Hybrid pylon
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A hybrid pylon is a pylon, which carries electric circuits for different types of current. Here are in principle the parallel guide of a three-phase alternating current line and a traction current line, a HVDC line and a three-phase alternating current line and a HVDC line and a traction current line possible. However only the first case is of practical interest, since until today no parallel guidance of traction current lines and HVDC lines and three-phase alternating current lines and HVDC lines on the same pylon were realized. As a rule a hybrid pylon carries two electric circuits for traction current and two for three-phase alternating current, whereby the lowest cross bars are for the traction current circuits, which are arranged in one level and the highest crossbars are used for the three-phase circuits. The arrangement of the three-phase circuits can take place in one, two, or three levels. In order not to get an excessively pylon height and width, usually the arrangement of the three-phase circuits in two levels is used. In Switzerland there are also hybrid pylons with three crossbars, carrying on one mast half a three-phase system and on the other two traction current circuits. If the traction current line shares the pylon with a three-phase alternating current operated by more than 220kV, the isolation of the traction current line has to be designed for 220kV, because in some failure cases of the 380kV-line along the traction current line over-voltages can occur, which overtax an insulation for 110kV.
