08. Arapuni
Efficiency Upgrade
A major overhaul of the oldest and largest single station on the Waikato, Arapuni, was completed in 2002. The overhaul included the stations generators and turbine runners, to improve both its efficiency and environmental performance. This project is the latest in a line of sustainability enhancements at Arapuni, and in the Waikato Hydro System, illustrating Mighty River Power's long-term commitment to the sustainable use of resources.
The station has undergone numerous enhancements over the years, including a new forebay and intake structures, new spillways, new governors and control systems, automation, generator rewinds, modified draft tubes, circuit breakers and transformers. To improve its environmental performance, the station has been fitted with modern oil interceptors and the original septic tank discharges into the river have been discontinued. In 1989-90, the turbine runners in units G1 to G4 were replaced.
In 2001-02, Mighty River Power undertook an upgrade of the second four units G5 to G8 and replaced the turbine runners in units, with a view to maximising efficiency and improving environmental performance.
Why was the upgrade needed?
Computer model of Francis Turbine runner
The G5-8 units were all in need of a major overhaul. Two machines, G5 and G8, required the head gate to be lowered to stop the machines because the wicket gates links, levers and bushes were excessively worn. G7 and G8 were derated 3MW each due to cracks in the runner. G8 had a severely eroded headcover which had been patch-repaired. G7 had a 'do not stop' operating restriction because it lost large quantities of oil from the turbine bearing into the head cover on stopping. All of G5, G7 and G8 had high thrust pad temperatures due to warn crown wear rings.
The overhaul of the units also provided an opportunity to replace the existing turbine runners with more efficient, higher power runners. The condition of the original runners was poor. They contained 'significant cracks' in the blades with some up to 270 mm long and others having evidence 'of being cracked through the width of the blade'. The cracking resulted in the units being de-rated five years ago from 24 MW to 22 MW. Frequency keeping was not available with the old runners due to increased stress on the runners and the effect on existing cracks.
What was involved?
The project took several years from tendering in 1997 to completion in 2002. International competitive tenders were called for the design and manufacture of the four 15 tonne Francis turbines. Voith Hydro in Pennsylvania fabricated the turbines. Altsrom NZ completed the turbine installation.
State-of-the-art computer design, based on computational fluid dynamics (CFD) and modern manufacturing techniques and materials, were used in designing and manufacturing the new turbine runners.
The units and turbines were dismantled. The turbine and generator components were refurbished, then reassembled, the units realigned. The turbines and generators were then recommissioned. The project also included work that was additional to what would be carried out on a conventional overhaul. This work included: replacement of the runner wear rings, installation of wicket gate lube free bushes and installation of high pressure oil injection.
08Results
- The upgrade has breathed new life into the aging plant, adding additional capacity and value, and maximising efficiency and environmental performance.
- Prior to the upgrade, capacity had been downgraded from 24 MW to 22 MW. As a result of the upgrade, peak output increased to 27 MW, meaning an extra capacity of 2.4 MW per machine or 10 percent.
- The upgrade has also resulted in the following performance gains: an increase in turbine efficiency of 2 percent, reduced maintenance due to superior materials, removal of down rating due to cracks in the old turbine, increased flexibility for frequency keeping, peak efficiency of 94.2 percent, and better hydraulic stability.
- Leading-edge design, materials and manufacturing technology were used, enabling the new turbines to be successfully integrated in the old generating plant. As a result of this new technology and smart asset management, the oldest and largest single power station on the Waikato, is now the most efficient.
- Almost 10,000 kW of additional renewable energy generation capacity has been added at Arapuni. This increase is the equivalent of building another small hydro station.
- The increased capacity produces a total of 18 GWh of renewable energy a year - enough to supply a town the size of Morrinsville.
- The return on investment from hydro generation is better than building new non-renewable fossil fired plant. Hydro also provides a renewable sustainable energy, helping to reduce New Zealand's reliance on thermal plants using non-renewable energy sources.
- The increase has been achieved without altering Lake Arapuni, an important fishing and recreational lake or the historical features of the dam and power station. More electricity is now generated from the same amount of water and within Arapuni's existing "environmental footprint".