[HN Gopher] A train-sized tunnel is now carrying electricity und...
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A train-sized tunnel is now carrying electricity under South London
Author : zeristor
Score : 119 points
Date : 2025-12-20 08:26 UTC (14 hours ago)
(HTM) web link (www.ianvisits.co.uk)
(TXT) w3m dump (www.ianvisits.co.uk)
| zeristor wrote:
| Impressive piece of work, first time I've heard of this.
|
| I had heard that tunnels were a good first step for rolling out
| super conducting cables, but that doesn't seem to be a thing.
|
| Superconducting cables have progressed a lot. I'm assuming that
| setting up a cryogenic system to keep cables cool enough, in a
| confined space wasn't thought to be worth it.
|
| The tunnels look tight enough, and boiling liquid nitrogen from a
| leak could cause asphyxiation I imagine.
| lukan wrote:
| "I had heard that tunnels were a good first step for rolling
| out super conducting cables, but that doesn't seem to be a
| thing."
|
| Yeah tunnels underground would be better for superconducting
| cables, but it is indeed not really a thing as the cooling and
| installing and maintainance would be waaaay more expensive,
| than just using higher voltage. Or if one really cares about
| the loss, use direct current - but we are talking aber very
| small distances here.
|
| If superconducting would be easy, we likely just would have
| fusion plants everywhere with no need for transporting
| electricity long distances.
| ferfumarma wrote:
| > Or if one really cares about the loss, use direct current
|
| I thought direct current had higher transmission losses vs AC
| tallowen wrote:
| AC has higher losses over a transmission wire because of
| the changing magnetic field that it induces which creates
| losses.
| HPsquared wrote:
| There's a cool effect where if you hold a fluorescent
| tube under a high voltage power line, capacitive coupling
| from the varying electric field causes it to light up.
| Some energy is continually leaking out via this route,
| the tube just reveals it. (Magnetic induction too)
|
| https://youtu.be/0D50Dcvzkr4
| afandian wrote:
| You can feel it yourself if you have a piece of metal,
| like a bike.
| LargoLasskhyfv wrote:
| Not if it's really high direct current, like
| https://en.wikipedia.org/wiki/High-voltage_direct_current
|
| Problem is they need these
| https://en.wikipedia.org/wiki/HVDC_converter , for these
| https://en.wikipedia.org/wiki/HVDC_converter_station which
| are expensive. So it depends on circumstances, how and when
| its economically feasible to use HVDC, or not.
| teo_zero wrote:
| Tesla vs. Edison is back! :)
| mr_toad wrote:
| The tunnels are just for ease of maintenance.
|
| https://en.wikipedia.org/wiki/London_Power_Tunnels
| locknitpicker wrote:
| > The tunnels are just for ease of maintenance.
|
| The fact that the tunnels are 50 meter underground leads me
| to wonder if their main requirement comes from national
| security needs.
| defrost wrote:
| London underground is _prime_ real estate - to find a level
| and avoid
|
| * the recent new massive and extensive sewer tunnels,
|
| * the secret basement extensions of the ultra rich, multi
| story archival storage, vaults, etc,
|
| * the new underground tunnels (rail / subway for US
| readers),
|
| * old roman and other buried but still 'conserved'
| architecture,
|
| * crypts, graves, plague pits,
|
| * WWII UXB's etc. etc
|
| is a hell of a 3D needle to thread - there's > 2,000 years
| of urban layering in that small area.
| OJFord wrote:
| And unmapped (and changing, inadvertently diverted)
| rivers
| ant6n wrote:
| There's Probably simply too much city in the way on higher
| levels.
|
| If you wanna knock out the grid, hit the substations and
| power plants.
| everfrustrated wrote:
| The way the regulator regulates capital returns incentivises
| overspending on lavish infrastructure works as a way to
| return more profits back to the shareholders.
| zeristor wrote:
| From Gridlock to Grid Power: The Promise of Superconducting
| Cables
|
| https://ee.eng.cam.ac.uk/index.php/2025/09/22/from-gridlock-...
|
| An interesting article, I'll download the IoP report and maybe
| read it.
|
| But it talks about doing the hard work to improve the
| Technological Readiness Level from 7 to 9. Although these
| cables need rare earths so might be problematic.
| BrtByte wrote:
| I think tunnels like this are more about future-proofing access
| than betting on a specific technology
| quickthrowman wrote:
| > Superconducting cables have progressed a lot. I'm assuming
| that setting up a cryogenic system to keep cables cool enough,
| in a confined space wasn't thought to be worth it.
|
| Yeah, the cost isn't worth it.
|
| Buying two transformers to step up the voltage on one end and
| step down the voltage at the other end is going to be several
| orders of magnitude cheaper than actively cooling cables to 20K
| for their entire length.
| luma wrote:
| Anyone have any idea why the cables are arranged like this?
| https://8400e186.delivery.rocketcdn.me/articles/wp-content/u...
|
| What's the zig-zag pattern for, seems like a fair bit of extra
| conductor.
| probablypower wrote:
| First guess (may be wrong) is to manage thermal
| expansion/contraction constantly on a micro-scale.
| nraynaud wrote:
| I would also add that there some slack for repairs.
| jo909 wrote:
| This view is just very extreme, it is much less zig zag. It is
| just mounted to the wall at the high points and slack in
| between. Certainly there is also a reason for the exact amount
| of slack like thermal expansion.
|
| https://cdn.ca.emap.com/wp-content/uploads/sites/9/2018/04/l...
| markdown wrote:
| A thousand words but 200 of them are bullshit, and they
| didn't even have to use Photoshop.
| defrost wrote:
| There are three reasons:
|
| * Cable thermal expansion under current load:
| https://www.ahelek.com/news/cable-thermal-expansion-and-its-...
|
| * The amount pictured is in excess of that required for thermal
| expansion. The excess is to have some spare length in case of
| modifications. For example if you have to replace the
| transformer and the terminals are not in the same location. You
| cannot extend a massive cable like that easily or without
| degrading its specs.
|
| * The sine wave pattern makes it into AC of course (/s)
| locknitpicker wrote:
| > Anyone have any idea why the cables are arranged like this?
|
| I think that's just cables sagging, which is a requirement to
| accommodate thermal and seismic displacements.
| Reason077 wrote:
| Is that a tandem bicycle? Cute.
| _trampeltier wrote:
| Yes. First time I see a tandem in such tunnel. Until know I
| just saw and used normal bicycles in such kind of tunnels.
| zeristor wrote:
| Tandem bike or a new SCP variant.
| bregma wrote:
| Do you mean the catenary [0]?
|
| [0] https://en.wikipedia.org/wiki/Catenary
| chadcmulligan wrote:
| Maybe to reduce electromagnetic coupling? Seems they're offset
| a bit.
| BrtByte wrote:
| My understanding is that it's mostly mechanical and thermal,
| not electrical cleverness
| nkrisc wrote:
| That's just slack. You're seeing a very long distance with
| extreme foreshortening in this image.
| 1-6 wrote:
| It looks like AI slop to be honest. My second best guess is
| that it could be arrayed transformers.
|
| I don't think a utility company in their right mind would allow
| workers to bicycle inside a tunnel powering the grid.
| ryanjshaw wrote:
| Great photo from an artistic POV, completely useless to get a
| sense of the main subject of the tunnel: the cables carrying
| electricity.
| W3zzy wrote:
| It's the lens distorting the view.
| jimnotgym wrote:
| So Everywhere else in Britain it is 'too expensive' to put cables
| underground. Just goes to show how London centric this country
| is.
| jasoncartwright wrote:
| Sounds like you are trying to compare the many hundreds of
| miles (thousands?) of transmission cables needed to cope with
| the massive geographic change of generation sources to this
| ~20mi cable system.
|
| There are many examples of how the UK is London-centric. This
| isn't one of them.
| VBprogrammer wrote:
| It sounds like sour grapes. London contributes nearly a third
| of the UKs tax income. It has a higher population than the
| whole of Scotland.
|
| Not to mention that over ground wires are manifestly better
| in every dimension except for aesthetics.
|
| This is a great example:
|
| https://youtu.be/z-wQnWUhX5Y?si=qdqrpJ-zS7lh2J8Z
| jimnotgym wrote:
| >contributes nearly a third of the UKs tax income.
|
| Because it contains all of the financial services business
| that screw money out of all of the real businesses in the
| rest of the UK.
| ra wrote:
| I know what you're saying, but maybe there is a tipping point
| when the economics is worth it.
|
| Cool to see cycling down there - much safer than on the roads
| above.
| nkmnz wrote:
| What are the costs of an underground cable in the cities you're
| considering, expressed as:
|
| 1. Cost per kWh transmitted?
|
| 2. Cost per person served?
|
| 3. Cost per pound of GDP generated?
|
| Please provide this for London and the other locations you have
| in mind.
| docflabby wrote:
| London and the south east is the UK in economic terms. Its the
| only those 2 regions which are net contributor to taxes
|
| https://www.ons.gov.uk/economy/governmentpublicsectorandtaxe...
| nephihaha wrote:
| This region is by far the most heavily subsidised in the UK
| in reality, which is confirmed by the number of expensive
| infrastructure projects there such as this one.
|
| London is effectively kept going by these infrastructure
| projects and so many UK government agencies and businesses
| being headquartered there. Even the monarchy plays a role, as
| a massive gravy train mostly based there. All that money
| keeps other businesses in London going. Every time someone
| pays UK taxes in any form they are supporting jobs and
| physical facilities based there. The BBC is another one.
| People throughout the UK are forced to pay a licence fee that
| is mostly used to produce content in and about London.
|
| This is part of a repeating pattern. London took massive
| amounts of resources such as coal, metals and manufactured
| goods from other parts of the UK which are now in poverty.
| The North Sea Oil boom of the 1980s, was used to prop up the
| London stock market, and only a fraction of that money stayed
| within Scotland which was suffering industrial decline at the
| time. (Aberdeen has surprisingly little to show for the oil
| boom and is now a city in heavy decline.)
| jimnotgym wrote:
| It would be nice to have a similar study that corrected for
|
| 1) the amount of wealth claimed to be generated in London,
| that was actually just financial services sucking in money
| from the rest of the UK
|
| 2) The cost of all the pensioners that left London when they
| retired, claiming their pension and health care in a
| different part of the country
| onraglanroad wrote:
| That's because all the wealth of the UK is leached towards
| London and the SE.
|
| Then a small bit of what is leached is paid back in taxes and
| you pretend that means the leeches are subsidising the actual
| workers.
| zeristor wrote:
| Manchester is burgeoning, still someways behind London.
|
| It'd be super, smashing, great! for the cities to be far better
| connected together across the Pennines.
| walthamstow wrote:
| You're lucky to have us mate. London is the only thing going
| for this country. We should be betting /more/ on London.
| immibis wrote:
| On the flip side, that's mostly because London takes all
| their wealth and crowds out wealth generation
| walthamstow wrote:
| People move to where the jobs are. That's how most English
| towns came to exist in the first place in the industrial
| revolution.
|
| We're ~30 years into a new information/digital revolution
| and London is a world centre of it. There's plenty of
| wealth generation happening. People are welcome to sit and
| wait for it to come to them if they want.
| jimnotgym wrote:
| Can you give me some examples of wealth generation from
| London?. You can exclude the massive amount of Financial
| Services that they shout about so much, that is just a
| way of skimming wealth of real businesses.
| walthamstow wrote:
| Oh very cute you've got special criteria about what's a
| real business and what isn't. Any other rules I need to
| be aware of?
|
| Go to the last HN hiring page and search for London.
| You'll find plenty.
| nephihaha wrote:
| The UK is run as a city state for London's behalf.
|
| The odd thing is that it makes fun of all those coal mining
| and oil producing areas whose wealth it has been only too
| happy to steal. A sort of internal colonialism.
| KaiserPro wrote:
| Yes.
|
| And I wish people would understand how costs work.
|
| Pylons need space right, they also need maintenance corridors
| and access. Every ~360m you need a space to put a pylon[1]. Can
| you imagine the cost of buying 400m2 every 360 in zone 1?
|
| what about the scaffolding when you need to re-string the
| cables? can you imagine how expensive that would be? what about
| if a lorry smacks into it? Its just not practical.
|
| I grew up in norfolk, next to a bunch of HV pylons. No-one
| commented on them, because they were always there. THey are
| going to put some more in, and suddenly "its a blot on the land
| scape" and its "ecological damaging" Then its proposed that the
| cables are buried. apparently a 200 meter clearing 30km long is
| more ecologically friendly than pylons ever n hundred meters.
|
| but thats an aside.
|
| [1]https://www.quora.com/What-is-the-distance-between-
| electrici...
| jimnotgym wrote:
| I wish that people understood how costs work? Does that count
| as polite conversation in London? It is about typical of how
| Londoners sneer at the rest of the UK.
|
| London is the place that costs don't work. It is full of
| things that don't need to be there, driving up the real
| estate prices. The port closed decades ago, so let's move the
| insurance and FX markets somewhere cheaper? Let's move out
| the government departments too.
|
| Modern London is all about MPs who are physically tethered to
| Westminster by in person voting, being surrounded by the kind
| of people who want direct access to MPs...
|
| All of the companies who specialise in creaming money off
| real companies gravitate towards that pig trough. Today they
| need data centres as close as possible, so they can cream
| money off faster. And we now have to feed those parasites
| with extra power lines.
|
| Don't lecture me on costs, while London continues to try so
| hard to inflate the price of the tidal mud it resides on, by
| sucking in the rest of the UKs wealth.
| bboreham wrote:
| FX trading has been completely online for about 20 years.
| jimnotgym wrote:
| You would think so, wouldn't you? Then why is the FX
| department at my bank based in London? As a mid market
| client they occasionally used to send one of them out to
| see me. They told me all about their office and team in
| London...sat in front of screens. Their system aggregated
| the banks position onto the screen of the master trader
| for each currency, sat at a screen in the next office.
| What utter waste
| KaiserPro wrote:
| > Does that count as polite conversation in London?
|
| no, because this is mostly a logical fallacy.
|
| > It is full of things that don't need to be there
|
| Like what specifically? Most heavy industry moved out in
| the 70/80s, the port moved to essex in the 80s. The fish
| and meat veg moved out to canary wharf, and are going to
| move further out soon. what else should be taken away?
|
| > Today they need data centres as close as possible,
|
| This is a cable replacement, but yeah, lets go with new
| datacentres in london (as we know planning is quick and
| simple, so they are popping up weekly....) However the key
| issue is that population density is now back to somewhere
| like it was in the 40s
| (https://trustforlondon.org.uk/data/population-over-time/)
|
| So whilst power demand is largely flat, infra doesn't last
| for ever, so even power cables need to be replaced.
|
| > Modern London is all about MPs who are physically
| tethered to Westminster by in person voting, being
| surrounded by the kind of people who want direct access to
| MPs...
|
| Yup, all 8.9 million people that live there are entirely
| there to do lobbying. I to am a lobbyist.
|
| > Don't lecture me on costs, while London continues to try
| so hard to inflate the price of the tidal mud it resides
| on, by sucking in the rest of the UKs wealth.
|
| Well, as you've neatly avoided any mention of costs of
| tunnel vs pylons, its not really a lecture is it. I hope
| you come to peace with the general concept of london.
| 7952 wrote:
| Well they are put underground sometimes when there is a sound
| reason to do so. But mostly there is not a good reason to do so
| across hundreds of miles of agricultural land.
| nopurpose wrote:
| Cables on overhead high voltage lines are mounted using stacks of
| ceramic insulators, but here they seemingly just sleeved in some
| protection and hang on a tunnel wall. Why is that?
| jo909 wrote:
| Overhead high voltage conductors are not insulated with a
| coating, probably for many reasons but certainly for cost and
| heat dissipation.
|
| That means the path through the air to some conducting
| materials needs a certain distance, and that even when wet or
| iced over or whatever can happen up there.
| VBprogrammer wrote:
| Overhead conductors use air as the insulator. Underground
| cables use an insulating jacket. In the past it was really
| difficult to build cables with voltage ranges in the 10s of
| thousands of volts without additional complexity like a
| dielectric oil being pumped through the cable. I think modern
| dielectrics are significantly better though.
| quickthrowman wrote:
| Modern cables with XLPE insulation can handle very high
| voltages without active oil cooling, here's a 345/400kV rated
| underground cable assembly rated for 90C: https://assets.sout
| hwire.com/adaptivemedia/rendition?id=332f...
| VBprogrammer wrote:
| Yeah, the wires in the new London tunnels are XLPE. Despite
| being first used in the late 60s it took a long time to be
| commonly used. Though much of the infrastructure around is
| still very old.
| KaiserPro wrote:
| Cost, mainly
|
| The cost of oil insulated cables that can do 132kv is about
| PS900 a meter. Whilst there are HV cables that exist on the
| outskirts of london, they are much rarer in zones 1-3.
|
| I assume that the cost of pylons with raw cables is much much
| cheaper. The problem is planning permissions, physical
| clearance. planning permission and now one wants to live near
| HV cables (that they know of. There are a bunch of 33kv cables
| buried outside posh people's houses in zone 5, and a bunch in
| canals.)
| BrtByte wrote:
| Overhead lines need big ceramic stacks because the air is the
| insulation. In tunnels, the insulation is in the cable itself,
| and the tunnel just provides structure, cooling, and controlled
| geometry
| quickthrowman wrote:
| The tunnel is too small to use air as an insulator so they use
| cable assemblies with multiple layers of insulation.
|
| Here's some high-voltage cable spec sheets that show a cross-
| section of the assembly for voltages above 69kV:
| https://www.southwire.com/wire-cable/high-voltage-undergroun...
| metalman wrote:
| essentialy no choice in putting infrastructure underground as the
| cost's and delays in putting a power corridor right of way
| through is unthinkable, they will almost certainly be useing old
| established locations for transformer substations that have the
| required set backs and other services, which must from time to
| time, come to the frenzied attention of developers, agast ,that
| they cant relieve someone of this "vacant" land
| BrtByte wrote:
| So tunneling isn't just an engineering choice, it's a planning
| survival strategy
| nephihaha wrote:
| "In total, the PS1 billion London Power Tunnels 2 (LPT2) project,
| which began in 2019, spans 32.5km across seven South London
| boroughs from Wimbledon to Hurst."
|
| In spite of devolution and the so called "levelling up" programme
| for other parts of the UK, London obviously continues to be
| heavily subsidised by the rest of the UK.
| EmbarrassedFuel wrote:
| "London obviously continues to be heavily subsidised by the
| rest of the UK"
|
| This is a farcical comment. Were you being sarcastic? The tax
| revenue from London massively subsidises the rest of the UK.
| The investment happens in London because you can guarantee it
| will make a return, and quickly.
| nephihaha wrote:
| The real reason London is rich at all is because it was a
| trading depot with the continent. It made money from goods
| leaving England, and entering England. Later on, like Paris,
| it became wealthy off running an overseas empire, and when
| that empire vanished it turned to nearer territories.
|
| London has centuries worth of investment from everywhere else
| based on that. That money has stayed there, and money is
| spent constantly on infrastructure which helps it make more
| money. Contrast this with Liverpool, Cardiff or Belfast which
| suffered decades of decline for various reasons and a
| fraction of the investment.
|
| If the capital had been moved to Liverpool back at some point
| in the Middle Ages, then that would have remained a wealthy
| city instead of becoming a basket case in the eighties. The
| presence of the civil service and government alone would have
| kept Merseyside wealthy, and would have made it a huge
| tourist centre. Bigger than now, and even that was mostly to
| do with the Beatles.
|
| By the way, the state funded Wembley refit cost more than the
| construction of the Scottish Parliament. Guess which one got
| all the negative press?
| nephihaha wrote:
| Let me put this another way. If I got given a very well paid
| civil service job, I would end up paying a lot of tax in
| return. And if someone paid for my house to be renovated and
| build the best utility and transport connections, then the
| value of it would go up.
|
| And if mass media continued to promote my area continually
| then the value of my home would also go up. I would get given
| higher wages to cope with the increased cost of living there.
| We would get more tourists visiting my area, and firms and
| non-doms would set up there because of the positive image.
|
| Much like London.
| hshdhdhj4444 wrote:
| You're only explaining potential reasons why London is
| generating higher value.
|
| Which is still implicitly accepting the fact that London
| does create more value that then goes to the rest of the UK
| rather than the reverse.
| BrtByte wrote:
| No flashy product launch, no app, just "we quietly replaced a
| 1960s failure point before it failed."
| 9NRtKyP4 wrote:
| It's not just a tunnel, it's a game-changing lifeline
| swarnie wrote:
| I'm shocked it only took five years and only cost 4 month's
| worth of bomalian budget.
|
| Great job whoever was in charge.
| zimpenfish wrote:
| I'm slightly confused by the "New Cross substation in Southwark"
| whilst the map immediately above clearly shows it in
| Lewisham[0][1].
|
| Best I found was a page talking about vertical boreholes on Old
| Kent Road (opposite Commercial Way which is just inside
| Southwark) but nothing about a substation there on any page.
|
| [0] Estimating off the National Grid map, it's roughly vertically
| centred between Lewisham and Greenwich DLR stations - absolutely
| not in Southwark!
|
| [1] I feel like I'm going mad - the number of pages I found
| whilst trying to find the exact location that said the same thing
| under the same map is honestly discombobulating.
| fanf2 wrote:
| It's easy to find on https://openinframap.org
| zimpenfish wrote:
| If only the search engines had bothered to return that as a
| suggestion!
|
| But yeah, that's the place on OKR I mentioned - just inside
| the Southwark boundary, not really New Cross. Nowhere near
| the dot that nationalgrid have on their map either.
| sowbug wrote:
| I'm looking forward to the next Amendment to the 18th Edition's
| conduit-fill factors.
| blackhaj7 wrote:
| Anyone know why the power cables in the image zig zag up and
| down?
|
| Seems like it would require more cable than a straight line so I
| am guessing there is a reason for it
| dmoy wrote:
| https://news.ycombinator.com/item?id=46334725
|
| TL;DR
|
| Camera makes it look more than it is, it's mostly just sag
|
| If you want to see electrical with significant zig zag, open up
| the wall of a house that was built without very detailed plans,
| but still hired an electrician with a lot of prior experience
| being told to move stuff after the fact. They just zig zag it
| like crazy under the drywall, so there's an incredible amount
| of slack to pull wire to new and exciting unplanned locations.
| 7952 wrote:
| My guess is that the droop is just due to gravity. And it is
| easier to affix in fewer places. But the style of photography
| exaggerates the effect.
| sokka_h2otribe wrote:
| Some good reasons:
|
| -gotta hang them somehow, and in a very controlled way.
|
| -thermal expansion, very important not to cause axial strain on
| a cable, which happens on tight bends.
|
| You might think it would be enough to just have slack
| "somewhere" but I think you get to have many many micro
| adjustments when you have it across the entire length.
|
| Why don't HV telephone lines do this?
|
| I have no idea. Maybe because they can hang and droop more
| easily. I hope someone more knowledgeable gives a real answer.
| Neil44 wrote:
| Things like this are useful when for example you want to charge a
| fleet of electric busses overnight.
| mfateev wrote:
| That sounds like an excellent use for narrow boring company
| tunnels.
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