Transition underground cable - overhead line

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Toliman
380 kV
Berichten: 669
Lid geworden op: 28 apr 2020 01:47

Transition underground cable - overhead line

Bericht door Toliman » 22 okt 2020 01:17

Why are transitions from underground cable to overhead lines for lines with voltages below 100 kV nearly always on the terminal pylon, for 110 kV sometimes on the terminal pylon, but sometimes on the ground as shown on http://www.hoogspanningsforum.com/downl ... p?id=34312 and for voltages over 200 kV always realized in a switchyard?

Would it be possible to realize the cable-overhead powerline transition of "Baltic-Cable", which is shown on https://files.structurae.net/files/phot ... in_s13.jpg and which works with 450 kV DC, on the terminal tower? If not, why?

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Hans
Site Admin + actief in het mainsite team
Berichten: 3824
Lid geworden op: 03 dec 2011 20:49
Dichtstbijzijnde trafo: Veenoord 110/10 kV

Re: Transition underground cable - overhead line

Bericht door Hans » 22 okt 2020 14:18

Several reasons for. But the most important two are mechanical forces and the possibility to mount surge arrestors or devices cancelling out radio interferrances.

Extra high voltages usually come with more than just a ceramic or plastic terminator to lead the conductor in the insulated cable. Already starting at 110 kV but rather from 220 kV and up, usually the termination ends are equipped with extra gear such as the mentioned surge arrestors. Breakers are unusual, but separators (not to be operated under load, but for a branch during maintenance they can serve a good job) are not uncommon. Mounting and connecting this devices require additional engineering and also space.

For the Baltic Cable, but also comparable DC-links such as the Sydvästlänk, Cross-Skagerak, Fenno-Skan or Konti-Skan, the termination points where the DC goes underground are all equipped with -at least- a bunch of surge arresters. For DC that is a critical requirement to make it all work (or more exact: to keep it all at work when Thor has a bad day). Mounting surge arrestors high up in the pylon or even at a kind of terrace half-way the tower is challenging and it also visually pretty much a consideration when suspending all that gear at display some tens of meters above the grass.

The other factor is in the mechanical forces. 110 kV does not require heavy-duty gear: material strength, pylon height and the weight of the termination components are well within the "common scale" of what is accepted in everyday engineering.
When terminating 380 kV, this grows out of control. Imagine the tensile forces in the huge bundled conductors. It makes quite some difference whether the termination pylon must sustain the full forces, or just a portion of it when spanning them down to the ground. (For example, an AMS-620, in The Netherlands kind of the 'golden standard' between the mid-nineties and the rise of post-AMS materials and/or HTLS-conductors of today), a quadruple weighs as much as three metric tons for a typical span of about 400 meter. Multiply that times six, increase that with the catenary parameters to obtain tension, and add to that some worst-case scenarios for icing, snow, calamities or asymmetrical stresses in the pylon, and the net result would be a pylon requiring such a brute desigm that it is simply not worth while the effort of saving some land to obtain form the local farmer for a ground termination yard.

However, this still does not mean it doesn't happen at all. When there are proper reasons, there are different decisions. For example, this one in Berlin or a bizarre case in Latvia, mapped by Peter. If there is no space available and the cables can get quite low to the ground before leading them down, it can be worth the effort anyway.
Halfverankering is net als Volbeat: het kan altijd

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