Annual Planning Report 2012 - Transpower

Chapter 3: Facilitating New Zealand’s Energy Future2.2.1 Variable line ratings - operating assets to ratingsThe initiativeMost of our transmission lines are operated to pre-calculated, seasonal ratings.Pre-calculated ratings are design ratings which reflect a least favourable combinationof operating conditions (such as ambient temperature and wind) for that season. Ifthe ambient conditions at a point in time are more favourable than assumed for thedesign ratings, then transmission lines can carry more power than the design rating.In November 2011 (a month ahead of schedule), we implemented interim variableline ratings (iVLR) on six transmission circuits. The use of iVLR provides the circuitswith up to 84 different ratings at different times of day and month in the year. Theambient conditions are determined for a 500 metre grid along the line using historicweather data from NIWA. The ratings are then determined from the maximum designoperating conditions and line ground clearances measured from aerial laser surveys.iVLR provides an average capacity gain for each circuit of between 8% and 16%.The trial is expected to continue for at least two years, with the experience helping toframe the rollout of variable line ratings across the entire transmission network. Weexpect to roll out variable line ratings for all lines in conjunction with the next majorupgrade of the System Operator’s software tools, within the planning period. Theambient conditions used to calculate these ratings will be based on shorter periodsand actual regional data and forecasts.Operating experienceThe following circuits presently have VLR applied:Clyde–Roxburgh 1 and 2 – part of the 220 kV grid in the Otago-Southland areaWairakei–Ohakuri and Atiamuri–Ohakuri – part of the 220 kV “Wairakei Ring”northwest of Taupo, andOtahuhu–Whakamaru 1 and 2 – part of the 220 kV grid into Auckland.VLR assists in management of the low hydro generation in the Otago-Southland areaby increasing the average transmission capacity from the Waitaki Valley to the Otago-Southland area during the time before the capacity of the Clyde–Roxburgh 1 and 2circuits is increased through the installation of duplex conductors.The Wairakei Ring area has significant hydro and geothermal generation, with moregeothermal generation under construction or planned. VLR allows more flexibledispatch of generation with the existing transmission system, applying downwardpressure on energy prices. There is an approved project to increase transmissioncapacity by building a higher capacity transmission line in the Wairakei ring. VLR onthe Wairakei–Ohakuri and Atiamuri–Ohakuri circuits will enable additional capacityand better utilisation of these existing lines. VLR will also further increase the overalltransmission capacity through the Wairakei ring.VLR on the Otahuhu–Whakamaru 1 and 2 circuits is primarily intended to provideincreased transmission capacity during maintenance outages of other 220 kV circuitssupplying Auckland.Planning experienceVLR also requires a change in transmission line rating methodology. The change inrating methodology increases the average capacity of a transmission circuit, but mayalso decrease the rating of some circuits for certain periods during the day and timesof the year. The new rating methodology is yet to be applied to transimission circuitsthat are not part of our iVLR work.2012 Annual Planning Report © Transpower New Zealand Limited 2012. All rights reserved. 17