Cwm Dyli hydro-electric power station was built in 1906 and was remarkable as the first instance in Britain of the use of Alternating Current. The power station was built by the North Wales Power and Traction Co, and was conceived to serve the Oakley, Dinorwic and Pen-yr-Orsedd quarries. The company had been established in 1904 by Platt and Tomkinson; the electrical engineers Bruce Peebles of Edinburgh received the contract for the turbines with Harpur Bros. as consulting Engineers. Current was derived from harnessing the water from Llyn Llydaw and energy was transmitted at 10,000 volts. The power station was much lauded in its day as a 'triumph of modern transmission' and with its bold basilica-like design soon became known locally as 'the chapel in the valley'.

Source: Cadw list description
RCAHMW 2008