| Civil works


have been working on the project to replace cables originally installed in the 1970s and which are now coming to the end of their operational life. The first of three new circuits – comprising three 12.2km cables each made from 14 sections joined together and weighing an estimated 1500 tonnes – were installed under sections of main roads in nearby fields and inside Dinorwig. Each new circuit takes a different route from the original cables, ensuring the existing connection can remain in service and continue transmitting vital clean power from Dinorwig while the upgrades take place. The new energised circuits will help create a more resilient power network for the UK, enabling it to better handle sudden changes in power demand and unexpected outages.


Phil Rose, National Grid’s Lead Project Manager for the Dinorwig to Pentir cable replacement project, said: “Completing the first of our three connections marks a really significant milestone in this crucial project to futureproof the infrastructure carrying clean power from Electric Mountain.


“It’s also a fantastic achievement for the team


despite some very difficult conditions, with extremely hard rock and unprecedented rainfall challenging our progress at times,” he added.


Bat protection


As part of its work at Dinorwig, National Grid has also been safeguarding critical bat habitats in the cable tunnels. Dinorwig hydroelectric power station is made up of 16km of underground tunnels deep below Elidir Fawr mountain in Gwynedd. Through the 40-year life of the power station, these tunnels have become an important maternity


and hibernation roost for the rare lesser horseshoe bats and a day roost for Soprano and Pipistrelle bats. Several other species also use the tunnels. At its peak, 191 lesser horseshoe bats were recorded making the site regionally important and of high conservation significance.


Charlie Roberts at National Grid said: “Bats are a


protected species in the UK. It’s amazing to think that the cable tunnels provide a safe haven for so many bats and we are committed to keeping it that way. At Dinorwig several mitigation measures have been put in place over the years and, more recently, have included the installation of a permanent acoustic barrier alongside the maternity roost to reduce any noise and disruption during construction works.


Above right: Marchlyn Mawr, the feeder reservoir for the Dinorwig power station in Snowdonia, North Wales


Below: Overlooking Llyn Peris reservoir at Dinorwig


Above left: Tanygrisiau lower reservoir of the Ffestiniog pumped storage project. © Phil Silverman / Shutterstock. com


Facts and Figures from North Wales


Ffestiniog, one of the first pumped hydro systems in the UK, was opened in 1963 by Her Majesty, Queen Elizabeth II. As the older of the two plants, refurbishment is already underway. Dinorwig, the largest and fastest-acting pumped storage station in Europe, followed in 1984 and was opened by HRH King Charles III. It is regarded as one of the world’s most imaginative engineering and environmental projects, capable of delivering full output in just 30 seconds Situated in a former slate quarry, which is a UNESCO World heritage site, the station itself is built deep inside the Elidir


Mountain in a network of tunnels. The lower reservoir, Llyn Peris, is positioned at the bottom of the quarry, while the upper Marchlyn Mawr reservoir sits higher up the mountain. Dinorwig is capable of generating up to 1800MW of power – enough to supply approximately 1.5 million homes at peak output. The entire facility required the excavation of 16km of tunnels and the removal of 12 million tonnes of rock. The low pressure tunnel line is 1695m, transporting water between the reservoir and high pressure shaft at a maximum of 390M3


/sec. Over 180 people are employed across Dinorwig and Ffestiniog,


www.waterpowermagazine.com | August 2025 | 35


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