Places To Visit: Electric Mountain

The experience of being a teenager leaves a host of memories, of social awkwardness in the difficult process of not quite being a child any more, of tedious school days, and of team sports seemingly enjoyed only by the few. Wherever in the world you grew up will have lent a particular flavour to your recollections of that period of your life, whether your memories are good or bad.

One surprising common theme in British teenage memories, at least those of a few decades ago, are power stations. In the 1970s and 1980s, the Central Electricity Generating Board had a PR effort that involved bringing parties of teenage school geography students in for a tour of their local electricity plant, so if you talk to a British person of a certain age you’ll probably find they’ve been up close and personal with a coal-fired power station.

The true power station marvel of the age would have been too far away to tour for most kids at the time, though our geography teachers expounded on it at length. Dinorwig pumped-storage power station in Wales was opened in the early 1980s, and is a hydroelectric plant that uses excess grid generating capacity in the middle of the night to pump water into a lake at the top of a mountain, from which it can later be released at very short notice to respond to demand surges in a matter of seconds. The oft-quoted example is that when an episode of Coronation Street draws to a close there are several million British kettles turned on simultaneously, at which point Dinorwig comes online to rapidly make up the resulting shortfall.

As teenagers we were too far away for our school to make the trip to see Dinorwig, but happily as an adult there is no need to rely on a geography teacher’s abilities with a Transit minibus. The station markets itself as a tourist attraction, Electric Mountain, or Mynydd Gwefru in Welsh (Welsh pronunciation help). There are pre-booked guided tours, so it was in the car and off up to North Wales to finally see the subject of those geography lessons from decades ago.

There is an 1800MW power station in this picture.
There is an 1800 MW power station in this picture.

The station itself is almost entirely underground, it lies inside the Snowdonia (Eryri in Welsh) National Park, and thus its construction was required to impose minimal visual impact. The Dinorwig slate quarry is an abandoned industrial site on the slopes of Mynydd Elidir Fawr, and the power station itself sits almost entirely inside the mountain with only some unobtrusive structures on the surface at the bottom of the quarry’s terraces. Besides this, the other parts that are visible are the upper reservoir, the mountain top lake Marchlyn Mawr, a surge pond at the top of the high pressure downward shaft, and the lower reservoir, another lake, Llyn Peris.

The company operating Dinorwig runs a visitor attraction offering tours of the station from a visitor centre in the nearby village of Llanberis, and it was one of these that allows us to describe its workings in more detail. So it was that on a damp June day we found ourselves queuing up with a group of fellow visitors, ready to head forth into the mountain.

It’s a tour aimed at the general public rather than engineers, so the first port of call on the tour is a film explaining  some of the history of the quarry and the plant, and giving an overview of its operation. There is a real sense of pride in the achievement that comes forth in this video: there are other hydroelectric stations and even other pumped-storage stations, but nothing quite like this one. Then the tour moves to a waiting bus to take you into the mountain, with a stop to deposit belongings in a locker; bags, phones, and cameras are prohibited underground.

A cutaway view of the Dinorwig complex.
A cutaway view of the Dinorwig complex.

Beneath the surface, there are 16 km (about 10 mi) of tunnels in the complex, and some of the largest man-made underground caverns in the Europe containing the valve gear, turbine and generator assemblies, and transformers. And when we say tunnels it is important to understand that these are not stoop-unless-you-are-a-hobbit sized tunnels, but huge brightly-lit tunnels containing full-size roads, large enough for specialist haulage trucks to have delivered monster-sized power station transformers and other machinery into the middle of the mountain. The tour bus is an ordinary road vehicle, looks small in the tunnels. The rock from which they are cut is the same slate that originally brought the quarrymen to Dinorwig, we are told that since it is a brittle material all the surfaces have been given a spray concrete treatment.

One of Dinorwig's massive inlet valves.
One of Dinorwig’s massive inlet valves.

The first port of call for visitors is the inlet valve cavern. The water from Marchlyn Mawr, 568 metres above us and to our left, emerges here into six hydraulically operated valves, one for each pump/turbine. The valves themselves are straightforward ball valves as you might even find in your home plumbing, but such a description does not convey their size. If you could imagine a casting in which an elephant could comfortably take shelter, you wouldn’t be far from the mark. Each valve is operated by hydraulic rams working on a pair of arms with 16-tonne counterweights, these can be opened to their half-way point within five seconds and closed from full flow within twenty seconds. The viewing platform is at one end of the cavern with the valves stretching away into the distance, in our case we were lucky that the valve closest to us was in the open position.

One of the Dinorwig pump/turbine assemblies from above.
One of the Dinorwig pump/turbine assemblies from above.

A short walk through a connecting tunnel brings visitors to the machinery cavern, to a viewing platform immediately above one of the pump/turbine assemblies. These are Francis turbines, mounted vertically with the shaft going upwards to the motor/generator assemblies unseen on a level above us. The water enters the turbine from a spiral tube surrounding it, and the turbine is controlled by a set of moveable vanes. From the viewing platform the hydraulic motors to control these vanes are clearly visible, as are the lubrication systems for the massive bearings in which the shaft sits.

On our visit, we were lucky that the turbine closest to us was running, as its corresponding valve in the previous cavern had been open. Its shaft was spinning before us, and in close proximity to a 300 MW turbine the noise level was high, though not uncomfortably so, and with a sense of hot lubricating oil in the air. The viewing gallery is as you might expect separated from the machinery by the appropriate distance and handrails, but it was very pleasing that this was not to the extent that it obscured any of the machinery or its subsystems from view. There is plenty here for an engineer or other mechanically minded person to study, your only problem is likely to be that the rest of your group will tire of the spectacle before you do.

One of Dinorwig's six generator/motors. Sadly the tour can't show you these.
One of Dinorwig’s six generator/motors. Sadly the tour can’t show you these.

The tour then boards the bus once more for a trip to the top level of the machinery cavern, which sadly for us leaves out the generator/motors, and transformers. We can understand that high voltage generation and tourists would not make a good combination. It is however worth describing this part of the pant even if we were unable to see it for ourselves.

The generators produce 18 kV AC at the UK line frequency of 60 Hz synchronised to the grid. This is converted through a transformer to the grid transmission voltage of 400 kV, then sent through an underground transmission line to a substation a few miles away. Dinorwig’s very fast start-up time is achieved by keeping some turbines constantly spinning in compressed air using power from the grid, in this way they avoid the extra wait to spin up the very heavy shafts to operating speed when the valve is opened. The station also has a set of diesel generators to allow the turbines to be spun up in the event of there being no grid power, this would allow it to be used to restart the UK grid in the event of a complete power loss.

The Dinorwig generator cavern.
The Dinorwig generator cavern.

Returning to the tour, at the next stop you leave the bus to emerge on a viewing platform at the very top of the generator cavern. Visible are only the covers of the very top of each generator, their real workings are out of sight a level below. The six generators stretch away into the distance, with a full-height cavern that would dwarf the naves of most cathedrals to accommodate a pair of maintenance cranes. The roof sports a pair of curved shields to catch any falling rocks, we are told that these were designed by the British TV personality [Carol Vorderman] in her pre-media career as an engineer. Here you gain a sense of the size of the construction more so even than in the valve cavern, because of the commanding viewpoint and the lack of distraction from machinery close-up.

A picture of part of the construction at the Dinorwig site.
A picture of part of the construction at the Dinorwig site.

The tour then concludes with another short film in an underground cinema, wrapping up what you have just seen and talking about the environmental impact of the station and its construction. Then it’s back to the bus to return to the daylight, concluding with a drive over the curved bridge spanning the station’s tailgates into Llyn Peris.

It is difficult for an attraction aimed at general tourists to maintain enough of an interest level to captivate an engineer, but in that aim the Electric Mountain tour succeeds admirably. It is extremely rare to find yourself at this proximity to industrial plant of this nature, and rarer still to do so with a plant that is very much in front-line operation. Thus it is an attraction that most Hackaday readers are likely to find extremely interesting, and should be well worth adding to your itinerary should you find yourself on a British holiday. Tours must be pre-booked, but that is easily done either online via their website, or in person at the visitor centre.

The Snowdonia region is one of the more spectacular parts of the British Isles, and as a visitor to that part of Wales you will both see and hear the Welsh language in everyday use. If you are used to the tourist hotspots of London or other cities, you are likely to find it to be (quite literally!) a breath of fresh air. If you need to convince your family members that the extra distance is justified, take a look at the North Wales tourism website for the area’s other attractions. And if as I did a few years ago you come away with an interest in learning Welsh then I can recommend trying an audio course. Enjoy your trip!

Our thanks to the Electric Mountain staff for the tour, and to the Electric Mountain visitor centre for granting us permission to use the underground images in this article.

62 thoughts on “Places To Visit: Electric Mountain

    1. In the 90s I had the privilege of taking a tour at a nuclear power plant here in the US as part of a freshman level college class. Have the lawyers since killed even that off?

        1. Not sure you are allowed beyond the visitors centre these days ?
          Back in the 80’s when I was in the Cub Scouts we managed a tour of a UK AGR Nuclear generating facility, including walking round the top of it where the rods are inserted, turbine hall, control rooms, the full works.
          Different times…

          1. Drax, burning biomass that we import from US and Canada – it would be interesting to see if the transportation footprint makes it worthwhile burning a “sustainable” feul.

      1. Back around 2007 while I was in college we got to tour a local coal-fired plant; including standing at the top beside the “oh shit valves” that served as the emergency pressure relief for the steam. I have a suspicion that our professor had some industry perks, however.

        In Scouts I got to go to a nuclear plant here in Kansas, but if I recall right we were limited to a visitor center which was located a short distance off site. They did, however, have the old control room set up for people to play in and try to “break” the reactor. I got parked at the station that had manual control over some valve and a multi-turn pot that I couldn’t find the end of over the two minutes we were given.

        Once reset, they kindly asked me not to play with that one again as it took too long to put back in operating position.

  1. Dang it! I was right there a couple years ago, but didn’t go in. There’s some old ancient watchtower ruins nearby that I saw it from, but just assumed it was a slate mine, not knowing what lay beneath. We spent a morning in Snowdonia; went up the train to the summit since we didn’t have 3 days afterwards to recover from walking.

    If I go back one day hopefully I will have more time to look around. I was born in Chester, and my wife did a semester of college in Bangor, so I suspect we will make the trip across to Old Blighty again at some point.

    Thanks for the tip!

  2. Jenny, you write the most in-depth and best researched articles on here. Also, unlike a lot of other contributors, you actually proof read your material. This is excellent, very well done and I enjoyed reading it.

      1. “It is however worth describing this part of the pant even if we were unable to see it for ourselves.” I’m guessing it’s plant? :p
        Nice article, I too have had the pleasure of visiting.
        The national slate museum near by is also worth a visit, not just slate there were engines too!

    1. Actually now I’m reading I keep finding odd bits:
      “It is however worth describing this part of the pant even if we were unable to see it for ourselves.”
      Plant not pant

      “Its shaft was spinning before us, and in close proximity to a 300 MW turbine the noise level was high, though not uncomfortably so, and with a sense of hot lubricating oil in the air” I lost the plot a little here

      All of this is not meant to be disparaging. I love reading about huge power stations, or any huge machinery to be honest and would much rather have the article in its current state than no article at all.

  3. Quote:
    “Dinorwig pumped-storage power station in Wales was opened in the early 1980s….”
    There is another one (possibly the same one with foorestry grown since the pictures here?) around the devils bridge area and aberystwyth ways….
    Over the otherside of the mountain is a HAARP facility, brothers’ been there and they’re open to the public just like the water storage dam has a public open building.

    Thus that proves the “HAARP conspiracies” to be a pack of lies… by guess who???…..
    Mostly Religion pushers in tin-foil clothing (i.e. sheepin wolves clothing)!


    (Keyboard needs fixing, spelling issues may not been caught)

    1. Here it is new age hippies, UFO and Bigfoot and Shadow People enthusiasts, and Chem Trail, Gluten, and evil Monsanto/oil/nuclear power/”processed food” Hollywood ninnies.

      HAARP fits best with the “NASA faked the Moon landings” people.

    1. Not strictly correct either. 50Hz however bang on.
      The EU harmonises power at 230V AC since 2003, however the UK nominally uses 240V and Europe uses 220V.
      There is an allowed deviation of +/- 10% @ 230V which covers the gap.

      But appliances can be designed to work at 220V, 230V, or 240V, and thus the upper and lower voltage range bounds will be set by that design voltage target.

      Obligatory XKCD:

    2. Hz should be 50 but the long line transmission voltage is typically in kilovolts to reduce transmission losses. It gets stepped down a lot for dispersal to buildings, then each building and home, or groups of them, gets it stepped down in volts again with more transformers.

    1. My 3 guesses:

      1) Flash photography may cause temporary blindness, people might trip, engineers working might also have an accident – why take a risk.
      2) Because terrorism – makes it harder to plan an attack without photos. – why take a risk
      3) Mobiles – Because the equipment predates mobiles and perhaps isn’t tested (certiified) against RF at that frequency, and they have cameras. – why take a risk

      1. *throws phone at window* “There was suppose to be an earth-shattering KABOOM?” But really even when I toured a power plant, either there were a lot of people around so such a terroristic act wouldn’t have been possible, or too far away even with a good pitching arm to make a difference (turbine hall was a lot warmer than I expected, and loud too).

  4. For those in / visiting the US, if you find yourself in Vegas, take the ~1hr drive to the Hoover Dam. They have a tour of the the passages within the dam, and the generator hall (from a viewing platform). Would have loved to get a more in-depth tour, but still cool to see.

    1. Unfortunately, they discontinued the extended “hard-hat” tours after 9/11. I was lucky enough to go a year or two before that. Fascinating place and surely worth the tour regardless of length.

  5. My Mum’s family are from Betws-y-Coed in Snowdonia, so I’ve visited that area many times, but never thought to visit Dinorwig. The nearest I’ve been is the Trawsfynydd Magnox power station. That was in the early 90s soon after it was shut down. There were no other visitors that day, so I got a personal tour with the stations Medical Officer, who was not only a medical doctor, but also a physics PHD! It was fascinating, and is the 3rd tour of a nuclear plant I’ve managed to get on. Back in 1960s I went on a school trip to Calder Hall, the first commercial nuclear power station. In 1983 I visited Sizewell, another Magnox, on the Suffolk coast. Again, being midweek, I was the only visitor and got a personal tour. At the time only the A station was in operation, but the B station was in the planning stages and some ground had been prepared. Sizewell B is the only PWR in the UK. A C station is planned.

    1. Yes! I’ve visited both Dinorwig and Cruachan. Cruachan is smaller and the tour I went on was less extensive: the raceways and valve gear are not accessible but you still get a good view of the turbine hall. Definitely worth a visit if you’re in the area though.

      Like Dinorwig, the Cruachan plant is almost entirely underground and the fact that the whole space is mined out of granite is impressive in itself.

    2. I was a bit dissapointed by cruachan. the view of the turbine hall was ace, but I just wish they showed a little more, you could see loads of cool stuff thro windows in doors as you passed, if they just had a small chained off area so you could stand just inside and look that’d have been amasing. That said, if your in the area its worth stopping in.

  6. This is just like Grand Coulee near me in Washington State. It is also a pump/plant style, but in addition to that the lakes generated are used as irrigation for eastern Washington.

    To those wondering, tours are much more limited after 9/11, but you can go on limited tours. Coulee even does a night time laser show. If you are in the power sector you can apply for a work sponsored trip deep into the plant (like the old tours).

  7. Finally! Got an entry for my bucket list. I’ve GOT to see this (and I’ve always wanted to hear Welsh spoken up-close-and-personal). I think I’ll add a second item because of the proximity factor: a ride on the steam-version Flying Scotsman.

  8. Before I was even a teenager (12 I think), I did a tour of the Snowy Hydro Electric Scheme here in Australia. Multiple dams, powerplants, including underground, and one I think was just such a pump-up, generate-down. Very impressive indeed, even as a 12 year old who didn’t understand much of it. Sadly it seems that tours of such facilities are now a thing of the past, due to (1) liability and (2) terrorisim.
    I am saddened that my kids (of whom are now 10 and 8) will probably never get to see this sort of thing.

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