Transposition tower

In electrical power transmission, a transposition tower is a transmission tower that changes the relative physical positions of the conductors of a transmission line in a Polyphase system. A transposition tower allows these sections to be connected together, while maintaining adequate clearance for the conductors. This is important since it distributes electrical impedances between phases of a circuit over time, reducing the problem of one conductor carrying more current than others.

Double-circuit lines are usually set up with conductors of the same phase placed opposite each other. For example, a section of a line may be (top-to-bottom) phases A-B-C on the left, also phases C'-B'-A' on the right. The next section may be B-C-A on the left, also A'-C'-B' on the right. Therefore, the rotation on each side of the tower will be opposite.¹ Transposition helps to reduce the mutual coupling between conductors and between conductors and ground.² It also useful in mitigating issues like induced voltages in nearby telephone lines.³

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  Here, on the right the phases are rotated; on the left, two phases are swapped, thus the rotation sense changed.
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  Transposition tower. On the right side, the phases are rotated upward, on the left side downward.
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  Transposition in the span field of a 420 kV-line in Sweden

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  Transposition on a single-phase line
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  Transposition by using multiple towers
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  Transposition by using elongated traverses

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  154 kV double-circuit transposition tower.