Pumping up power down under

THE Wivenhoe pumped storage plant is of strategic importance to the electricity network in northeast Australia. Owned and operated by Tarong Energy, it was once part of Queensland’s state-owned electricity utility but since 1997 has been a state-owned corporation operating in Australia’s national electricity market (NEM).

In Queensland, coal is cheap and readily available. As a result, Tarong’s major capacity is coal-fired – it has 1400MW of existing plant and 450MW under construction. But the two 250MW units at Wivenhoe are the biggest standby generators in Queensland and therefore are of great importance in the NEM. Apart from dealing regularly with peak loads, Wivenhoe provides a measure of security in the state’s vulnerable electricity grid. While Queensland has no shortage of power from several generating companies, the major load centres are strung along a 275kV backbone that extends 2000km from north to south of the state, and is as far again from other load and generating centres in New South Wales and the rest of the NEM.

According to Tarong Energy ‘the physical operation of Wivenhoe could not be said to have altered radically with the advent of deregulation’, although operators say that the number of plant starts and stops has increased. The key is the abundance of cheap coal-fired electricity available in the state, which in the days of central dispatch meant ‘there was little difference between the marginal fuel costs of the cheapest and most expensive generation’, according to company statements.

Tarong says that now, with the move to an energy market, the peak/off peak price differential is generally greater than that previously implied from marginal fuel cost differentials alone. However because the peak and off peak prices are not relatively fixed and known in advance, the capture of this margin is less of a science than it once was.

While capacity satisfies demand in Queensland at present, the state’s population is growing rapidly and power demand is growing with it. That growth will represent an opportunity for Wivenhoe: as capacity margins shrink the NEM should ensure that peaking capacity becomes more valuable and the price differential will increase.

Tarong has invested in this valuable asset and at the end of the 1990s both units underwent upgrading to solve problems that had stopped them from operating at low loads since the plant started up in 1984, and others that had reduced the plant’s maximum power by between 20 and 40MW.

  ‘We have Francis turbines so we had cavitation at low load. In generating mode we could only operate with the guide vanes at 40% and above, otherwise the vibration might have damaged the turbine,’ says plant manager Trevor Lush. That meant the plants had to be ramped rapidly up to 120MW. Now, however, tests have shown that vane positions below 25% were also acceptable, so the governor was modified to keep the vanes in the correct ranges. Each unit can operate in the 0-60MW band, so using the two units in combination or singly allows the entire range of output, except the 120-160MW band, to be covered.

Problems operating at high power were discovered in 1987 when Wivenhoe dam filled up for the first time. Previous owner, AUSTA Energy, traced noises when the turbines were operating at full load to high load on the draft tube bearing. These were exacerbated by a phenomenon known as the Magnus effect which occurs when the spinning pump shaft is subjected to a cross-flow inside the turbine draft tube. When shaft velocity and water velocity are in the same direction a lower pressure is formed and the new pressure differential increases bearing load on the shaft by some 50t. This resulted in loss of clearance, severe rubbing and material loss. Tests in co-operation with the manufacturer Toshiba confirmed the Magnus effect. It was decided that the effect could not be removed and instead a solution should deal with its results.

A group under Jurgen Sprengel at AUSTA Energy developed a solution in the form of a water injection system, which uses a variable frequency controlled motor coupled to a single stage injection pump. The system flow rate – 13.5litres/sec at a pressure range of 1.8 to 3.1 MPa – is measured by a magnetic flow meter and maintained by the variable speed drive. It comes on automatically above 75% turbine guide vane opening. As the generating load increases, the turbine shaft moves into the draft tube bearing in an area where nine water injection points have been installed. As the reduced bearing clearance increases backpressure and reduces injection flow, the system responds by increasing pump speed to maintain the set system flow. This balances the turbine shaft load and maintains clearance.

The system was installed at unit 1 in 1994, and following some modifications to deal with power spikes, at unit 2 at the end of the decade. Each unit can now operate at up to 312MW.

With Wivenhoe’s major problems addressed, Tarong is now focusing its efforts on improving day to day operation of the plant. Over the years, upgrading work is adding instrumentation that will allow far more detailed information to be collected during plants operations. ‘The data logger was changed in 2000,’ Lush mentions as an example. ‘The new one will monitor and model the plant and we can plug in more field devices. It means we can do remote examinations, and also reduces callouts because if there is an alarm we can “dig in” to the data remotely.’

That work is continuing, Lush says. ‘The existing flow meters were difficult to calibrate. More modern meters can trend data, so failures can be predicted.

‘Since the market was developed in the state we have run with lots more stops and starts,’ says Lush. ‘There is a new focus on reliability, partly because the station is unmanned.’

Wivenhoe can now operate at its full power range, and its reliability is high and increasing. For Tarong Energy and the Queensland electricity grid, it will continue to be of strategic, and growing, importance.

Tables

wivenhoe power station
wivenhoe dam

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