How much current flows through pylons?

[James Wilkinson Sword]

Despite extensive googling, there seems to be nothing that tells me how much current flows along wires on a national grid pylon. They only list voltages. Anybody know?

[Brian-Gaff]

Well it depends on the load one supposes. It also then depends on the voltage on thewwires as the whole idea of using high voltages is to reduce losses due to disipation when the system is under load. The answer basically is there is no answer. Might find more if you asked the max current of the one at xx to yy. Brian

[spuorgelgoog]

A 400 kV National Grid circuit may carry 1 kA in each of its three phases, thus transmitting a power of 700 MW. A 132 kV distribution circuit may carry 300 A in each of its three phases, thus transmitting a power of 70 MW. An 11 kV distribution circuit may carry 150 A in each of its three phases, thus transmitting a power of 3 MW. A 400 V final distribution circuit may carry 200 A in each of its three phases, thus transmitting a power of 150 kW.

(Remember, these voltages are phase-to-phase voltages, the phase-to-earth voltages are 1.73 times lower. Thus (400 kV/1.73) x 1kA x 3 = 700 MW.)

(site maintained by National Grid)

[Pylon type] L12 is effectively the L6 replacement will take twin conductors up to 850mm2, but all aluminium conductor rather than the heavier steel cored kind formerly used.

Owain

[newshound]

And Supergrid pylons (400 and 275 kV) are normally double circuit, aren't they? Three wires on each side of the "tree".

[NY]

200A per phase at 400V (240V phase-to-neutral) doesn't sound very high. We have a 60 A "company fuse" and I presume our neighbours do too. With an electric fire (3 KW), an electric shower (maybe 8 kW) and an electric oven and hob (maybe 6 KW), you'd be getting towards that limit but still remaining legal. Now imagine lots of people roundabout doing that. It doesn't take many houses to run up 200 A - or a total of 600 A across all three phases. How many houses are typically fed from a single feed from the substation or 11 kV-to-400V pole-mounted transformer? What is the average current that is assumed per house when sizing up the number of houses that can be fed from one substation circuit? I presume it not the full 60A of the company fuse rating.

How much of the route from the power station to the consumer is redundant multi-circuit? At one point, typically, does it change over to a given house only being fed by one set of wires, and if that line develops a fault there is no backup circuit?

Is there a backup route as far as the final substation that transforms to 11 kV or 400V, or is it higher up the chain?

I presume for maximum redundancy they try to use feeds from different places rather than two sets of wires carried on the same pylons, in case an accident takes out *all* the wires (both circuits).

I'm intrigued at the way house gets its electricity supply. There is overhead mains on wooden poles (originally four separate wires, now a single fat cable with four wires) and our house is the middle house of two adjacent blocks of three houses. There is a single feed from the wooden poles to the end of one block, and then four wires running along the back of one block, overhead across the gap to the next block and along there, with each house taking its feed from neutral and one of the three phases - I think no two adjacent houses are on the same phase. I suppose this is less unsightly than every one of the six houses having its own single-phase feed from the street poles.

[Andrew Mawson]

They always used to rotate phases down a street, so (say) phase 1 -> house

1, phase 2 -> house 2, phase 3 -> house 3, then phase 1 -> street lighting, phase 2 -> house 4 and so on down the road to balance the load between phases.

We have the 11 kV to 415 v transformer on our land and are the first 'drop' of single phase at the farmhouse, but also take the three phase into the barn at 160 amps per phase. No street lighting though round here. The 11 kV can be fed from two points - we have an overhead HV line and an underground HV cable, but normally only one is active.

Andrew

[Harry Bloomfield]

Andrew Mawson was thinking very hard :

So as to as best they can, balance the load on all three phases, so as little current as possible appears on the neutral line.

[Harry Bloomfield]

[e-mail] submitted this idea :

Also remember it is a grid. Those are likely maximums and the actual current flow can be in either direction, depending upon where there is demand and what generating stations are online.

[ARW]

Yep, and it's not uncommon to see that in multiples of houses. So you may have 4 houses next to each other on phase one, the next 4 on phase 2 etc.

A far more common variant (on properties built in the late 1960's and early 1970's) is house 1 and 2 on phase 1, house 3 and 4 on phase 2 and house 5 and 6 on phase 3.

[Alan]

It's called Diversity. The DNO's know that everything will not be plugged in at the same time, so their network will cope for the vast majority of the time (and time has proven this, as there are very few blackouts caused due to DNO substation fusing blowing).Short term overloads dont stress the system too much, as can be seen on christmas day - when 50% of houses have their ovens on etc - but 50% of them are gas,then the oven is not on full power apart from the first 5 minutes, so the load isnt as much as you think I was told the typical demand for each house when the network is designed is around 5 to 10 amps.That'd give 120 houses to one substation feed at 5 amps - that seems about right.

[The Natural Philosopher]

Course not. Average power per household is 1-2KW.

Typically 11KV is run as a ring - it is here anyway, so the only single point of failure is the 240V stuff from the local substation.

As I said, round here 11KV is a ring.

[Brian Gaff]

The actual answer of course is none it goes through the wires, but that would be being pedantic. Of course if like me you stood near a pylon when it was struck by lightening you would see how well built they are! Brian

[NY]

I've never been near a pylon when it's been struck by lightning (*). I wonder if it was the pylon itself or one of the phase wires that was struck. I bet the bang is pretty impressive. The closest I've come was when a pylon (maybe 66 or 132 kV) about 300 yards away was struck - there was a ground-shaking bang and a very bright flash out of the window - much brighter than normal lightning - and the power went off for a couple of seconds and then came back on again as the circuit-breakers somewhere upstream tried restoring power. I couldn't see any sign of blackened pylon arm, so it may have been a strike on one of the phase wires.

Luckily I'd unplugged my PC a few moments before, as I noticed the storm approaching - distant thunder got louder and the flash-to-bang time reduced to a couple of seconds - so I escaped any damage.

Another storm, I wasn't quite so lucky: a minor flicker in the lights from a fairly distant storm took out the power supply of my PC. As luck would have it, I had a spare PSU on my desk which I keep in case any of my customers have a faulty PSU, so I was able to swap it over. I'm quite proud of myself: I have a weather station which updates its data in the log files every ten minutes. The PC went down just after one reading and I had the PC back up again, having swapped over the PSU, together with leads to each disk drive, the CD drive, and the three leads to the motherboard, in time (just - it was close!) for the next reading 10 minutes later. And probably about 2 minutes of the time was taken up with the interminable time it takes my PC to boot up and for the weather-station app to start reading data again.

(*) I have not-so-fond memories of cross-country running at school on a route that took us under pylons. It was a really obnoxious route. You had to endure the taunts of the local kids from other schools on the estate, then run along a muddy, puddly unmade road, past the factory where they boiled up animal carcases to make glue (trying not to puke at the smell), under the fizzing pylon wires which often glowed a pretty mauve on a foggy day and made your hair stand on end, and then try not to get savaged by the alsatian guard dogs from the car breakers yard - and all of that was within about 1/4 mile. After that it got easier!

shows the rusty girders which are all the remain of the glue factory, then the pylon and then (to the left of where the car is parked) the place where the car dump used to be. It all looks a lot less grotty than I remember it in the mid 70s.

[NY]

Out here in a North Yorkshire village there seem to be 11 kV spurs, each with a pole-mounted transformer to step down to 240V. Where I'm sitting I can see the end of the 11 kV line and its transformer. Its 240V cables go underground to houses on one side of the road (probably built in 1950s) and then come above ground to 3-phase 240V overhead wires to the houses on the other side of the road (1930s).

[Harry Bloomfield]

NY has brought this to us :

Are you sure it is 11Kv - more likely it will be 3.3Kv.

[NY]

Ah, I wasn't aware that there was an intermediate distribution voltage between 11kV and 240V. Are most power lines on pairs of wooden poles with big glass insulators and pole-mounted transformers 3.3 rather than 11 kV?

This is the pole

I hadn't spotted the four horizontal wires in the foreground. I was wrong: the 240V evidently goes underground to the back of just one terrace block and then 3 phases and neutral runs between the three terrace blocks, with alternating phases - same as in our older terraces on the other side of the road from the 3.3 kV.

[damduck-egg]

Wonder if H Bloomfield did a typo and meant 33Kv which is used frequently . The supply to our farm was be 11Kv , where we are now is also fed by 11Kv so to say it is more likely to be 33Kv or 11Kv would need access to official records but also where you are gives a bit of a clue, a few houses and a farm out in the sticks 11Kv but a more densely populated area with some industry 33Kv. There is some distribution done at a lower voltage usually in towns and nowadays at 6.6Kv but some supplies that have been in place since the 1920's or 30's may been slightly lower before things got standardised.

G.Harman

[Tim Streater]

I wondered that too but don't know enough about power distribution.

When I first saw this thread title, however, I assumed it was talking about how much leakage there was to earth from a pylon.

[Harry Bloomfield]

NY has brought this to us :

On wooden poles, I would suggest yes..

[Harry Bloomfield]

[e-mail] expressed precisely :

No, I meant 3.3Kv or 3Kv3. So far as I am aware, all 11Kv and up is on metal poles.

They can use what ever is the most cost effective/efficient for the load and distance involved.