Green energy for thousands of kilometres of railway track

The Austrian Federal Railways (ÖBB) set a tight schedule for the modernisation of three 10-megawatt hydroelectric generators, each over sixty years old, used to generate power for the railways. This was because the machines’ stators, weighing almost 80 tonnes, had to be back at the power station in time for the snowmelt so that they could be reconnected to the grid in the spring. Even for the staff at Baumüller Services, who are experienced in working with large machinery, overhauling these giants was a challenge.

Although the three single-phase generators from Brown, Boverie & Cie. at the “Uttendorf I” hydroelectric power station in the province of Salzburg were operating reliably and fault-free despite their age, ÖBB managers wanted planning certainty for future railway power generation: “The machines have been in service since 1950. We want to continue using them reliably for another 25 years,” explains engineer Bernhard Usel, project manager at ÖBB Infrastruktur AG, outlining the plan. The plan was to overhaul the machines in three stages between 2016 and 2019.

The modernisation involved the complete dismantling, cleaning and inspection of each stator, followed by the re-installation of the insulation and rewinding with Röbel bars. In addition, the retrofitting of press stud monitoring systems and temperature sensors was planned, to enable the stator winding, the laminated core and the insulation of the press studs to be monitored at all times during operation. Furthermore, the generators were to be converted to low-wear static excitation, thereby eliminating the need for the original exciter.

Experts in the modernisation of generators

Following a Europe-wide tender, Baumüller Austria, the Austrian subsidiary of the Nuremberg-based Baumüller Group, was awarded the contract in 2016. Its managing director, Michael Ziegler, is certain that the experts’ strong reputation for modernisation in the high-voltage sector also contributed to this decision: “We have already overhauled ten generators for ÖBB over the past 15 years, thereby demonstrating our expertise, adherence to deadlines and reliability.” Given the weight of the stators and their diameter of more than five metres, the Austrian team called on the support of colleagues from Baumüller Services at the Nuremberg headquarters for this latest project. Using the 60-tonne indoor crane there, they were able to move the stator halves, each weighing approximately 36 and 39 tonnes. The first generator was modernised between October 2016 and March 2017, with numbers two and three following at annual intervals. After the machines had been transported on flatbed lorries to Middle Franconia, the staff first mechanically removed the windings from each upper and lower half. They documented the winding circuits in detail for later reassembly, as the relevant records were no longer available. The stator halves were then dry-cleaned with ice, and the clean laminated cores were visually inspected for damage.

Surprises and spare parts that are difficult to source

“When we removed the sixty-year-old windings, it was only natural that some components, which had shrunk over that time, became warped. When reassembling the unit a few weeks later, we then had to go to great lengths to adapt some spare parts – such as various spacer plates, bolt insulators, switch connections and the like,” recalls Stefan Deß, the project manager and account manager responsible at Baumüller Services, which draws on decades of experience in winding high-voltage generators.

In the case of the Uttendorf stators, once the initial assessment had been carried out, work began on the – at times difficult – procurement of spare parts and the reassembly. First, the stator halves were primed with anti-corrosion paint and the insulated press bolts, which clamp the laminated core stacks together, were fitted with monitoring contacts. The Röbel-bar winding was then inserted into the grooves of the stator housing, where it was supported by insulating tubes.

“Despite all the detailed planning and buffer periods, we know from our many years of experience that projects of this scale always throw up surprises that can quickly throw the schedule off course,” reveals Deß. For example, the insulating tubes fitted to the first two generators did not fit the third machine. A different diameter was required there, which then had to be sourced quickly.

Deadline met: High praise

“During the five-month modernisation of a generator, five employees at Baumüller Services were constantly working on the overhaul. Thanks to their experience and expertise, they managed the project largely independently and on schedule,” emphasises Deß.

The overhauled stator halves were then transported back to the Uttendorf I hydroelectric power station. After the generators had been reassembled by ÖBB staff, the Baumüller team on site finally fitted the connectors for the various temperature sensors and press stud monitoring systems to the underside of the lower housing.

In April 2019, the last of the three machines was installed on schedule. ÖBB project manager Bernhard Usel is full of praise: “That was excellent work. Whenever problems arose, the experts reacted immediately and found solutions at short notice, ensuring that all challenges were overcome competently, swiftly and to our complete satisfaction. The quality of the work, the reliability and the adherence to deadlines were spot on in every respect.”

The Uttendorf Power Station: ÖBB traction power from renewable energy sources

Energy from renewable sources plays a major role at ÖBB. In Austria, around 90 per cent of the electricity used by the railways comes from hydropower. The railway’s eight own power stations make a significant contribution to this.

One of these is the Uttendorf I hydroelectric power station, situated in the village of the same name in the province of Salzburg. It is the lowest-lying site in the Stubachtal power station group. There, the three BBC generators produce traction current at a grid frequency of 16 2/3 Hz, which is typical for the rail networks in Germany, Austria and Switzerland.

The generators are driven by Pelton turbines using water falling from reservoirs in the Hohe Tauern mountains. At Uttendorf I, water is utilised from a head of 225 metres; each of the three machines can process up to 5.5 cubic metres per second. This enables the power station to generate a maximum of 26.7 MW of electrical power.