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Forty Years of Successful Generation at Wairakei: a Successful Match and Marriage

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Forty Years of Successful Generation at Wairakei: a Successful Match and Marriage New Zealand Workshop FORTY YEARS OF SUCCESSFUL GENERATION AT WAIRAKEI: A SUCCESSFUL MATCH AND MARRIAGE B. CAREY Wairakei Power Station, Contact Energy Ltd SUMMARY - 40 years of generation at Wairakei. Power generation commenced on 15 November 1958when Assistant SuperintendentC D loaded the 6.5 high pressure turbine generator, G2, to 1.7 Forty years of a match and a very successful between the Wairakei geothermalfield and the Wairakei geothermal power station have ensued. In, ‘Wairakei - The First Twenty Years’, Ian wrote “Wairakei, it is hoped, will be like grandfather’s axe - four new shafts, two new heads, but still the same axe”. There have been new shafts and new heads but Wairakei is Wairakei, is is Wairakei, is Wairakei .. .. There is no hoping after forty years, it is reality. This is a remarkable achievement fkom a plant that is now a consistent and reliable electricity generation facility. But it wasn’t like this. 1.0 THE ENGAGEMENT The design and procurement for station were progressing. In parallel, the steam The beginning of this remarkable marriage winning was continuing so satisfactorily that started with an engagement where the extensions to the power station were proposed matching process was anything but certain. in 1956. Two more 11.2 high turbines were to be installed in the ‘A’ There were many questions that rapidly station and a new ‘B’ station was to be emerged and were to answer: constructed with three 30 mixed pressure What is the size of this resource? sets (Figure 3). This was to take the How do we these wells and how big are installed capacity to 192.6 the holes to be? What size power station is to be installed ? 2.0 BUILDING THE SUCCESSFUL MARRIAGE The plant was to be 20 Then it was to be a combined power plant and heavy water Just like any successful marriage, there were production facility with a generating capacity issues that needed to be worked through. of 46.6 and a 6 tonne per year heavy Working through these issues has put water production capacity (Figure 1). The Wairakei on a sound footing. pulled out of the heavy water distillation facility and extra power turbines In the early years the plant was were installed to take the capacity to 69 temperamental, in modern idiom it was a How deep are the holes to be drilled ? “dog”. The plant was vulnerable to minor fluctuations in the steamfield pressures The pilot hot water scheme using water causing machine instability. The rotary gas produced a of wells in the eastern exhausters were unstable and unreliable. In borefield was proposed. Another 11.2 fact such was Wairakei’s reputation that the low pressure turbine was installed as Stage Minister of Electricity was to be personally to utilise steam produced this scheme informed of machine trips as soon as they (Figure 2). This was to take the installed occurred, as long as the house was sitting of capacity to 80.2 course. 255 4 3 a 256 and more efficient utilisation of As more wells were connected into the the geothermal energy and water have steamfield system so the stability improved. contributed to the success of the Wairakei The gas exhauster problems were solved by development. scrapping this equipment and installing steam jet ejectors on the ‘A’ station LP condensing It was recognised that additional steam could turbines. The last of the rotary gas exhausters be produced the discharged water. The was replaced in 1970. The 30 ‘B’ station pilot hot water scheme was one of the early sets came supplied with steam jet ejectors. efficiency projects. The scheme was designed to collect water from seven eastern borefield Corrosion was a problem in some areas. wells, transmit it to the power station and Electrical equipment was vulnerable but so produce additional IP and LP steam. The was the leading to the gas stacks scheme was installedbut as the enthalpy of the which corroded rapidly until resistant eastern borefield wells rose once production materials were sourced. commenced, the of water available for processing decreased significantly the The focus of the work was to improve the scheme never functioned properly. In April reliability of the power plant and to improve 1964 after about a year of operation the system its consistency in energy production. High was decommissioned. load factors and low forced outage rates were also objectives. These objectives started to be Further schemes were developed to utilise the achieved from the mid 1960’s. From then on separated geothermal water. A steamfield Wairakei has gone from strength to strength optimisation study was undertaken by J P F in achieving an enviable record of power plant Robinson in 1970. The design and performance. construction that followed was undertaken by the of Works and included ‘G’ line, a 3.0 ENRICHINGA SUCCESSFUL (48”) diameter low pressure steam MARRIAGE pipeline. Flash plants were installed as part of the optimisation to flash additional Wairakei was a cornerstone of electricity steam fi-om the geothermal water. ‘G’ line production in the North Island in its earlier was installed to bring this low pressure steam years. This was in the days before 1965 when the field down to the power station. It the North and South Islands grids were joined operated at a pressure of about 1 bar gauge at with the commissioning of the Cook Strait the power station where steam was passed cables. Over the Christmas period of the early through pressure reducing valves before being 1960s Wairakei supplied 50% of the total admitted to the low pressure steam system in electrical load of the North Island. The the power station. This equipment was contribution to the electrical energy supply in commissioned in 1974. A three pressure the North Island and New Zealand on an steam system was in operation in the Wairakei basis is shown in figure 4. In 1966 steamfield at this time with steam being Wairakei supplied about 19% of the North supplied to the power station at IP and LP Island’s total electrical energy consumption. conditionsfrom the steam field. Initially the Wairakei development was a two From the time that the power plant was fully pressure steam system with steam supplied at commissioned in 1963 up until the early HP and IP pressures (Turbine Inlet pressures 1980’s several wells in the western borefield of 12.5 and 3.5 bar g (IP).). were connected up into the steamfield pipe (20”) diameter and pipelines carried network. These were wells which had been steam the steamfield to the power drilled as part of the drilling but they station. Three larger (30”) diameter had not been connected up to the steamfield steam lines were installed as part of the ‘B’ pipe network. station development. The reservoir fluid was separated at wellhead separators, The high pressure turbine inlet pressure was with the steam transmitted to the power progressively reduced between 1963 and 1982 station and the water discharged to the to enable more energy to be extracted the drainage system. available wells to partly compensate for the decline in reservoir pressure. On the 30 November 1982 the steam system was 257 Wairakei Generationas a Percentage of North Island and New Zealand Figure 4. GenerationStatistics Power Station Station MW .. ..... ..... .. ..... .. A YW Figure 6. Field Energy Productionwith Time Figure 5. Power Plant 1996-1998 258 i decommissioned (Morris The low pressure steam system that were installed pipeline system was converted to IP and the 4 as part of the steamfield optimisation HP turbines (G2, G3, G5, G6) work with a turbine generator (Figure 5). A decommissioned. The installed capacity of the refurbished turbine and induction generator plant became 157.2 The steamfield was capable of producing about 3.5 now a two pressure system supplying and were connected to the national grid on the 5 LP steam to the power station. May 1996. The installed capacity of the plant increased to 161 Utilising wells drilled as part of the 200 series investigation programme commenced in the Additional steam and liquid wells in the Te early The commissioning of pipeline area have been drilled and connected extensions into the Te Mihi area occurred on into the power station steam supply system 15 June 1983. These extensions included since the end of 1995. The changing location flash plants 9 and 10 to enable wells 206, 207 of fluid production through the years is shown and 215 to be utilised. These three wells had in figure 6. This figure shows the decline of been drilled in 1960 and 1961 as part of the the eastern borefield area, changes in the 200 series investigation program but had not western borefield and the increasing been connected into the power station pipe production of energy from the Te Mihi area network. since the mid 1980’s. Production drilling at Wairakei recommenced The recent work during 1998 has fully loaded in 1985 some 30 years after the 200 series the power plant enabling it to produce up to investigation well drilling programme had 164 for the grid. concluded. A shallow steam well 228 was drilled in October 1985 to utilise the high 4.0 POURING COLD WATER ONTO pressure Te Mihi steam zone. Additional THE drilling to delineate the north western extent of the Te Mihi high pressure steam zone was This is known by geothermal undertaken in early 1987. Three specialists as injection or reinjection. 230,231 and 232 were drilled. Scientificinvestigationand testing of injection Additional steam production wells were of separated geothermal water has been drilled between August and December 1987. undertaken at Wairakei since about 1978.
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