11kV mystery object / taps

pc1966 said:

On the road from Perth to Dundee I frequently pass this 11kV (I presume) pylon and often wondered about the odd taps taken off two of the lines. So tonight I stopped to take some photographs and the mystery is even greater as it seems not to be a power take-off at all as no HV cable going off underground. The rear of the pole just has an earth cable that is joined to the box, metallic stand-offs, and (I guess) to the top bar.

Also a curiosity is the thin line wrapped around the main HV conductors, it looks almost as if it is a fibre optic cable changing phase, but why?

I wondered if someone like @Julie. might have an answer.

TL;DR What is the purpose of the strange taps?

View attachment 98127

Click to expand...

It looks like a fibre optic repeater, on 11kV...33kV there is a product called access wrap, similar to sky wrap at the higher voltages, which is an optic cable wrapped around existing ohl (rather than combining in the earth conductor, which doesn't usually exist at these lower voltages)

This fibre may be used for general communication, in addition to protection equipment, and scada.

The box looks like a repeater to increase the signal, plus perhaps tap-off for local communications, not really sure of that side of stuff.

The odd rings just above the insulators on the optic are to provide an even electric field to the insulators as the outside of the optic below the insulators is earthed.


EDIT

Should really say "smart grid" rather than scada now I guess!

Out of interest, any idea what they would change phase for the fibre, rather than just tap off and back on to the same conductor?

pc1966 said:

Out of interest, any idea what they would change phase for the fibre, rather than just tap off and back on to the same conductor?

Click to expand...

No idea, could be something mundane like it splits to a single phase (L-L) distribution further down

You just stopped on that road to steal some strawberry’s from the field…
Taking photos of a pylon was just in case the police saw you.

If the cable wrapped around the live conductor is a communications cable - most likely fibre optic - then it is non-metallic in construction. Nevertheless, the surface of it has dirt and water which under the right conditions and voltage will conduct a current along the outersurface. At 11kV and above this becomes problematic. There can be a problem where the surface insulation has small gaps which provide 'hot spots' for an electric field leading to local heating and ultimately arcing. Arcing then leads to carbonisation and over time the surface becomes more conductive over longer lengths. The comms cables are all dielectric self supporting (ADSS). See:

Study on development of dry band on ADSS fibre optic cable - https://ieeexplore.ieee.org/document/1453453

The electric field strength around the live conductors is highest at the insulators where the full potential difference exists between conductor and ground. I think the arrangement you show a picture of is to periodically insert some resistance into the communications cable. I suppose the circuit is something like my attached electrical art with current flows as indicated in red. I guess along long lengths of comms cable these currents are troublesome. Adding a periodic break in the outer insulation of the cable either stops these currents or introduces some resistance which reduces their magnitude and effect.

I think the reason the cable swaps from one live conductor to the next at regular intervals eg L1 to L2, L2 to L3 and L3 to L1 and L1 to L2 ....... is because the emfs in each length of comms cable between swaps is 120degrees out of phase with the emfs in the lengths before and after it. We know in a such a balanced arrangement of ac emfs the phasor sum of them is zero or something small at least - the posts are all the same distance apart this arrangement - which will ensure that either end of the line a large emf has not built up along the comms cable.

I think - not absolutely sure - that what look like two black bellows shaped insulators below the clamp ensure that the full line voltage is not connected across the section of comms cable which connects between L1 and L2 in your image. These 'insulators' I reckon are also what introduces the break and the resistance I mentioned earlier.

Or something along these lines....