Electricity Substations

[Tim Jenkins]

Just a question out of curiosity really.

We had our local 11Kv substation transformer replaced - the old one was 30 years old and decided it had had enough and blew up !

Southern Electric turned up after a couple of months with a large crane and replaced it with a refurbished one. (they had "back-fed" us from a nearby transformer in the meantime).

The bloke who installed the new one said it was rated at 400 amps on the Low Voltage side.

Considering there are about 50 or so houses in our street (each with a

100amp suppliers fuse) and 1 transformer - am I right thinking that we can only use about 8 amps each before problems start ??

Also, when we were "back-fed" from our next nearest substation, which presumably was a similar 400 amp transformer, this meant it was supplying twice its normal load, so presumably 4 amps each ??

Do they Electricity companies rely on us all not having an electric shower or cooking the Sunday roast at the same time ?!

Like I say, just curious !! How does it all work ??

[Wanderer]

Whilst individual houses may have a peak instantaneous demand of anything up to 20 - 25 kva, a notional maximum curent of 100 amps, in reality people don't have everything switched on at the same time, and thermostats start cycling in and out, so the real of after-diversity maximum demand for a house may not be anymore than perhaps 8 to 10kva.

Taken across an estate of 50 houses, this effect also happens between different properties, and the design admd for each house will actually be as low as 1.5 to 2kva per dwelling.

So what about Sunday lunch, when everyone's cooking? The distribution cables and mains fuses can withstand short-term overloads without any deterioration in insulation.

[Andy Dingley]

Yes. Search around for the term "diversity"

-- Smert' spamionam

[Colin Wilson]

Happy days - I used to work on HV plant, and they can make a mess :-p

We used 500kva trannies in ground mounted subs, which give out ~633A per phase on the LV side IIRC (gawd, it`s been ~10 years since I played on the network !)

Interconnection of the LV network is quite common, and you may find you were actually backfed from several subs, not just the one. This has the great advantage of less down-time if fuses blow (most of the time you`d never know), but faults can be a b*tch to track down :-}

[Sparks]

Just out of curiosity, how far apart are subs roughly?

Just thinking, if the LV side is interconnected, then they must use some pretty hefty cables to do so!

Sparks...

[Mike Harrison]

That is probably per phase, so output would be 3x that

[Chris Oates]

Which reminds me of reading about systems used in the USA on remote distribution - they have banks of 4 pole mounted fuses that are explosive - the fuse that fails explodes (gas) which closes the next one (short delay) and hopefully clears a fault such as animal intrusion or falling tree.

[The Natural Philosopher]

By statistics probably.

Only in Italy do they turn everything on at the sam time to blow th national grid :-)

[The Natural Philosopher]

There is certainly some such system in place in the UK, as we get a lot of trees down and lightning strikes on teh overheads here, and its very usual to get a cut, followed by after 3 seconds a flicker and maybe another cut...up to 3 times.

It is almost guranteed to wipe the OS and crash the disk of an auto booting PC if it goes all the way.

I had assumed it was more sophisticated that blowing fuses - some sort of overcurrent trip and a few time delays...but maybe its just fuses...

[Colin Wilson]

That varies on the amount of load in the area - over 250 in a single grid square in one area, and maybe one or two in others. Not all will be HV equipped, but many are.

185 / 95 probably accounts for most of it I think...

The interconnection allows for smaller cables to be used AIUI rather than larger, as the load can be fed from a couple of directions - a bit like a ring main. Can you imagine the volt drop on a long radial otherwise ? :-}

[Colin Wilson]

It depends on a few things - yes, autoreclosers are fairly common in overhead line areas for the HV side (AFAIK the LV side is only ever protected by fuses), and elsewhere there are overcurrent relays and other devices in use to figure out of there`s a fault.

A lot depends on the type of HV network - a Y type ring (of subs) is only switched at the remote ends (the TX itself might have fuse protection), whereas an X type ring can switch each sub independantly, which will mean less people go off supply in the event of a fault.

[Andy Wade]

To add to Wanderer's answer: that will be 400 A on each of the three phases (a 315 kVA transformer I assume). If it were only feeding your street then you'd be very generously supplied (6.3 kVA per house). The chances are it's feeding a wider area than that.

Ours has recently been replaced too - a nice old brick building with a Mansard roof replaced by a horrible plastic (GRP?) module plonked straight down on the street front, making something of a mockery of planning. They've sold off the land that the old substation stood on for development. Two observations: (1) the supply voltage at times of light load is still around 240 V - so they're not reducing to 230 V when new transformers are installed; (2) the change-over from old to new involved no interruption of supply, which I take to imply live cable jointing on both the LV and MV sides. Working on live 11 kV cables (6.3 kV to earth) in a damp hole in the road must be 'interesting'.

[Dave Liquorice]

A primative auto-recloser, works fine until the last fuse has gone but how do you know when your on the last fuse?

Auto-reclosers are very common over here, saves an awful lot of work for engineers coming out to reset/replace a single shot overload device. The one on our 11kV feed tries to reconnect 3 times over around a minute, after that it gives up and leaves the circuit open.

This works fine for preventing short term overloads such as trees touching the line or nearby lighting strikes taking out the supply for hours at a time.

[Christian McArdle]

They can't really change the voltage. Incandescent lightbulbs are still sold according to the likely actual voltage, not the nominal 230V. So UK lightbulbs are designed for 240V and European ones are designed for 220V. By changing the actual supplied voltage, lightbulbs will be either inefficient and dim (UK), or blow too quickly (Europe).

Christian.

[Andrew Gabriel]

Ring mains are used too -- that's actually where the term comes from. (The one in your house is really a 'ring circuit' or ring final circuit'.)

[Andrew Gabriel]

I recall reading a report of the 1965 US blackout some time ago. One of the issues identified in that was the lack of autoreclosers in the US, verses UK where they were common even back then on HV distribution.

[Dave Liquorice]

Probably not there will almost certainly be more than one way of feeding the area. By closeing normally open switches and opening normally closed ones there will be no supply interuption but the kit they are working on will not be live.

[Tim Jenkins]

(2) the change-over from old to new involved no interruption of

Same with us. There was no interruption to service when they did the changeover. The incoming 11kV feed came via some sort of switchgear which isolated the transformer from its feed. They removed the cable between the dead side of the switch and the transformer. Then they removed the 3-phase

415v feeds from the transformer LV side. Along came the crane, winched out the old transformer, dropped the new one in, put a new red cable in from the switch to the HV side of the new transformer and re-connected the LV side.

Then they energised the transformer by throwing the switch but we were still back fed, then they removed the back feed by inserting some fuse links on the LV side which I think meant we were effectively fed by both transformers simultaneously, then they went round the corner to the other transformer and removed some links.

Whilst we were back fed, we were getting about 230volts, but when the fuse links were re-instated, it jumped up to 244volts.

Tim.

[Colin Wilson]

Often known as an RMU - ring main unit

[Wheelbarrowbob]

All UK 275 KV and 400 KV circuits have delayed auto reclosers ( which can be taken in and out of service remotley ) which will attempt to restore power to a tripped circuit 3 times in a matter of seconds, but I am unsure of the RECs protection at 132 KV and lower, although I know Aquilla in the Midlands are installing DARs on some of there 11KV pole lines as opposed to the old drop out fuses