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How power plant infrastructure can disappear inside a mountain

What is currently becoming Italy’s largest power plant project is proceeding almost invisibly. Ultimately, the new St. Anton power plant is being constructed in the depths of Hörtenberg,

... Bozen’s local mountain to the north of the capital of South Tyrol. The private entrepreneurs of the project, Karl Pichler and Hellmuth Frasnelli, are investing around 55 million euros to place the power plant concept for the facility, which has existed since 1951, on an entirely new footing. The key aspect of this construction project is to eliminate the surges in the Talfer River, which have so far claimed the lives of 21 people since the existing power plant has been commissioned. In addition, the efficiency of the plant will increase and the installed machine output will rise from the previous level of 72 MW to 90 MW. All of the electromechanical equipment for this project is made in South Tyrol. The renowned hydropower specialist Troyer AG from Sterzing is supplying the equipment ready to use, and these are the largest turbines the company has ever delivered. The units have just been switched on for the first time and they have been generating power since early May.

As one of the most important local sites for recreation in the Bolzano municipal area, the meadows along the banks of the Talfer have always attracted large numbers of people, particularly in summer. “This is understandable. When the Talfer carries only 1 m3/s of water, people like to go in the river and enjoy sitting on the rocks. Unfortunately, in the past barely a month went by without people being rescued from the torrential Talfer because suddenly a surge of 16 m3/s dashes down,” says Karl Pichler from Eisackwerk GmbH outlining the well-known, established problem with St. Anton power plant. In its history, since it began operating in 1951, 21 people have lost their lives in the Talfer. The reason: the surge which transformed the small stream into a torrential river in just 40 seconds. It is also obvious that such fluctuations in the flow of water have a devastating effect on the fish population and the fauna in the bed of the stream. But this downside to St. Anton power plant will be a thing of the past, as Karl Pichler explains. For this purpose, the new operators from Eisackwerk GmbH constructed a large underground equalizing basin in the turbine outflow which will ensure a continuous outflow from the power plant in the future. A cubic volume of 150,000 m3 has been blasted into the mountain to create the new demodulation basin. The basin, which is 900 m long and 13-15 m wide, can hold around 95,000 m3 of water. The construction works for it have already been completed.

Environmental impacts eliminated
However, the project's leaders Karl Pichler and Hellmuth Frasnelli established the most important milestone in the new construction project back in February 2015. After a legal dispute lasting more than four years, Eisackwerk GmbH was officially declared the winner of the tender offer for St. Anton power plant. A short time later, the private company was able to acquire the power plant from SE Hydropower. With an installed power output of 72 MW and a standard capacity of 270 GWh, the plant was and still is the fifth largest hydropower plant in South Tyrol. But there is still room for improvement. Pichler and Frasnelli have set a target of increasing the yield by 10 per cent. The focus in redesigning the plant was on boosting efficiency, higher environmental compatibility and operational safety. “We are nullifying all environmental impacts by moving the whole power plant into the mountain to a depth of 300 m”, explains Karl Pichler, adding: “We have already gained a great deal of experience on how this can be done in construction terms from Mühlbach power plant.” The Mühlbach HPP officially began operating in 2012 and all of its infrastructure is   likewise housed inside a mountain. It is still regarded today as a truly pioneering feat of power plant construction, and it has delivered many benefits for the community of Mühlbach and its residents.

Special recognition
The positive response to the new construction of the traditional St. Anton power plant has in fact been even stronger, as Karl Pichler confirms: “The project was recognised by the President of Italy, Sergio Mattarella, as one of the ten most environmentally friendly projects in Italy. It is a first in Italy for a hydropower project to receive such recognition.” This of course is no coincidence. There probably is no other power plant in Europe where such an effort has been made to eliminate the negative effects of hydropeaking. It is no wonder then that the St. Anton project has also received the highest praise from fishermen, who are often obviously sceptical towards power plant projects.

Favourable construction process
The majority of the excavation and concrete works on the equalizing basin, the machine cavern and the access tunnels were completed back in the middle of last year. “Thanks to the favourable progress of the construction, we were able to meet our timetable precisely,” Karl Pichler outlines with delight. The underground construction works were facilitated by the general geological conditions. The Bozener porphyry rock is as hard and stable as granite, moving forward through blasting therefore works smoothly. At the height of the excavation works, the team advanced through the rock up to 12 m per day while approximately 1,800 kilograms of dynamite were exploded daily and more than 1,500 m3 of excavated material were transported out of the mountain. In total, up to 270,000 m3 of rock have been removed. The geological stability means that even the large caverns require hardly additional reinforcing concrete arches for stabilisation. Prior to these works, the reservoir situated on the plateau above was also emptied and cleaned up. The entrepreneurs also relied on an innovative method for this operation. “A specialized company that we commissioned flew floating platforms onto the reservoir by helicopter. Powerful pumps were attached to them using suction pipes to convey mud and water onto the floating platforms where the mixture of mud and water was passed through a Coanda screen to remove coarse debris. During the water abstraction, the water containing the fine sediment was released through the turbines before the catchment and thus channelled through the turbines. We found a sediment load in the works water of no more than 3%, which is fine. During storms, the works water is contaminated with much higher loads of suspended solids. Using this environmentally friendly method, we have cleared up the whole area in the water catchment in the reservoir. The coarse-grain sediments have been deposited safely away from the water catchment,” says Karl Pichler. The reservoir holds around 350,000 m3, 320,000 m3 of which can be utilised to produce hydropower.

Vertically through the mountain
The excavation of the shaft for the vertical penstock also proceeded on schedule using a raise boring method. This involves first drilling a pilot bore from top to bottom, in case of St. Anton power plant over a vertical distance of 523 m. The hole was then widened to a diameter of three metres from the bottom upwards – a proven and efficient method. The pipe shaft was excavated by the end of July last year. The rest of the procedure, in which the penstock has been installed in the shaft, proved to be particularly challenging. As with the construction of the vertical penstock at Mühlbach power plant, the steel penstock was lowered into the shaft from above as well. The 12 to 13 m long pipe sections were welded onto the piece of pipe below, the welded seam was checked and treated with protection against corrosion – and then lowered further down. This process was continued until the pipeline had reached the level of the demodulation basin. “For this purpose, a stable tower that can support the biggest loads has been built at the top end of the shaft. In the end a mass of around 800 tonnes hung on this tower. There are huge forces at work at the plant,” explains Karl Pichler. From the beginning of February to the beginning of April, the pipeline was cast in concrete in the vertical shaft. The pressure test was carried out successfully in the second half of April.

Largest turbines in the company’s history
While the excavation works for the pipe shaft were still well underway, the machine installation work had begun last year's summer. At a depth of 300 metres inside the mountain, three identical turbines were installed which deliver a power output of 90 MW totally. The contract to supply all the electromechanical equipment at the power plant was secured by a company that is based not far from the power plant construction site: Troyer AG from Sterzing, which previously supplied the machines for Mühlbach power plant. With an output of 30 MW each, these are the biggest and most powerful turbines that the experienced hydropower specialist has so far manufactured in its history as a company. “Never change a winning team,” says Karl Pichler, referring to the “very good experiences gained in Mühlbach”. “We have seen that Troyer is in essence more than a match for the very big players in the industry – and also delivers the added benefit of implementing the plant so that it is fully ready for use, including transformers and generators.”

Limits reached
For the medium-sized family enterprise, the project does of course represent a major challenge. “In terms of design, there were fundamentally no big differences compared to small-scale hydropower for us. But the situation is definitely different as far as the internal logistics and manufacturing technology are concerned. Due to the significant dimensions of the individual components, we encountered a few limits – particularly in relation to gates or the handling of the large parts,” explains Thomas Fiechter, Troyer AG's project manager. The three turbine housings were completely preassembled at the Troyer AG factory in Sterzing and delivered to the construction site along with the ring line and the shut-off valve. By August 2018, all three turbine housings had been delivered to the St. Anton cavern. They were followed by the generators in mid- September, which have been transported from Dresden to South Tyrol. The medium-voltage installations were then delivered and mounted. They are designed for short-circuit protection for currents of up to 63,000 A. The cabling for the installation proved to be a particular challenge. Thomas Fiechter says: “In total, 186 medium-voltage end terminals were implemented with more than 16 km of medium-­voltage cables. Moreover we supplied and provided the cabling for all the control and automation cabinets.” In addition, two high-­voltage transformers were supplied, fitted and wired up along with all the control, automation and protective fields.

Designed to meet the highest requirements
The installed turbines are specifically 4-nozzle, vertical-axis Pelton turbines with brake jets. Rotating inside the turbine housing are rotors with an external diameter of 2,080 mm on which a total of 19 buckets have been fitted. The rotors were milled from a monobloc of highly alloyed steel and naturally configured in terms of their hydraulic design to cater for the high demands and requirements in the optimum way. The works water does at any rate overcome a drop of around 595 m from the reservoir down to the machine cavern. This means that normally a pressure of around 60 bar acts on the shutoff valve and turbines and the water leaves the nozzle needle at a speed of around 385 km/h. In this way, the rotor drives the directly coupled 3-phase synchronous generator via a free-flying shaft at 600 rpm. Each of the turbines is designed for an absorption capacity of 6 m3/s. These are high-end machines, both in terms of their robustness and availability and as far as efficiency is concerned reflected the cutting edge of today's hydropower technology. “We did of course carry out numerical studies and analyses beforehand. We reckon the maximum turbine efficiency level is 0.915,” says Thomas Fiechter.

Improvements to all components
The machine shut-off valves in use are spherical valves with a mobile operating and inspection seal of dimension DN1000 and pressure class PN80. They were designed and optimised for operation at the new St. Anton power plant by Troyer AG's engineers, likewise conducting numerical calculations. This is also true incidentally for the ring line and the pipe distributor. In each of these components, flow-related losses can of course occur if they are not optimally adapted. As the water level in the demodulation basin rises with a fill level of 8.10 metres, the turbines must also be positioned higher. “There is another drop in height as a result of the 362 m long return channel. Despite this, through careful planning we succeeded in limiting the loss in the drop compared to the old structure to 4.96 m,” says Karl Pichler, explaining how the target for an increase in efficiency of 10 per cent should still be achieved: “Thanks to the new equalizing basin, the new power plant will be able to process 18 m3/s rather than the previous figure of 15 m3/s. It is also possible to reduce friction losses thanks to the new penstock. Compared to the old structure with DN1700/1600, the new penstock now has a clear width of DN2200. What is more, we now have more efficient and powerful turbines and will be able to reduce the downtimes to a minimum.  And not to forget: The drop at the equalizing basin is also utilised by two Kaplan turbines from Troyer AG. All  these measures combined we will achieve a 10 per cent increase in yield that we are looking for. We are in fact secretly hoping for an increase in production of up to 20 per cent.” With an electrical bottleneck capacity of 90 MW, in an average year the new St. Anton power plant will in future generate around 300 GWh of clean electricity from the River Talfer.

Power plant as grid stabiliser
The new cavern power plant in the north of Bozen is designed as a peak-load power plant and is also intended to be operated as such. Thomas Fiechter says: “This fact does of course place the biggest demands on the generator and the medium-voltage installation. Both have been designed for repeated start-up and phasing out of the turbines – generally up to five times a day – is guaranteed without any problems.” The electricity which is generated is brought out of the mountain at a medium-­voltage level of 13,800 volts and then increased up to 220 kV.
The importance of the fifth-largest hydropower plant in South Tyrol for the stability of the grid cannot be overestimated. Particularly in relation to the sharp rise in volatile sources of electricity, such as photovoltaics and wind power, there is a need for powerful facilities which can, when required, deliver the necessary balance at the frequency band of the supply network.
But a new era will also shortly be dawning as far as operational safety is concerned. The water level in the River Talfer may of course still vary a little in the future – specifically at a ratio of no more than 1:4, with the rise and fall in the level taking place gradually. But this will be quite harmless compared to the previous hydropeaking ratio of 1:16. This means that the power plant will not only be safer for nature lovers along the River Talfer, but will also be more beneficial ecologically for all of its residents. The first electricity was generated in early May 2019.

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