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Looking back at major winding refurbishment and upgrade

  • October 2016
  • Number of views: 191
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John Allen
Sheppard Engineering

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Clachan Hydro Power Station (HPS) went into service in 1956. Clachan HPS is located on Scotland’s west coast about 40 miles north of Glasgow. The underground power station is at the head of Loch Fyne sea loch. See Figure 1. The tailrace discharges into the Fyne River, a salmon fishing river. Loch Fyne has a renowned fishery and seafood restaurant within a mile.

The 900 ft (275 m) head vertical shaft Francis turbine driven 50 MVA, 40 MW 428.6 rpm 11 kV generator was designed by English Electric. The generator stator was recored and rewound in 1984 by Peebles Field Services (acquired by Dowding & Mills in 1998).

During the 1984 rewind, the original split core stator was rebuilt as a complete annulus. And the Class B winding was replaced by a resin rich Class F epoxy winding. The turns were insulated with Samicaflex insulation tape and the slot cell would typically have been an S5 mica tape; this is a 180g/m2 epoxy mica tape on a glass fabric. The dielectric stress for the slot cell (wall) insulation was very conservative at 41.9 v/mil (1,650 v/mm).

The rewind used 5 mm (0.197”) epoxy glass wedges with Nomex 410 packing and 4 mm (0.157”) phenolic glass coil separators. The punched slot width was 22.15 mm (0.872”) which would typically have resulted in a built slot width of 21.8 mm (0.858”) and the specified slot cell width was 21.1 mm (0.831”).

As part of Scottish & Southern Electric (SSE) program of power station refurbishment, Clachan HPS was refurbished in 2000. The program included refurbishment of the generator stator with an option to rewind if the partial discharge levels could not be improved. 

Dowding & Mills refurbished the generator stator, re-insulated the rotor and replaced the DC exciter with a brushless excitation system. They also replaced the complete station control systems together with all the low voltage (LV) and high voltage (HV) electrical installations.

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