Short Piece of Cable Used in Outdoor Substation

Because a long one isn't needed.

Do you know why some power is generated at 42 Hz?

Also, when they used a long piece of cable, people kept tripping on it.

Just adding to JAE's excellent explanation, the last pole or structure is called a "Dead End" structure, it is strong enough to hold the full tension on one side without a balancing load on the other side; the short piece of cable is referred to as a "Slack Span". The slack span is short so that it does not have to be tight to keep it at sufficient clearance above other equipment and people: because it isn't tight, it doesn't pull the connections off the transformer.

And I might add the slack span line drop into the substation should include a connection to the line end of a station class surge arrester ahead of, and in parallel with, the transformer bushing.

Transmission lines are never connected directly to equipment due to forces acting on the conductor. The droppers, open ali conductor is terminated on a landing structure and these short ali conductors can be removed to isolate the line from equipment.

Connecting the indoor switchgear to the transformer outside, one cannot use open conductor for obvious reasons, BUT, if the conductor is going inside a building, a rather large wall mounted busing is required, and these are expensive, very large, heavy and a wall needs reinforcing to support it. Inside the building one requires a lot of clearance for the indoor conductor so that makes it expensive to build a huge building to offer clearances to ground. ABB makes these bushing for HVDC converter stations and the bushing that was delivered today on a semi trailer is longer than the trailer itself and approximately 1.4m in diameter at its thickest point.

If it is indeed an insulated cable of 1 or 3 core construction, it does the same thing, offers an point to isolate the equipment in the yard to the indoor equipment and offers a circuit to the indoor equipment. Outdoor equipment is normally connected via a strung ali busbar or an ali tube busbar which runs overhead. I have built subs with busbar going from outdoors to indoors, via an insulated wall section, bushings and onto the indoor busbar system, on to the switchgear. It is costly and cumbersome and easily short circuited by birds, cats, rats, people, vehicles and ladders.

That is the gist of it, costs, maintenance, turn around time for failures, protection of equipment from fault forces and it a line falls down, it does not wreck the equipment by ripping out bushing, The line and droppers snap normally and save the expensive yard equipment, CB's, Tfx, Isolators,surge arrestors etc.