11. The National Grid

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.
For most of us, pylons are the symbols of the National Grid - Image: Heurisko Ltd.
Three of the transformers in the switchyard of the Wairakei Power Station that link it to the National Grid - Image: Heurisko Ltd.

Electricity is carried in two ways on the National Grid.

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 movement occurs exactly 50 times a second (50 Hertz, 50Hz).

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

The electricity leaves the Wairakei Power Station at 220 000 Volts and then

  • Joins the National Grid
  • Is transformed down to 110 KV, 66 KV, 50 KV, 33 KV or 11 KV near towns and cities
  • Is transformed down to 230 V AC near houses
  • Travels through electrical appliances and equipment
  • Travels back to the Wairakei Power Station.

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 1040 MW of power at 500 KV, 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.