Verbund and Salzburg AG celebrate inauguration of joint hydropower plant Gries

It was in mid-June under a glorious blue sky when VERBUND and Salzburg AG celebrated the official inauguration of their new joint power plant…

… in the Gries district of the city of Bruck an der Glocknerstrasse. With its two vertical Kaplan turbines and a combined maximum capacity of 8.9 MW the new power plant on the Salzach is expected to supply more than 10,000 households with clean energy. Extensive structural and technical adjustments reduced the overall costs by 20 percent to ensure the project’s economic viability. As for electromechanical equipment and hydraulic steelwork engineering, the operators relied on the competence of well-established hydropower specialists.

With the environmental impact assessment completed already in the summer of 2010, hydropower plant Gries was finally given building permission in January 2013. In view of the low electricity tariffs at the time, however, it turned out that the construction costs would have rendered the approved power plant concept unprofit­able. To be able to go ahead with the project after all, the project team had to work out the constructional and technical adjustments needed to cut the costs by about 20 per cent. In addition to reducing the required cubature, the constructional modifications had to enable the installation of cost-efficient equipment. The revised power plant concept called for two vertical Kaplan turbines, each in its own power house on either side of the river, with two weir gates in the middle. Not only did this concept provide the required economic advantages, it also allowed a considerable optimisation of the construction process. What is more, the updated construction plans opened up more flexible design options in terms of the machine arrangement and the corresponding overall constructional design of the facility. Where the plant’s operation was concerned, the operators expected the symmetric architectural geometry to ensure significant advantages in terms of bedload transport while the plant is in operation. In 2015 the planned changes were submitted for permission, which was granted already in April 2016, allowing construction work to commence soon after in June.

OPTIMISED SUCTION PIPE SAVES COSTS
When the water flow rate rose above average levels during the inauguration ceremony, that provided an opportunity to verify that the floodway was working properly. Although each of the weir gates is designed to handle the maximum flood water flow on its own, both locks were fully opened for security reasons during the ceremony. Thanks to the arrangement with a power house on either side of the river, it was possible to install two Kaplan units with vertical shafts and directly coupled generators instead of the originally planned horizontal pit type bulb turbines. Switching to this turbine model extended the range of possible suppliers to include a number of internationally renowned small-scale hydropower specialists. At the same time, this turbine version required a considerably lower constructional effort compared to vertically aligned machines. A further reduction in construction costs was to be achieved by modifying the design of the draft tubes to reduce the required excavation volume. The calculations and model designs for the alternative draft tube concept were provided by the Institute for Hydraulic Fluid Machinery at the Graz University of Technology. To ensure that the intended changes would not deteriorate the turbines’ performance, the modifications were tested out by way of a numerical fluidity simulation. In the end, the engineers settled on a draft tube design based on a trade-off between acceptable turbine efficiency and sufficiently low construction costs. Each of the turbines is optimised for a design flow rate of 57,5 m³/s and a gross head of 8.9 m, resulting in a bottleneck capacity of 4.45 MW under maximum flow rate conditions. Thanks to their special design the double-regulated machine units cover a wide operational range both under full and partial load, which ensures a high level of efficiency at varying flow rates.

CUSTOM-BUILT GENERATORS
Two synchronous generators coupled directly to each of the turbine shafts ensure the efficient transformation of the turbine runners’ kinetic energy into electric energy. The generators were manufactured by electrical engineering specialist ELIN Motoren GmbH, who enjoy a world-wide reputation as providers of high-quality machines for industrial and power plant environments. Weighing around 60 tons each, the generators are among the heaviest components of the facility. Like the turbines, the multi-poled generators run at a speed of 150 rpm, which allows them to achieve an apparent power of 5,200 kVA and a nominal voltage of 6,300 V. Cooling is by way of an air-to-water heat exchanger, which uses forced ventilation to provide the cooling air stream for the generators. The project managers at ELIN Motoren GmbH emphasize that the generator for the Gries power plant was a highlight in Austria‘s hydropower sector. Custom tailored to the limited height of the power houses, the generators also had to be easy to maintain. Due to the structurally optimised height of the power houses, the machines had to be dimensioned as small as possible, which required a solution other than the usual standard designs. The result was a modified technical design where the exciter and the support and guide bearings are built into the machine casing. As another special feature, the axial bearings for the drive unit featured a split design and were installed accordingly. In consultation with the customer, the design of the cooler unit was adapted to the limited spacial conditions in the power houses. Once assembled, all the high-tech devices had to undergo a series of elaborate tests at ELIN Motoren GmbH’s ultra-modern testing facilities. Transporting the machines from Styria to the Pinzgau region in Salzburg in vertical position required a specially built transport frame.

CLEAN ENERGY FOR 10,000 HOUSEHOLDS
Based on a classic block diagram, a Siemens transformer was used for each of the machine units to step up the 6.3 kV of generator power to the 30 kV line voltage required for the grid of Salzburg Netz GmbH. To connect the plant to the grid and to deliver the power, a 1.5 km underground line was installed, which provides the required connection to the near­est distribution station. Thanks to the outstanding coordination and cooperation be­tween the almost 100 project partners, hydropower plant Gries went into regular operation a few months ahead of schedule. In a normal year the new facility on the mid-­section of the Salzach river generates around 42 GWh of clean energy, which corresponds to the average annual energy requirement of 10,000 households.
The new power plant in the Pinzgau region, which is operated jointly by VERBUND and Salzburg AG, has many exemplary qualities, in addition to its exceptional electrical engineering equipment. Excellent communication with the official bodies and local residents throughout all project stages al­lowed a series of additional tasks to be completed as well. These include, first and foremost, several ecological compensation measures as well as infrastructural traffic measures around the facility, in addition to required flood protection structures. With the completion of hydropower plant Gries, VERBUND and Salzburg  AG have underscored their leadership in the responsible expansion of renewable energy sources, while contributing significantly to the progress towards meeting Austria’s ambitious climate and energy goals.