The Whakamaru to Brownhill Road transmission line is a double-circuit 400 kV-capable transmission line constructed by Transpower to increase the capacity of the National Grid between the southern Waikato and the city of Auckland. The line runs from the Whakamaru sub-station near the Whakamaru Power Station, over a distance of to the new Brownhill Road substation near Whitford in southeastern Auckland. The line will initially be operated at 220 kV. From Brownhill Road, 220 kV underground cables connect the line to the Pakuranga sub-station in eastern Auckland. The project was the subject of considerable controversy and protest during the planning and approval stages. Construction of the line started in February 2010, and the line was commissioned on 30 October 2012. The transmission line forms the major part of a wider North Island Grid Upgrade project with a forecast cost to completion of $894 million.
Overview
The line was a major enhancement to the National Grid and was designed to increase electrical supply and security to Auckland and Northland. The line provides more transmission capacity between the Waikato River hydro stations and Auckland, and takes load off the existing 220 kV lines. Prior to the commissioning of the new line, all existing major transmission lines into Auckland converged on the Ōtāhuhu substation, creating a concentration of risk to the security of supply to Auckland and Northland. The new line was terminated at the Pakuranga substation instead, creating diversity of electricity supply to Auckland.
The line was designed and built for 400 kV operation, but has operated since commissioning at 220 kV. In future, possibly by 2030, the installation of 220/400 kV transformers at Whakamaru and Brownhill Road would allow the transmission line to be operated at 400 kV, but the high cost of the required 400 kV transformers are unlikely to make that upgrade feasible. If upgraded to 400 kV operation, additional underground cables would be needed to connect Brownhill Road directly to Ōtāhuhu substation.
Background
There are very few electricity generating stations north of Auckland, which means that the electricity requirements for Auckland city and Northland must be supplied from the south - especially from the hydroelectric power stations on the Waikato River and the dual-fuelled Huntly Power Station.
Prior to the commissioning of this new transmission line, there were six lines running north into Auckland:
A double-circuit 220 kV line from Ohinewai (near Huntly) to Ōtāhuhu via Huntly and Takanini, with one circuit also going via Drury. A double-circuit 220 kV line from Whakamaru to Ōtāhuhu via Ohinewai, with one circuit also going via Hamilton. Two single-circuit 220 kV lines from Whakamaru to Ōtāhuhu direct. A double-circuit 110 kV line from Arapuni to Ōtāhuhu, via Hamilton, Bombay and Wiri. A single-circuit 110 kV line from Arapuni to Bombay direct
The Ohinewai-Huntly-Ōtāhuhu line was the last of these transmission lines to be built into Auckland, commissioned in the late 1970s to export electricity out of the then under-construction Huntly Power Station. Since then, the population of the upper North Island has more than doubled, and electricity use has trebled.
Power supply to the Upper North Island is dependent largely on two major factors: the generation from Huntly and the transmission capacity between Huntly and Ōtāhuhu; and generation from Whakamaru and south of Whakamaru, and the transmission capacity between Whakamaru and Ōtāhuhu. For the transmission system as at 2010, issues that could arise during periods of high demand and low generation in the Auckland area included: an outage of a Huntly–Ōtāhuhu circuit may overload the other Huntly–Otahuhu circuit an outage of a Huntly–Ohinewai circuit may overload the other Huntly–Ohinewai circuit the two 220 kV Whakamaru-Ōtāhuhu circuits may overload during a contingency an outage of the Whakamaru-Hamilton circuit may overload the two 110 kV Arapuni–Hamilton regional circuits an outage of the Hamilton–Ohinewai circuit may cause low voltage at the Hamilton 220 kV bus.
The North Island Grid Upgrade (NIGU) project, including the Whakamaru to Brownhill transmission line, was intended to address these issues, although forecasts predict the last issue (Hamilton–Ohinewai outage causing low voltage at Hamilton) will reappear towards 2025. The project also increases the power transfer capacity into Auckland, and reduces the loading on the existing 220 kV Whakamaru-Ōtāhuhu and Huntly–Ōtāhuhu circuits, with a consequential reduction in the reactive support needed in the Upper North Island.