Distribution
Electricity from Transpower's national grid is distributed to local lines companies and large industrial users via 180 grid exit points (GXPs) at 147 locations. Large industrial companies, such as New Zealand Steel at Glenbrook, the Tasman Pulp and Paper Mill at Kawerau, and the Tiwai Point Aluminium Smelter near Bluff, draw directly from Transpower substations and not the local lines companies' local grids.
Distribution of electricity to local consumers is the responsibility of one of 28 local line companies. Each company supplies electricity to a set geographic area based on the grid exit points they draw from. The companies are as follows (those in bold serve an urban area with a population over 100,000):
| North Island | South Island |
|---|---|
| Centralines | Alpine Energy |
| Counties Power | Aurora Energy |
| Eastland Network | Buller Electricity |
| Electra | Electricity Ashburton |
| Horizon Energy Distribution | Electricity Invercargill |
| Northpower | MainPower |
| Powerco | Marlborough Lines |
| Scanpower | Nelson Electricity |
| The Lines Company | Network Tasman |
| Top Energy | Network Waitaki |
| Unison Networks | Orion |
| Vector | OtagoNet Joint Venture |
| Waipa Networks | The Power Company |
| WEL Networks | Westpower |
| Wellington Electricity |
The local line companies draw electricity from one or more grid exit points at various voltages between 11 kV and 110 kV, with voltages 22 kV and over usually used for sub-transmission between towns and other communities. Sub-transmission voltages are stepped down to distribution voltage at zone substations. Some medium-sized industrial consumers draw directly from the subtransmission grid, and smaller power stations up to around 60 MW in size usually connect to the sub-transmission grid for distribution and connection to the national grid.
Subtransmission and distribution voltages differ from area to area. Most areas use 33 kV subtransmission and 11 kV distribution, although there are multiple variations: for example, Auckland uses 110/33/22 kV subtransmission and 22/11 kV distribution, most of Canterbury uses 66/33 kV and 22/11 kV, Dunedin used 33 kV and 11/6.6 kV, and the Waipa and northern Tararua districts have 11 kV distribution with no subtransmission network.
Distribution voltage lines run from zone substations (or the grid exit point) to the streets of consumers, and to some large local businesses, such as supermarkets, schools, large farms. Three phase power is available in all urban and most rural areas. Single phase supply utilising only two phases or single wire earth return systems are used in outlying and remote rural areas with light loads. Local pole-mounted or ground-mounted distribution transformers step-down the electricity from distribution voltage to the New Zealand mains voltage of 230/400 volts (phase-to-earth/phase-to-phase) for distribution into local homes and businesses.
A major failure of distribution systems occurred in the 1998 Auckland power crisis - two 40-year old cables connecting Penrose and Auckland's central business district failed in January to February 1998 during unseasonally hot weather, causing strain on the two newer remaining cables which subsequently failed on 20 February 1998 and plunged central Auckland into darkness. The failure cost businesses NZ$300 million, and resulted in central Auckland being without electricity for 66 days until an emergency overhead line could reconnect the city - the longest peacetime blackout in history.
Read more about this topic: Electricity Sector In New Zealand
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