Chapter 2: The Upper North Island Grid
The National Grid
22. The National Grid is infrastructure of national importance. It involves over 12,000 route-kilometres of transmission lines, about 25,000 towers, 16,000 poles, and 173 substations. It is supported by its own information technology and telecommunications.
23. Transpower’s responsibility for maintaining and developing the grid involves applying good transmission planning practice, by which use of existing assets is maximised (where practically and economically feasible) before constructing new transmission assets. Use of existing transmission assets can be increased by enhancing capacity, improving power sharing across parallel circuits, and increasing the voltage stability limit. Capacity can be increased by raising the operating temperature of conductors, by adding a conductor (eg, duplexing), or by installing larger conductors. Power sharing can be improved by adding extra reactive equipment to direct power flow through higher rated circuits.
24. Good industry practice, and the most cost-effective, is to provide new transmission with sufficient capacity for the future. This also reduces the number of low-capacity lines needing to occupy multiple corridors. It calls for long-term planning by Transpower.
Upper North Island Grid
25. Electricity for the upper North Island is supplied through the National Grid by 220-kV and 110-kV networks. The primary supply is two separate 220-kV paths, between Whakamaru and Huntly respectively, and the Otahuhu Substation in South Auckland. Each path consists of three 220‑kV circuits. So, in total, there are four 220-kV transmission lines supplying Otahuhu Substation from the south: two single-circuit, and one double-circuit, transmission line from Whakamaru; and one double-circuit transmission line from Huntly. The western path continues south from Huntly to Stratford in one double-circuit transmission line.
26. These paths are supported by two 110-kV circuits: one between Bombay and Otahuhu, and the other between Arapuni and Pakuranga. These 110-kV circuits make only minor contributions to transmission capacity, less than 10 per cent of the total power flow.
27. The Otahuhu Substation is critical to the security of supply to the Auckland and Northland regions, because of the number of lines leading to and from it, and the capacity of those lines.
28. Up to 30 per cent of the winter peak load in the upper North Island can be supplied by local generation in the Auckland area. Of that 30 per cent, over half is supplied by a single combined-cycle, gas-fired generator at Otahuhu.
29. In addition to the grid upgrade that is the subject of the requirements and resource consent applications that have been called in and referred to the Board, Transpower proposes other associated works and projects for improvement of the Upper North Island Grid, that are not before the Board.
30.. Transpower described the grid as a dynamic working system; and explained that routine operational requirements result in constant change in the network as power is switched around the system. This has physical results: conductors sag as they heat relative to power flows, and swing to a greater or lesser extent due to ambient conditions; and towers supporting the conductors may be reconfigured, strengthened or added to for improving security (eg, from lightning) or to cope with increased electrical load.
31. The grid upgrade referred to the Board, and the associated works and projects, are intended together to provide additional capacity for electricity transmission, and to enhance diversity and security of supply.
Associated works
32. The associated works are intended to enhance the capacity of the existing grid while the more substantive works of the Grid Upgrade Project itself are completed.
33. In summary the associated works are:
- Report and Decision of the Board of Inquiry into the Upper North Island Grid Upgrade Project | Ministry for the Environment
34. Transpower contended that those associated works need to be completed by 2010, and will provide sufficient interim capacity to allow the first stage of the Grid Upgrade Project to be completed by 2013.
Other projects
35. The other projects are the Otahuhu Substation Diversity Project and the North Auckland and Northland Project.
36. The Otahuhu Substation Diversity Project aims to enhance diversity and security within Auckland. Transpower has the consents needed for this project and work is underway.
37. The Otahuhu Substation Diversity Project comprises construction of a separate Gas-Insulated Substation within the boundaries of the existing Otahuhu Substation. This will provide increased reliability and security of supply by providing physical diversity at the site. It will also provide for cabling overhead lines to remove line crossings. About half the transmission circuits will be terminated in the new substation, and the remainder will continue to be terminated at the existing substation.
38. The North Auckland and Northland Project will connect the modified Pakuranga Substation to the existing Penrose Substation, and then through to substations on the North Shore. By winter 2013, the load of the Northland region is forecast to reach 957 megawatts (MW), and the transmission reinforcement provided by this project will then be required to ensure secure capacity is available. Transpower is requesting the Electricity Commission’s approval for this project.
