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The 'National Grid' is the 12,175km network of power pylons, poles, cables and 170 electricity sub-stations throughout NZ. Transpower, on behalf of the NZ Government owns and manages the National Grid.

Brian Ultee at work inside the Control Room of Station B at Wairakei Power Station. This room still contains the original meters and control equipment giving information about turbines, generators and electrical output to the national grid. It has been replaced by a much smaller, computer based system in the administartion building, that controls all of Wairakei, Poihipi and Ohaaki power stations with the click of a mouse - Image: Heurisko Ltd.

Electricity is carried in two ways on the National Grid.

For most of us, pylons are the symbols of the National Grid - Image: Heurisko Ltd.

1. An Alternating Current system

Alternating Current (or AC) is the type of electricity where electrons move back and forth along a wire. This back and forth movement transmits the electrical energy, but individual electrons do not travel far. In NZ this back and forth movement occurs exactly 50 times a second (50 Hertz, 50Hz).

Alternating Current is transmitted at a high voltage but a low current, so that heat (energy) loss from the wires is reduced. Transformers are an easy way to raise or lower the Voltage.

Electricity leaves the Wairakei Power Station at 220 000 Volts (220kV) and then:

• joins the National Grid.
• is transformed down to 110kV, 66kV, 50kV, 33kV or 11kV near towns and cities.
• is transformed down to 230V AC near houses.
• travels through electrical appliances and equipment.
• travels back to the Wairakei Power Station.

Three of the transformers in the switchyard of the Wairakei Power Station that link it to the National Grid - Image: Heurisko Ltd.

2. The Direct Current System

Direct Current (or DC) is when the electrons move in one direction along a wire. For convenience, this is considered to be from positive to negative.

Direct Current is only used in one situation in the National Grid, which is the 575km DC cable from Benmore, Otago to Haywards, Wellington. This cable joins the power supplies of the North and South Islands. Ninety five percent of the time the current flows North.

The cable carries up to 1040MW of power at 500KV, hence its name "High Voltage Direct Current" - or HVDC for short.

When built in 1965, the HVDC link was the world’s largest and longest DC cable, incorporating the world’s largest submarine cable. The success of this engineering feat has earned it a Millennium Award from the Institute of Professional Engineers of NZ (IPENZ).