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In the autumn of 2020, after comprehensive refurbishments the Swedish Duved storage hydropower plant run by energy provider Jämtkraft AB was reconnected to the grid.

The powerhouse at the plant located around 50 km from the Norwegian border was refitted with state-of-the-art infrastructure as part of the modernisation process. The internationally renowned Austrian hydropower experts in this field, GUGLER Water Turbines GmbH, were chosen to deliver a directly-coupled synchronous generator – and the most powerful Francis turbine available to guarantee an installed capacity of around 6.5 MW. Although the discharge capacity and head remained unchanged, the installation of modern technology raised the efficiency of the Duved plant by a remarkable 8%.

In Sweden, the generation of electrical power from renewable resources is very impor­tant. Traditionally, by far the largest share of Sweden’s energy mix is accounted for by hydroelectric power. EU commission statistics compiled in 2017 stated that 65.9% of the country’s electricity production was generated from renewable sources, highlighting Sweden’s pioneer status in this regard within the EU. Jämtkraft AB is an energy supplier based in Sweden’s second-largest province, Jämtland, and only generates power for heating and electricity with the environmentally sustainable resources biomass, photovoltaic, wind and water. Overall, Jämtkraft runs 17 different types and power categories of hydropower plant across the Province of Jämtland, with an annual total production volume of 940 GWh of green electricity. This accounts for 8% of all the electricity from hydropower produced in Jämtland.

In order to maximise the efficiency of its water power plants, some of which have already been in operation for over 100 years, Jämtkraft is continually investing in the ongoing technical restoration and renovation of the existing infrastructure. In 2020, the Duved storage power plant, just 50 kilometres to the east of the Norwegian border in the famous skiing area of Åre, was considered ready for a general overhaul. In an average year the Duved plant generates 10 GWh of electricity. According to Jämtkraft it is one of 255 storage power plants in the country that produce the majority of the available operating reserve. After approximately 60 years of service, the original machine group consisting of a Francis turbine and a synchronous generator had, technically, reached the end of its useful life. This necessitated a decision by the operators in favour of comprehensive renewal. It was also considered essential to fundamentally update all the electrical infrastructure, control and feedback technology. No renovation work was required on the reservoir dam or the penstock.

At the end of 2019, following a European call for bids, the Austrian hydropower industry experts GUGLER Water Turbines GmbH were awarded an order to supply a new Francis turbine and the accompanying generator. The internationally respected hydroelectric experts had already proven their expertise in Sweden four years previously, having worked on the revitalisation of the Fäbodbäcken power­­ plant. Roland Fleischmann, GUGLER’s project manager, explains: “The Duved project was already our second contract in Sweden. The outbreak of the Corona pandemic in the spring of 2020 meant work had to be carried out under extraordinary circumstances. Fortunately, when the travel bans and restrictions were lifted in the summer the situation became more relaxed again. In fact, the overall restrictions caused by the pandemic were not as tough in Sweden as in the rest of Europe, which made many tasks a lot easier.”

GUGLER began engineering work on the horizontal-shaft Francis turbine the moment the order had been awarded. The consignment was transported from Denmark to Sweden by truck and sea freight half a year later in June 2020. In order to take full advantage of the voluminous supply of melted water during the thaw, the old plant was kept in operation through the spring. GUGLER mechanics finally shut down and deinstalled the old equipment last May. Roland Fleischmann outlined the importance of the challenge of designing a new machine that would fit into the limited space available in the ­powerhouse. The toughest on-site structural task was extraction of the original suction pipe. The new intake conduit was passed into the original channel which was especially adapted for the purpose. The turbine spiral delivered with the pre-installed guide vane component was installed in the place in the powerhouse where the original had been. Space limitations meant the new generator had to be mirror-image positioned on the opposite side from the old generator. More or less at the same time as the machinery was being installed, work also commenced on the integration of the electro-technical infrastructure supplied by a Swedish business. The medium-voltage set-up, the switch cabinets and the control PC were relocated to an adjacent room in the powerhouse. Control infrastructure based on Siemens SIMATIC technology was installed to guarantee fully-automated plant operation. The transformer alongside the power plant central office was also renewed and the power conduit now travels underground.

Roland Fleischmann detailed the benefits: “In comparison to the original infrastructure, the new machine brought with it numerous improvements. Optimised-flow runner geometry enabled the turbine to work significantly more effectively, despite an unchanged head and the same discharge capacity. The new machine set-up is far more compact now the runner is directly coupled with the generator shaft. The old turbine had its own bear­ing and was connected to the generator via a flywheel. The bearing of the new machine is accommodated on the same side as the generator and is fitted with an automatic lubrication unit. The solution without an additional turbine bearing, and with ‘no-maintenance’ bearing cages, clearly has a positive effect on the amount of servicing required. What’s more, the new machinery achieves a higher rpm count than the original machines, in turn allowing the structure to be kept more compact. This is also only possible because of the excellent cavitation properties guaranteed by our turbines.” GUGLER’s scope of delivery was completed with the provision of a stopcock, a pressure relief valve and a hydraulic power unit to control the guide vane unit.

The gross head of 204.4 m and discharge capacity of 3.45 m³/s now enable the turbine to generate a maximum power output of 6,483 kW. The runner was milled from a single block of stainless steel and has a diameter of 670 mm. The generator was manufactured by the French engineering company Leroy-Somer,­ set to 6,300 volts and offers nominal apparent power of 7,920 kVA. Mr. Fleischmann added that in response to a request by Jämtkraft the generator with a slide bearing was to be fitted with a heavier flywheel. This enables the performance of the plant to balance out public grid frequency fluctuations in a matter of seconds. The optimum operating temperature of air-cooled energy converter is assured by a large-scale venting conduit leading outside into the fresh air.

The recommissioning process at the Duved plant was initiated at the end of September, once all installation work had been completed. The first tests of the comprehensively modernised plant were already held at the beginning of October. The people at Jämtkraft were delighted that the performance and power generation capacity of the plant were enhanced by around 8 %. Usually, a revitalisation project results in a 3 – 4 % improvement in efficiency. “The entire project went smoothly and was completed without any problems. Cooperation and coordination with the representatives of Jämtkraft was immaculate, and good project management among all parties meant that not even the emergence of the Corona crisis managed to break our stride”, summarised Roland Fleischmann.

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