Der Platzbedarf von gasisolierten Schaltanlagen beträgt ca. 1/10 dessen von Freiluftschaltanlagen, wodurch diese Anlagen in Hallen untergebracht werden können. Dafür sind Kosten und Wartungsaufwand höher als bei Freiluftschaltanlagen, weshalb gasisolierte Anlagen vor allem in dicht bebauten städtischen Gebieten mit geringem Platzangebot Anwendung finden.
https://de.wikipedia.org/wiki/Schaltanlage

Elements of a substation
A: Primary power lines' side B: Secondary power lines' side
1. Primary power lines 2. Ground wire 3. Overhead lines
4. Transformer for measurement of electric voltage
5. Disconnect switch 6. Circuit breaker
7. Current transformer
8. Lightning arrester
9. Main transformer
10. Control building
11. Security fence
12. Secondary power lines
https://en.wikipedia.org/wiki/Electrical_substation

überspannen / span
(Dictionary of Exact Science and Technology)

Critical span analysis of overhead lines.
https://smartech.gatech.edu/handle/1853/16430

Application of the ruling span concept for overhead lines in mountainous terrain
https://www.google.de/url?sa=t&rct=j&q=&esrc=s&source=web&cd...

The lines spanned between the towers possess no insulating cover in contrast to cables.
https://www.google.de/url?sa=t&rct=j&q=&esrc=s&source=web&cd...

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Note added at 10 Stunden (2018-04-28 10:18:11 GMT)
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Substantiviert: "overspan of plant, installation or substation"

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Note added at 2 Tage 8 Stunden (2018-04-30 08:41:50 GMT)
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Other than insinuated, "overspan" is a genuine and well-established english term:
"overspan" About 17.700 results (0,37 seconds)

Definition of overspan
1 : to reach or extend over
overspan so many local variations —Ruth Benedict
2 : to erect or throw a span over (as a space)
https://www.merriam-webster.com/dictionary/overspan

Verb
overspan (third-person singular simple present overspans, present participle overspanning, simple past and past participle overspanned)
1. (transitive) To reach or extend over.
https://en.wiktionary.org/wiki/overspan

overspan
verb

[with object] To extend above and across (something); to span completely from side to side. Also occasionally without object.
https://en.oxforddictionaries.com/definition/overspan

Stage 1
- Installation of overspan from 500kY OHL Stepantsminda gantry to the A T4 transformer gantry
:
Stage 2
- Removal of overspan from 500kY OHL Stepantsminda gantry to AT4 transformer gantry
https://www.google.de/url?sa=t&rct=j&q=&esrc=s&source=web&cd...

Transmission Line Reconstruction Project Hrazdan to Shinuhair Corridor, Armenia
:
Identify exact location with local experts from Department of Protection of Monuments during design phase and bypass or overspan relevant site.
https://www.google.de/url?sa=t&rct=j&q=&esrc=s&source=web&cd...
(Issued by World Bank)

For bay 3 no rehabilitation works will be necessary except for the installation of the overspan between bus bar 1 and bus bar 2.
https://www.google.de/url?sa=t&rct=j&q=&esrc=s&source=web&cd...
(Other World Bank project)

4.2.4 Wetlands
No construction of operation would occur in wetlands. No equipment would enter wetlands, and poles will be placed outside wetlands so conductors would overspan them, conductor stringing operations would work from outside wetland areas.
https://www.google.de/url?sa=t&rct=j&q=&esrc=s&source=web&cd...
(Interational Atomic Energy Agency)

EUROPEAN STANDARD EN 50341-1
Overhead electrical lines exceeding AC 1 kV - Part 1: General requirements - Common specifications
:
5.9.2 External clearances to ground in areas remote from buildings, roads, etc.
Table 5.10 Minimum external clearances to ground in areas remote from buildings, roads, railways and navigable waterways
:
In some countries it is normal practice to overspan forests to avoid lopping and in this case maximum future height of the trees should be allowed for.
https://www.google.de/url?sa=t&rct=j&q=&esrc=s&source=web&cd...

Further to the temporary bridge at Ballyvourney, the most notable location on the route is the bridge at Ballyfinane. Here, the works discussed with Cork County Council comprise a saddle to overspan the bridge and raised parapets.
https://www.google.de/url?sa=t&rct=j&q=&esrc=s&source=web&cd...

This is overtensioning I think. You can safely ignore the /Anlagen/ part of the word. This is typical German nomenclature redundancy.

The dead-end tower or dead-end portal (Freileitungs-Abspannportal) are placed in intervals to support the weight of the cables. The dead-end portal has strain insulators to carry the mechanical pull of the conductors/cables. If there is a great distance to cover like a body of water, for example, the right combination/number of towers with the right construction will ensure that there is no system failure: the cables can come into contact with each other when windy, the cables might sag over time because of mechanical failure of components.

You text states that if the dead-end tower is not in the immediate vicinity of some field (Feld - does your text define what this field is?), overtensioning occurs. This happens when the span is too wide causing too much of a mechanical dynamic load (downwards, swinging). A span that is too wide is present when the distance between successive towers is not optimal.

Just a start. Can look for references tomorrow. Read about strain insulators. That will point you in the right direction.

definition from the International Electrotechnical Vocabulary (IEV 604-03-09); available on Electropedia

Various types of overvoltage
An overvoltage is a voltage pulse or wave which is superimposed on the rated voltage of the network.