A 132/33 kV route with a directional drill part way along it. The drilled section and the open-cut sections are different thermal and mechanical problems, and treating them as one route gets both wrong.
The Brief
- Cable ampacity and pulling for a 132/33 kV connection, where the route passes under an obstacle by horizontal directional drill.
- The installation method changes along the route: a common duct through the drill, individual ducts either side.
- The client had a drawing and a proposed formation, and needed to know whether both worked.
How It Was Done
- The route was split into three pull sections rather than treated as one: the drilled length, and the runs either side of it. Each has its own tensions and sidewall pressures.
- The thermal case was run on the as-drawn formation, three single-core cables in trefoil in individual ducts at 300 mm circuit centres, taken off the client’s own CAD rather than an assumed arrangement.
- Trench sections were drawn from the same source as the ratings, so the drawing and the calculation cannot disagree.
What Came Out Of It
- A rating and a pulling assessment per section, with the drilled length treated on its own terms.
- A revision followed when the client changed the duct arrangement, and the sections were re-run rather than interpolated.
Standards applied
IEC 60287 · CIGRE TB 889
Published without naming the client. The technical detail is exactly as delivered.
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