The "Glide Path"

In 1997, Transpower adopted a strategy known internally as “the glide path”, and minimised spending on the grid and renewing assets. The rationale for this strategy was based on the expectation that there would be widespread installation of distributed generation (electricity generated close to where it is used), and that this would significantly reduce the need to expand and renew the grid.^[12]

However, by 2003, it had become clear that the glide path strategy was unsustainable. Many of the grid assets were approaching the end of their useful life, and at the same time had to carry higher loads than previously experienced to meet the demands of a growing economy and population. Transpower identified that the grid backbone was nearing its capacity and that investment was needed in many other parts of the grid. The transmission lines into and around Auckland were of particular concern, having remained largely unmodified since the late 1970s while the city's population has doubled. The aging and near-capacity infrastructure has caused several high profile failures, including the 1998 Auckland power crisis, where aging cables caused a cascading failure and blacking out the CBD for five weeks (strictly speaking, this was a distribution system failure, not a transmission failure); the 2006 Auckland Blackout, where a corroded shackle broke and caused a seven-hour outage of the inner city, and an October 2009 incident where a forklift accidentally knocked out power to northern Auckland and the whole of Northland.

Major grid developments commence

In 2008, Transpower turned its attention to the need for replacement and refurbishment of the aging grid assets.^[13]

A new strategy was developed that focussed on advancing investment proposals to increase the capacity of the grid. Some of the main grid upgrade projects are described in the following section.

Auckland and Northland

Auckland's power grid has suffered several famous blackouts, such as the 5-week long 1998 Auckland power crisis caused by failure of cables in the Mercury Energy distribution network.

Because of the location of the major load centres in Auckland city, the geography of the Auckland isthmus, and the historical development of the grid, all but one of the transmission lines from the south converge on Otahuhu substation creating limited redundancy in the network.^[14]

A major transmission failure occurred at Otahuhu substation on 12 June 2006, leading to the 2006 Auckland Blackout. It started at 8:30 am local time, with most areas of Auckland regaining power by 2:45 pm local time. It affected some 230,000 customers directly and at least 700,000 people in and around the city indirectly.

On 11 December 2006, the Electricity Commission (NZ) received an application from Transpower for the establishment of a new 220 kV gas insulated switchgear (GIS) facility adjacent to but geographically separate from the existing outdoor 220 kV switchyard at Otahuhu. This project was described as the Otahuhu substation diversity project, and included transferring approximately half of the circuits from the existing switchyard to the new GIS switchyard, to improve network resilience. The project was approved in August 2007.^[15]

On 30 October 2009 at around 8:00am, power was cut to the whole of Northland and most of the northern half of Auckland, affecting 280,000 consumers (14.5% of the country). A forklift carrying a shipping container accidentally hit one of the Otahuhu to Henderson 220 kV circuits while the other circuit was out for maintenance, leaving the region supplied by four low capacity 110 kV circuits. Power was restored to the entire region around 11:00am.^[16]

Wairakei ring

The Wairakei ring is a set of transmission lines north of Lake Taupō linking Wairakei with Whakamaru. These lines connect several hydro and geothermal power stations into the national grid, enabling their power to be exported to the major centres of electricity demand. A number of new geothermal power stations are being built or are planned for the area, and a higher capacity line is needed to transport their power to market. In December 2008, Transpower submitted a Grid Upgrade proposal to the Electricity Commission to build a new double circuit 220 kV line, Wairakei to Whakamaru C, to replace the single circuit Wairakei to Whakamaru B line. that was intended to help facilitate the connection of up to 1000 MW of new generation expected in the region over the next 5–7 years. The Commission announced their intention to approve the investment on 20 February 2009.^[19][20] Transpower completed the project in mid-2013. The estimated cost of the project was $141 million.

In October 2021, Transpower submitted a resource consent application to the Taupō District Council to build a series reactor on the Wairakei to Whakamaru A line at Ātiamuri. The reactor will re-balance the electricity flow between the A and C lines to allow an extra 500 MW to be transmitted towards Whakamaru, especially with the construction of Tauhara Power Station (152 MW) and Harapaki Wind Farm (176 MW).^[21]

Lower South Island

Transmission in the region is a mix of core grid (220  kV) and smaller (110  kV) lines. The 220  kV grid supplies major loads at Dunedin, Invercargill and Tiwai Point, from generation at Roxburgh and Manapouri. The 110  kV lines supply smaller load centres throughout the region including some larger industrial loads (Brydone fibreboard plant and Edendale dairy factory). Power transfer into and out of the region is primarily through the two Invercargill–Roxburgh circuits. There are two issues in this region:^[22]

• The available 220 kV transmission capacity from Roxburgh south to Invercargill and North Makarewa is limited. Additional capacity is required to maintain security of supply to Invercargill and North Makarewa during low hydrological inflow or low wind periods
• The 110 kV network is nearing capacity due to increasing demand in the region. It is vulnerable to loss of supply or low voltage when any 110 kV circuit or 220/110 kV inter-connecting transformer is out for maintenance. The smaller lines can also constrain the import of power into the region when Manapouri generation is low

The 220  kV and 110 kV circuits were also not inter-connected at Gore.

HVDC upgrade project

In May 2008, Transpower submitted a Grid Upgrade Plan proposal to the Electricity Commission for a major project to upgrade the HVDC system including:

• new HVDC converter stations at Benmore and Haywards to replace the existing mercury-arc valve converters
• seismic strengthening works for existing and new switchyards at both locations
• extensions to 220 kV switchyards at both locations
• replacement of control systems for the existing Pole 2 converter stations
• new unit connection transformers for four synchronous condensers at Haywards

On 25 September 2008, the Commission issued its final approval for the project, with an approved maximum cost of $672 million.^[28][29]

The project replaced the aging mercury arc rectifier Pole 1 converter stations with a new thyristor valve pole (known as Pole 3). Upgrades at the converter stations at Benmore and Haywards allowed the link to carry up to 1000 MW with future provision for 1400 MW.^[30]