performance.