In 1961, the completed cascade hydropower development of the Kiewa Valley added about 200 megawatts (MW) of capacity to the Victorian electricity system. Forty-six years on, a missing link in the cascade is now under construction. For compatible construction in a pristine Alpine National Park, challenges in the concept design resulted in some innovative solutions.

The Bogong Power Development Project, in the Kiewa Valley, Victoria, is the largesthydroelectric power plant to be built in mainland Australia in over 25 years. The construction of a hydroelectric power station and a tunnel linking two existing hydroelectric power stations will add 140 MW, creating a 340 MW renewable power development.

Approval to proceed with construction was granted in September 2006 after achieving environmental and regulatory approvals. In November 2007 tunnelling works began on a 5 metre diameter tunnel that will convey water from the existing McKay Creek Power Station to two 70 MW Francis turbine units in a new subsurface powerhouse – the Bogong Power Station. The scheme is expected to be on line by the end of 2009.

One of the key requirements for the project’s development is for there to be no impact on the Alpine National Park. The development’s design avoids a surge chamber and construction adits on the tunnel, eliminating surface construction activity in the National Park. The buried powerhouse will avoid sound and visual impacts on the nearby Bogong village and the peaceful lakeside environment.

The project is being developed by AGL after purchasing the assets of Southern Hydro, which had formed from the former State Electricity Commission of Victoria.

From Kiewa No 2 to Bogong Power Known as Kiewa No 2, the scheme was initially planned as part of the power development of the Kiewa Valley in the 1950s when the McKay Creek Power Station upstream – now 150 MW – and the Clover and West Kiewa power stations downstream – totalling 90 MW – were constructed.

The scheme was to involve a dam below McKay Creek Power Station (the top station), and an additional diversion intake in an adjacent valley from the Clover Power Station (the middle station). The headrace was to have a surge shaft, and a surface power house was to be built at the river confluence just upstream of Lake Guy – the intake to the Clover Power Station

However this 95 MW final scheme did not proceed, leaving a gap undeveloped between the top station and the middle station.

A 2001 forecast of a shortage of peak generation capacity in Victoria, coupled with the launch of the Federal Government’srenewable energy incentives, led to a preliminary study assessing whether there might be an economically viable option for developing the scheme.

Developing the Bogong Power project – some innovative solutions The development of the Bogong project saw numerous changes and enhancements to the initial Kiewa 2 scheme to develop ahighly efficient scheme that has minimal impact on the Alpine National Park.

No new dam The initial scheme included plans for a new dam, planned for downstream of the McKay Power Station. However it was found that this dam would not collect any more water than would need to be released back to the river for environmental flow. Avoiding the significant cost and the likely opposition to the construction of a dam in the Alpine National Park, a new concept was developed to feed the McKay discharge directly into the Bogong tunnel, leaving the river to run naturally.

Using the same flow as McKay, the Bogong station would also have the same plant factor, which is typically less than ten per cent. While this provided good hydrology data for feasibility assessments, this presented the challenge of developing scheme concepts that would maximise output and allow construction at a cost that such a low plant factor could support.

Olympic headpond The McKay tailrace will be diverted into a small head pond about half the size of an Olympic swimming pool.

SKM analysed the interactive operation of the Pelton turbines at the McKay stations and the Francis turbines at the Bogong station to establish the minimum practicable head pond size and corresponding control concepts.

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.

Maximising output 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 tail race water level below the Pelton turbines at McKay.

With a typical gross head of about 420 metres, either Francis turbines or a single 6-jet Pelton turbine were the plant options. 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 unit maximum output or peak efficiency, so the high peak efficiency of the Francis turbine is a significant advantage over the flatter efficiency of the Pelton.

The power station will use two Francis units to allow the McKay and Bogong stations to be operated jointly with greater flexibility when supplying the Australian National Electricity Market, and to reduce the impact of a unit trip during a market contract period.

Tunnel boring With the strict environmental requirements and all construction limited to the existing lease areas, very little exploratory drilling could be done along the proposed tunnel route.

The three schemes already existing in the Kiewa Valley all have significant tunnels, which were constructed by manual drill and blast in the 1950s. In the same period there was extensive exploratory drilling along the tunnel route originally proposed. Although some data has since been lost, there was a reasonable indication of the geological profile along the original route. It comprises predominantly hard granite rock (fresh granodiorite) with numerous wide fault zones across the route, raising concerns about the proposed tunnel route and the viability of using a tunnel boring machine (TBM).

An alternative tunnel route, which lay to the west and had a similar geological profile to the original route, was chosen. TBM tunnelling was chosen for the main headrace tunnel, comprising most of the tunnel length and a system of drilling ahead was adopted to identify poor ground conditions before the TBM head reaches it.

The TBM tunnel, which is now is 4.61km into the mountain, 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.

The High Pressure Headrace Tunnel will have a 3 metre diameter steel liner where ground cover and rock quality are insufficient for the internal pressure.

Tunnel pressure control A critical component of the scheme’s design was controlling the pressure rise at the steel liner end in the event of a station trip. The final concept was influenced by a set of five critical factors, including the project’s cost-efficiency, the requirement to avoid a conventional surge shaft due to its environmental impact and the need to minimise pressure rise. These issues and the highly important risk factors were worked through extensively.

Ultimately, the development’s electrical and mechanical contractor, Toshiba 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 company’s hydraulic transients analysis, using turbine flow characteristics determined from the turbine model tests, has shown that pressure rise can be maintained within allowable limits for the conduit system.

Hidden switchyard A hidden switchyard was chosen to minimise visual impact in the Lake Guy area. The power station output will be connected to the Mt Beauty Terminal Substation via unitised indoor generator stepup transformers, indoor switchgear and a single-circuit 220 kilovolt (kV) underground cable connected directly onto the existing McKay 220 kV transmission line on the opposite side of Lake Guy.

Looking ahead Marking the start of tunnelling works in November last year, Victorian Premier John Brumby recognised the Bogong Power Development’s contribution to the state’s renewable energy needs.

He said “Delivering the power we need for a strong economy and growing population is all about working with what we’ve got, while reducing our impact on the environment.”