Annual Planning Report 2012 - Transpower

Chapter 3: Existing National Grid3.1.2 The HVDC LinkThe HVDC link connects the North and South Island transmission networks.This bi-directional link runs from Benmore, in the South Island, where there is anAC/DC converter station. There is a 534 km transmission line between Benmore andFighting Bay (Marlborough), a 40 km submarine cable between Fighting Bay andOteranga Bay across the Cook Strait, and a further 37 km transmission line intoHaywards substation north of Wellington. At Haywards substation, there is anotherAC/DC converter station.HVDC power flow is predominantly from the South Island to the North Island. Powerflow is from north to south when it is necessary to conserve South Island hydroresources as part of an efficient generation process, or to supply South Islanddemand during dry South Island periods.The HVDC link now consists of one permanently operating pole: Pole 2(commissioned in 1991) operating at 350 kV, which uses thyristor conversiontechnology. An older technology (mercury arc valve) pole (Pole 1), operating at270 kV, was stood down in September 2007, with half being totally decommissioned,and the remaining half pole to operate on a limited basis. 10 We are also mid-waythrough construction of a $672 million project to replace Pole 1 by 2012 with a newpole (the HVDC Inter-island Link Project).Table 3-1 lists the pole capacities for converting power from AC to DC and from DCto AC for both poles. Total pole capacity equates to the total capacity of the link.Table 3-1: Converter ratings and pole capacitiesPole Commissioned Converter type TransmissioncapacityOperationPole 1 (half pole) 1965 Mercury arc valves 270 MW 1 Available for limitedpeak operation onlyPole 2 1991 Thyristor valves 700 MW 2 FullTotal possible transmission capacityNotes:970 MW1. In December 2007, Transpower announced it would decommission half of Pole 1, after standingdown the full Pole 1 in September 2007.2. In November 2007, Transpower reconfigured the three operational undersea cables of the HVDC linkto increase the capacity of the south to north transfer of Pole 2 to 700 MW.3.1.3 Transmission network asset profileTable 3-2 provides a summary of the transmission network’s assets.Table 3-2: Transmission network assetsAsset descriptionDetailLength of HVAC and HVDC transmission line11,730 route kmNumber of substations (including HVDC) 178HVAC transmission line voltages220, 110, 66, 50 kVHVDC transmission line voltages350, 270 kVHVDC link capacity 700 MW 110The remaining half of Pole 1 is available under limited conditions: for normal operation, in responseto Grid emergencies, and for testing. The conditions include north transfer between 130 MW and200 MW, with automatic controls unavailable (except frequency modulation). Other conditionsinclude a limit on the number of starts, minimum operating time per start and cumulative operatingtime.262012 Annual Planning Report © Transpower New Zealand Limited 2012. All rights reserved.