The St. Margaret's Bay Hydroelectric System consists of three hydroelectric power plants (two collocated in the same building) and the related lake, dam, and river systems driving them. The system is situated within both Lunenburg County and the Halifax Regional Municipality, beginning approximately east of the Head of Saint Margarets Bay and approximately southwest of Hammonds Plains. The system is part of the East Indian River watershed and drains , with water storage provided in Lunenburg County by Mill Lake, Coon Pond, Sandy Lake, Wrights Lake, Big Indian Lake, Five Mile Lake and, at times, an overflow from Pockwock Lake in the Halifax Regional Municipality. The system is owned and operated by Nova Scotia Power Inc.
System assets associated with the St. Margaret's Bay Hydro System, which represents six generating units with an operating capacity of 10.2 MW, include Beeswanger Dam, Five Mile Dam and Wing Dam 4, Mack Lake Main Dam, Five Mile Wing Dams 1,2 and 3, Big Indian Dam, Wright's Lake Dam, Coon Pond Dam, Sandy Lake Dam, Sandy Lake and Coon Pond Pipeline, Mill Lake Plant and Surge Tank, Little Indian Crossover, Mill Lake Dam, Tidewater Plant and Surge Tank, Tidewater Pipeline, as well as associated headponds. Outflow is into St. Margarets Bay, Nova Scotia between Head of St. Margarets Bay and Tantallon.
Commissioned 8 June 1922, St. Margaret's Bay Hydroelectric System is the oldest hydro plant in Nova Scotia.
Technical characteristics
Dams, Wing Dams, Spillways, and Canals
There are 19 dams and wing dams of various heights, lengths and construction types holding back the lakes and reservoirs that make up the St. Margaret's Bay Hydroelectric System:
Water for the system comes from Five Mile Lake, Big Indian Lake, Sandy Lake, Wrights Lake, Coon Pond, Mill Lake and, at times, overflow from Pockwock Lake. The two principal storage areas for the Sandy Lake Development are Five Mile Lake and Big Indian Lake.
On the Eastern side of the system, drainage into Wright's Lake and Coon Pond, in addition to surplus water from Pockwock Lake (owned by Halifax Regional Municipality), supplies the Mill Lake Plant. The new Coon Pond Dam has water continually flowing out from a maintenance chute to ensure water is always feeding a stream leading to St. Margaret's Bay to maintain fish habitat in the stream.
Output water from the Sandy Lake and Mill Lake plants then flow into the Mill Lake Head Pond to supply the Tidewater Plant which outputs at sea level into St. Margaret's Bay.
The total elevation from the highest reservoir to sea level is . The St. Margaret's Bay Hydroelectric System has an annual output of 30 Gigawatt hours of electricity from two .6 MW generators at Mill Lake and two 2.9 MW generators at the Tidewater plant.
Mill Lake Generating Station
Located on the Northeast River. Two vertical francis turbine units, shaft direct connected to the generators, with individual direct connected exciters, static head, 514 rpm, , for a total of . Two generators, 1,600 kVA each, 13,200 volts, 3 phase, 60 Hz. The original penstocks were built out of wood, using tongue & groove staves held together with metal bands (the same technology used in wood barrels), diameter, long, later replaced in part with fibreglass penstocks. More recently (2010) the penstocks from Coon Pond to the Mill Lake Station were replaced with black polypropylene, buried underground.
Sandy Lake Generating Station
Sandy Lake Generating Station was added to the system in 1927. The power house was built beside the Mill Lake Plant. Two vertical francis turbine units, shaft direct connected to the generators, with motor generator set excitation, static head, 514 rpm, , for a total of . Two generators, 2,000 kVA each, 13,200 volts, 3 phase, 60 Hz. The original Penstocks were built out of wood tongue & groove staves with metal bands, diameter, long, later replaced in part with fibreglass penstocks.
Tidewater Generating Station
Located on the shores of St. Margaret's Bay. Two vertical francis turbine units, shaft direct connected to the generators, with individual direct connected exciters, static head, 300 rpm, , for a total of . Two generators, 3,900 kVA each, 13,200 volts, 3 phase, 60 Hz. The original Penstocks were built out of wood tongue & groove staves with metal bands, diameter, long, later replaced in part with fibreglass penstocks.
2012 Upgrade
The system was upgraded in 2012 with $17.8 million of work to replace the old wooden penstocks in fibreglass, a new water surge tank, a new dam at Mill Lake, a new gatehouse and gate at Mill Lake, and new floating booms to keep debris away from the dams.
