AGL links Kiewa gap10 January 2008
The Kiewa Valley cascade was built in the 1950s, and AGL Energy is now completing the 'missing link' with construction of Bogong. SKM's Paul Caplen tells how mainland Australia's biggest hydro plant for more than 25 years is being developed
The Bogong Power Development project is the largest hydropower plant to be built in mainland Australia for more than 25 years and is being developed by AGL Energy (AGL). Construction got underway in September 2006 after the appropriate environmental and regulatory approvals were obtained. The 140 MW (two 70 MW Francis units) scheme is expected to be online by the end of 2009.
Located in the Kiewa Valley, some innovative design solutions were needed to enable it to be approved for construction. While comprising 7 km of 5 m diameter pressure tunnel, sub-surface transformers and switchgear, and a buried 220 kV transmission link, the design has not required a new dam, surge chamber or adits for the tunnel so as to eliminate major construction activities in the Alpine National Park.
Formerly known as Kiewa No 2, the scheme had been planned as part of the Kiewa Valley development in the 1950s, when the McKay Creek power station upstream (now 150 MW following a recent upgrade) and the Clover and West Kiewa power stations downstream (totalling 90 MW) were constructed.
The No 2 scheme was to involve a dam below McKay Creek power station, and an additional diversion intake in an adjacent valley. The headrace was to have a surge shaft, while a surface power house was to be built at the river confluence just upstream of Lake Guy – the intake to the Clover power station. The proposed installed capacity was approximately 95 MW.
However, this final scheme did not proceed due to other competing state projects at the time. Although the cascade development did add 200 MW capacity to the Victoria state electricity system upon completion in 1961, there was an undeveloped gap between the top station (McKay) and the middle station (Clover). In 2001, a forecast of a shortage of peak generation capacity in Victoria, coupled with a launch of the Australian Government's renewable energy incentives, led to a preliminary study by Sinclair Knight Merz (SKM) to assess whether there might be an economically viable option for developing the scheme.
AGL is developing the Bogong project following its acquisition of Southern Hydro in 2005, which included the Kiewa Valley cascade among its assets. Southern Hydro was formed from the former State Electricity Commission of Victoria (SECV).
A number of factors ruled against constructing a dam for the Bogong power project. Building such a structure downstream of the McKay power station would not collect any more water than the releases to be made to the river for environmental flow, and geological factors would also make it a costly venture. Not least, constructing a new dam in the Alpine National Park would likely be opposed.
A new concept was developed to feed the McKay discharge directly into the Bogong tunnel, which would leave the river to run naturally and, most importantly, would eliminate the need for a new dam.
Using the same flow as McKay, the Bogong station will also have the same plant factor, which is typically less than 10%. While this provided good hydrology data for feasibility assessments, the challenge was clearly going to be in developing scheme concepts that would maximise output and allow construction at a cost that such a low plant factor could support.
Flow at the new station will be dictated by plant capacity at McKay, so opportunities to maximise output lay in maximising gross head and minimising head loss. The former was achieved by locating the Bogong power house adjacent to Lake Guy to gain additional head when the lake was drawn down over its large operating range, and by maximising the tailrace water level below the Pelton turbines at McKay.
With a typical gross head of about 420m, either Francis turbines or a single six-jet Pelton turbine could be installed at the plant. The Pelton option was favourable for matching operation of the six Pelton units at McKay and to alleviate issues with hydraulic transients in the tunnel. However, it could not develop the full head even if installed below maximum tailwater level with the tailrace blown down.
Benefits of the Francis option were significantly greater than the Pelton, considering the greater output from higher efficiency and additional head although this was not entirely clear until the tunnel hydraulic transient analysis confirmed that no substantial mitigation provisions would be needed.
Two Francis units were selected for the Bogong project. This will provide greater flexibility when the McKay and Bogong stations operate together to supply the Australian National Electricity Market (NEM), and it will reduce the impact of a unit trip during a market contract period.
With such a low plant factor the McKay and Bogong stations will operate jointly at periods of peak power demand. Operation will typically be at maximum unit output or peak efficiency, which is advantageous for the high peak efficiency of the Francis turbine compared to the flatter efficiency of the Pelton.
To provide for the joint operation of the Pelton turbines at the McKay station and the Francis turbines at the Bogong plant, SKM analysed their interaction to establish the minimum practicable head pond size and corresponding control concepts. As a result, the McKay tailrace will be diverted into a small head pond for the Bogong plant.
With the geographical constraints at the exit of the McKay tailrace, careful attention to the shape of the head pond was necessary to achieve uniform flow conditions at the tunnel intake for Bogong. SKM initially tested the concept design with computational fluid dynamics (CFD). Refinements from the CFD work were then verified in a physical scale model using the laboratory at the University of New South Wales.
SKM continues to work with Toshiba to carry the control concepts into final design and to integrate the Bogong control system with AGL's overall system.
A critical condition of approvals to develop the project is for there to be no impact on the Alpine National Park. All construction is to be limited to specific areas at the McKay tailrace outlet, the powerhouse site and an area uphill from the power house that provides a launch site for constructing the main headrace tunnel. This meant very little exploratory drilling could be done along the proposed tunnel route.
The three existing schemes in the Kiewa Valley all have significant tunnels constructed by manual drill and blast in the 1950s. In the same period there was some extensive exploratory drilling along the originally proposed tunnel route. Although some data had been lost, there was a reasonable indication of the geological profile along the route, consisting predominantly of hard granite (fresh granodiorite) with numerous wide fault zones across the route. These fault zones gave the most uncertainty, mainly in regard to the viability of using a tunnel boring machine (TBM).
Although the data available were for the ridge to the east of the Pretty Valley Creek, a more cost-effective tunnel route lay to the west. Limited drilling was carried out at the edges of the park to get some guidelines on the geology of the western side and to compare with the data of the eastern side. The western side was assessed to be similar to the east, including similar large fault zones.
Mechanised drill and blast tunnelling was not an option due to the need to run multiple faces to achieve a suitable timeframe. The associated access and disruption in the Alpine National Park was not acceptable.
TBM tunnelling was chosen for the main headrace tunnel, comprising most of the tunnel length. A system of drilling ahead was adopted to identify poor ground conditions before the TBM head reaches it. Knowledge from other scheme tunnels and the limited geotechnical data were used to make an estimate of the likely proportions of the predicted ground conditions that might occur along the tunnel route. Tunnelling methodologies and support types were decided for the range of ground conditions and used to establish a base contract price. The owner is taking the geotechnical risk and the contract price will be adjusted according to the classes of tunnel support that will be installed.
The TBM tunnel will cross under the Pretty Valley Creek from a drop shaft that will connect it to the head pond on the McKay tailrace. Near the downstream end, another drop shaft will connect the TBM tunnel to the high pressure headrace tunnel which will be about 1 km long and is being constructed by drill and blast.
The high pressure headrace tunnel will have a 3 m diameter steel liner where ground cover and rock quality are insufficient for the internal pressure. The steel liner becomes a penstock beyond the tunnel portal and bifurcates to the two turbines.
Controlling the pressure rise at the steel liner end in the event of a station trip was a critical component of the scheme design. The concept was influenced by various critical factors:
• With an annual plant factor of less than 10%, developing the project cost-efficiently needed particular attention.
• The scheme could not have a conventional surge shaft, not only due to the cost, but mainly due to the environmental impact of the associated surface construction and access that would be needed in an undisturbed area of the Alpine National Park. A pressurised underground surge chamber was not favoured.
• Minimising pressure rise was important to minimise cost of the high pressure steel liner and penstock piping.
• The use of Francis turbines set well below the Lake Guy water level will maximise output and efficiency of the peaking power station and also minimise the disturbance of discharge to Lake Guy and the impact of noise to the adjacent Bogong tourist village. However, flow through the high head Francis turbines will be rapidly throttled when the rotational speed increases after a unit trip.
• Conventional turbine relief valves operated by mechanical linkage to the turbine inlet vanes would not provide sufficient protection. Also, a solution that eliminated relief valves was favoured for reasons of reliability, future maintenance and dissipating energy of the discharge to the recreationally popular Lake Guy.
These issues and the highly important risk factors were worked through extensively.
Ultimately, Toshiba International has developed an innovative design for the hydro plant at Bogong and is supplying turbines and generators with characteristics that allow for the elimination of turbine relief valves, while optimising project costs. The detailed hydraulic transients analysis, using turbine flow characteristics determined from the turbine model tests, has shown that pressure rise can be maintained within permissible limits for the conduit system.
The power station output will be connected to the Mount Beauty terminal sub-station via unitised indoor generator step-up transformers, indoor switchgear and a single-circuit 220 kV underground cable connected directly onto the existing McKay 220 kV transmission line on the opposite side of Lake Guy.
Visual impact in the Lake Guy area, therefore, will be minimised. The indoor space was readily incorporated into the powerhouse layout above excavation levels.
A unitised electrical system through to the HV switchgear works well for transport and installation, as well as operationally and for registering the units with the NEM.
AGL awarded the three main engineering, procurement and construction (EPC) contracts in September 2006.
McConnell Dowell Constructors (Australia) is responsible for the civil construction, including the powerhouse, tunnels, head pond and tailrace. Toshiba International is the main electrical and mechanical contractor, responsible for the main hydro electric plant. Corke Engineering is responsible for the installation of all balance of plant. The indoor high voltage switchgear and transformers are being supplied by ABB. Other small contracts make up the miscellaneous works.
AGL is managing the contracts with technical assistance from major international consulting engineering firms.
SKM carried out the preliminary feasibility assessments for Bogong, worked with AGL to develop the scheme concepts and contract specifications, and is currently providing general technical assistance in the management of the construction contracts. AGL was supported in the approvals process by GHD Pty Ltd which also worked with SKM on the conceptual tunnel design and specifications using its knowledge of local geotechnical conditions.
For further information, please visit Sinclair Knight Merz – www.skmconsulting.com, and AGL Energy – www.agl.com.au
In October, Paul Caplen presented a separate paper on the project at Hydro 2007.
The development of the Bogong project follows a successful string of enhancement projects and new plant by AGL in which SKM has assisted in the development of AGL's renewable energy portfolio. The projects include: