Benmore Dam

Benmore Dam
Location of Benmore Dam
Location	Canterbury Region, New Zealand
Coordinates	44°33′50.92″S 170°11′49.92″E
Construction cost	$62 million
Dam and spillways
Type of dam	Earth-filled dam
Height	110 m (360 ft)
Length	823 m (2,700 ft)
Crest width	10.6 m (35 ft)
Base width	490 m (1,610 ft)
Volume	28 million tonnes
Impounds	Waitaki River
Spillway capacity	3,400 m3/s
Reservoir
Creates	Lake Benmore
Capacity	12.5 million m3
Surface area	75 km2 (29 sq mi)
Max. water depth	120 m
Power station
Owner(s)	Meridian Energy
Commission date	1965
Hydraulic head	92 m (302 ft)
Turbines	6 x 93 MW
Installed capacity	540 MW
Annual generation	2,215 GWh

Benmore Dam is the largest dam within the Waitaki power scheme, located in the Canterbury Region of New Zealand's South Island. There are eight other power stations in the valley.

The dam is the largest earth-filled water-retaining structure in New Zealand. Its core is impermeable clay-like gravel, supported by two massive shoulders of river gravel. Lake Benmore has a volume of 12.5 million cubic metres, about 1.5 times as much water as Wellington Harbour. The dam's spillway can cope with 3,400 cubic metres of water per second, about 10 times the mean river flow. With a generating capacity of 540 MW, Benmore Power Station is the second largest hydro station in New Zealand.

The $62 million construction of the dam and hydroelectric station began in 1958. It was commissioned in 1965, and officially opened by Prime Minister Sir Keith Holyoake on 15 May that year. It was built for the New Zealand Electricity Department; since 1999 it has been owned and operated by Meridian Energy.

From 2008 to 2010 the six turbines are being refurbished at a cost of $67 million. This will enable a 5% reduction in water use for the same generation capacity, increasing annual generation by 70 GWh. New switchboards and an upgrade to the switchyard are also planned.

Benmore is the South Island terminus of the HVDC Inter-Island link between the North and South Islands of New Zealand. The static inverter plant is on the west side of the tailrace, and converts 220 kV AC power to +270/-350 kV DC for the 610 km (380 mi)journey to the North Island static inverter plant at the Haywards sub-station in Lower Hutt.

Otematata is the small town that supports the dam, although with advances in technology the staffing needed to maintain the dam is drastically smaller. Due to this the town is now a small holiday community, with only around 200 permanent residents. The dam is about 8 kilometres (5.0 mi) up the valley from the township.

Key statistics

• Lake Benmore area: ~ 75 kilometres square
• Lake Benmore shoreline: 116 km
• Dam crest length: 823 metres
• Dam crest height: 110 metres
• Dam width at base: 490 metres
• Dam width at crest: 10.6 metres
• Dam fill: 28 million tonnes
• Head (hydraulic): 92 metres
• Average river water flow: 340 m³/s
• Nominal annual generation: 2,200 GWh
• Installed capacity: 540 MW
• Machinery: 6 x 93,000 kW vertical Francis turbines (Dominion Engineering, Canada), coupled to 6 x 90,000 kW 112,500 kVA generators (Canadian General Electric); 6 transformer banks totalling 1,150 MVA
• Penstocks: Concrete

Benmore Dam from the south lookout, near the spillway

Transmission

Benmore is a major switching point in the New Zealand national grid and one of two major switching stations in the Waitaki Valley (the other being Twizel sub-station). The importance is largely due to the HVDC Inter-Island static inverter plant located at it.

The main AC switching station is on the east bank of the tailrace, and electricity generated by Benmore is injected into the national grid here. Three major 220 kV lines containing five circuits run from the Benmore substation. Clockwise from south, they are:

• A double-circuit line to the Twizel sub-station, with one circuit going via Ohau C power station and one circuit going via Ohau B power station.
• A single-circuit line to Islington sub-station in Christchurch, via Twizel sub-station and Tekapo B power station.
• A double-circuit line to Aviemore Dam

The HVDC Inter-Island static inverter plant is located on the west side of the tailrace, connected to the AC switching station by lines over the tailrace. The +270 kV mercury-arc Pole 1 has its valve hall on the north side of the site, near the powerhouse. The -350 kV thyristor Pole 2 has it valve hall on the south side of the site. A new +350 kV thyristor Pole 3 is currently under construction to replace the ageing Pole 1, and its valve hall will be located adjacent to the existing Pole 2 valve hall.

Further reading

• Martin, John E, ed. (1991). People, Power and Power Stations. Wellington: Bridget Williams Books Ltd and Electricity Corporation of New Zealand. pp. 316 pages.. ISBN 0-908912-16-1. 
• Reilly, Helen (2008). Connecting the Country – New Zealand’s National Grid 1886 - 2007. Wellington: Steele Roberts. pp. 376 pages.. ISBN 978-1-877448-40-9. 
• Sheridan, Marion (1995). Dam Dwellers – End of an Era. Twizel: Sheridan Press. pp. 392 pages.. ISBN 0-473-03402-6. 

External links

• Media related to Benmore Dam at Wikimedia Commons