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The Story Electrical I Left: ever wondered how big the insulators on 330kV lines are? This photo, taken during the construction of the 330kV line from the Murray 1 power station, gives the answer. Most power stations in the Snowy Mountains scheme are underground but the two largest are above ground, at Tumut 3 & Murray 1. Tumut 3 is also used during off-peak times for pumping water for energy storage. Leaving behind the Snowy uplands and Cabramurra (Australia's highest town), the discharge water from the underground power stations plunges downhill to fill Talbingo reservoir w~ich is 544 metres above sea level. Lying in the steep gorges of the upper Tumut river valley, the waters are impounded by Talbingo Dam, the most 88 SILICON CHIP massive structure in the Snowy system. Talbingo Dam is 162 metres high and over 600 metres thick at its base, and contains over 20 million tonnes of earth and rockfill. Its capacity is 921 gigalitres. The function of this reservoir is to provide the water head for Tumut 3 power station, the largest by far in the Snowy scheme. From the darn, water flows via a 945-rnetre long headrace channel to the pressure pipeline inlet structure. This channel, 92 metres deep and up to 190 metres wide, took three years to complete. The headrace channel feeds water into the six pressure pipelines which feed the power station. Each pipeline, 5.56 metres in diameter, is big enough to drive a doubledecker bus through. The pipes were constructed of rolled medium-tensile steel plate up to 30.2mm thick, manufactured in sections and joined by electric welding on-site. Each pipe has to withstand enormous forces in service - the pressure of the 150.9-rnetre head and the inevitable turbulence from the water flowing at up to 189 cubic metres per second. Therefore, the welding of pipe sections was a critically controlled operation. For stress-free welds, the pipes were heated to 66°C and held at that temperature, while electric welding was performed. Imagine, if you will, a tradesman arc welding inside that hot pipe section! But it had to be done at every joint in more than 10,000 tonnes By BRYAN MAHER of Energy, Pt.13 of steel pipe. N0 1 wonder the construction of the six pipelines took 4½ years! Tumut 3 power station Largest by far in the Snowy scheme, the above-ground Tumut 3 power station generates up to 1500 megawatts. The six 250MW alternators were the largest water-driven machines in Australia at the time of installation (between 1968 and 1972). Of the vertical shaft salient-pole type, each rotates ever so quietly at only 187.5 RPM. They have 32 poles and produce 15.4kV which is transformed up to 346kV for statewide transmission. The huge machine hall is 154 metres long and 60 metres high. The alternators are on original ground level, with the Francis type water turbine equipment extending 33 metres below. Two 130/20 tonne capacity overhead travelling cranes, used to construct the machines, are now used only for maintenance operations. For heavy lifts such as on the enormous generator rotors, both 130-tonne hoists are used in tandem using a special lifting beam. Unlike all the other power stations in the Snowy, the tailwater from the Tumut 3 water turbines flows uphill against a small head - 16.5 metres maximum. This water forms Jounama pondage, captured by the small Jounama Dam, a rockfill/ earthfill wall 44 metres high, 5.8km downstream from the power station. Pumped storage The eternal problem in all electricity generating systems is the daily cycle of peak and off-peak loads. Demand in NSW may be as high as 9 gigawatts during winter evening peaks but may fall to as low as 4.4 gigawatts around 4-5am the next morning: Steam-driven generating plants, with their slow thermal stabilisation time, cannot cope with such large variations. Start-up time is usually many, ma:µy hours and it is very difficult and inefficient to have boilers up to temperature, ready for load, but not yet needed. Large modern boilers have a minimum load value and cannot be operated at lower demand. Ideally, the thermal power stations should be used for supplying the constant base load 24 hours per day. The Snowy power stations can then be used at their full capacity only during the large morning and evening peaks. They can be brought on line very quickly, within a few minutes, and then taken off line just as quickly. Water turbines are large machines as this photo of a turbine spiral casing shows. Ten of these 97MW machines are installed in the Murray 1 power station. Water flows through each turbine at a rate of 241 cubic metres/second. SEPTEMBER1991 89 Tumut 3 power station is notable not only because it is the biggest power station in the Snowy scheme but because it also pumps water back up into Talbingo reservoir for use at peak times. It uses six 250MW alternators. Obviously, in dry times, there may be a limit to the amount of water available. We don't want to empty the dams just to supply peak hour electricity demands. To get around that problem, the Snowy scheme uses pump storage. To this end, three of the six machines at Tumut 3 power station are provided with extra equipment. As the cross section diagram shows, the three units at the western end of the station have much longer vertical shafts. In generating mode, the alternator is driven by the 254MW water turbine immediately below it. Further below 90 SILICON CHIP this , at the bottom of the shaft, is a huge centrifugal pump capable of pumping 100 cubic metres of water per second uphill against a 155-metre head. In pumping mode, the alternator is operated as a 250MW synchronous motor, driven by electrical power drawn from the state grid. This is done only during early morning, around 1-4am. In this mode the machine rotates in the same direction and at the same speed as it does when generating. Force pumping Lift pumping with the pump above water is unsatisfactory and severely limited due to cavitation (ie, water vaporising around the pump blades). Force pumping (with the pump below the water supply) is the only solution. It was for this reason that Jounama Pondage was provided. A section diagram of Tumut 3 shows that although it is an outdoor power station, when Jounama Pondage is at full service level (FSL), the alternators and much of the power station are actually lower than the level of the water outside, separated by a thick concrete wall. The sequence to start pumping is an interesting one. It goes as follows: (1) The turbine water inlet valve is opened and water flows down from Talbingo reservoir to drive the turbine and alternator, thus generating full voltage. (2) The alternator is synchronised with the state grid. (3 ) The turbine inlet valve is closed but the machine continues to rotate, the alternator now acting as a motor driven by the state grid. (4) The pump valve is opened and water is pumped from Jounama Pondage back uphill to Talbingo reservoir. Power to drive the machines when they are acting as motors is supplied (via the 330kV state grid) from the thermal power stations located in the Newcastle region. This pumping operation is only undertaken very early in the morning, when the state load is very light. In these dark hours , it is economical to keep the thermal stations running to provide the 750MW pumping power, as large boilers cannot be operated below some minimum load. Thus, the steam turboalternators and boilers at Eraring and Bayswater are kept thermally stabilised, ready for the morning peak which begins around 6am. In effect, electricity is being stored for later use. Snowy/Murray system To find the greatest water head of all hydroelectric plants on the Australian mainland, we must look to the Murray 1 power station. This is part of the southern half of the Snowy scheme - the Snowy-Geehi-Swampy Plains-Murray development. During wet seasons, the Murray 1 and Murray 2 power stations are driven by water caught in the precipi- ,\ TWO 130,'20 TON OVERHEAD TRAVELLING CRANES '- EARTHFILL FOUNDATION - ' EXCAVATION LINE RL 1197 PUMP SCA l f 01 i lf . SECTION ALONG TUMUT 3 POWER STATION RL 1152 Above: this cross-section diagram of Tumut 3 power station shows how three of the alternators double as pump motors. The turbines are immediately below the alternators, while the three centrifugal pumps are right at the bottom. Note that the tailwater from this power station is above the turbine, so that the pumps can operate. tous gorges of the Geehi River catchment. Geehi Dam, a rock/earth wall 91 metres high, impounds 21.1 gigalitres of water 1106 metres above sea level (higher than the Blue Mountains). The overflow spillway for this dam is unusual. A 32 -metre diameter bellmouth opens into an 8.84-metre diameter underground tunnel which passes under the dam wall. On the downstream side, this overflow tunnel exits to open air in a flip bucket which spouts the escaping water upwards to prevent erosion. Snowy-Geehi tunnel Another view ofTumut 3 power station, this time from above the headrace. The high voltage switchyard is in the background. At times , insufficient rain/snow falls on the western side of the ranges but the eastern Snowy River may be running full. In such circumstances, water pumped up from Lake Jindabyne flows via the 14.4km Island Bend-Geehi transmountain tunnel to refill Geehi reservoir. In still dryer periods, water is allowed to flow from the Lake Eucumbene mass storage via the EucumbeneSnowy tunnel to Island Bend; thence under the Great Divide (1000 metres SEPTEMBER 1Y91 91 provided for two pipes but the third was added during construction. These three pipes weigh 13 ,000 tonnes, plus the weight of water inside. At any one moment, those pipelines contain 43,000 tonnes of water rushing downhill at an average velocity of 93km/hour. At a bifurcation before the power station, the three pipelines divide into 10 conduits, each leading into a turbine. The 10 vertical shaft Francis turbines, each driven by water at the rate of 24 tonnes per second, provide a total of 950MW. Murray 2 power station Water is discharged from Murray 1 into the Murray 2 pondage, thence through a tunnel and pipelines to the Murray 2 power station, the last and most western on the Snowy scheme. This Murray 2 pressure tunnel is one of the largest in cross section, 7. 5 metres in diameter, steel reinforced and concrete lined throughout. Four machines, each rated at 137.5 megawatts, produce 17kV to feed the stepup transformers. Interstate electricity This is the Murray 1 power station which has the highest head of any hydroelectric station in Australia. It has 10 vertical shaft Francis turbines which provide a total power output of 950MW. below the mountain peaks above) to Geehi. From Geehi reservoir, water flows westward via the Murray 1 pressure tunnel towards the Murray power stations. The flow of water into and out of Geehi reservoir is controlled by separate valves, immense 7.6 x 3.7-metre bulkhead gates, stoplogs and guard gates. The mind boggles at the immense size of these valves - such is the amount of water used by the Snowy scheme. Murray 1 pressure tunnel The Murray 1 pressure tunnel carries water from Geehi to the heads of 92 SILICON CHIP the Murray 1 pressure pipelines. Steel and concrete lined throughout its 11. 7km length, this 6. 93-metre diameter conduit carries 250,000 litres of water every second! An 8.2-metre diameter vertical surge-relief shaft, 114 metres high, is situated half a kilometre back from the western end of the pressure tunnel. The top of this shaft opens to atmosphere into a 61-metre diameter surge pool situated on the mountainside above. The pressure tunnel leads through valves into the three pressure pipelines running 1.5 kilometres down the mountainside to the Murray 1 power station. The originai design The Murray power stations were designed to supply the first major interstate transfer of synchronous electric power in Australia. The Murray Switching Station is an immense array of 345kV busbars, circuit breakers and protection equipment. From here, the outputs of both Murray power stations are carried by three powerlines to Dederang, where they join the Victorian grid. In addition, 330kV interconnections join the Murray power station to the Upper Tumut and Yass substations. These links allow power to be interchanged between the thermal power stations of NSW near Newcastle and those in Victoria near Yallourn and Morwell. This, the first interstate synchronous interconnection, has since been extended to South Australia. SC .Acknowledgement Grateful acknowledgement and thanks to Libby Langford and the Snowy Mountains Hydroelectric Authority for data, photos and permission to publish.