The Lixhe hydro-electric power plant is situated some 10 kilometres south of Maastricht. Currently operated by the Belgian electricity supplier EDF Luminus, the plant commenced operations in 1980.

It was originally equipped with four identical Straflo turbines with horizontal axes. Their poor control characteristics, however, turned out a weak point with regard to the modern, adaptive operation of the power station. This is why as part of an elaborate construction project, two of the four Straflo turbines were recently replaced by double regulation Kaplan Bulb turbines from ANDRITZ Hydro. Meanwhile, Belgium’s biggest running-water power station has recommenced operations with all its generators. All thanks to its newly obtained flexibility, it is proving its worth in everyday service.

In the historical development of hydro-electric power in Belgium, 1980 was a year of great significance. With the Andenne and Lixhe power stations, two modern production plants commenced operation on the Meuse River. On average, they have jointly been providing some 96 GWh of clean electricity per year to the Belgian power grid. For a country on place 104 in the global ranking of installed hydro-electric capacities, these are two highly significant base load renewable energy producers. The Lixhe power station is even considered the most powerful running-water hydro-electric facility in all of Belgium. What the two plants had in common aside of the time they were built was their basically identical electromechanical equipment. Until 2013, Straflo turbines were the only design used in both the Andenne and Lixhe stations. The Andenne station had three with 3.05 MW each, while the Lixhe station featured for with 4.5 MW each. At the time, Straflo turbines constituted a highly advanced form of low-pressure turbine with an outstandingly short design as its key advantage. This significantly lowered erection costs. In the Lixhe power station, savings due to the reduced building volume amounted to 15 percent. There is, however, a downside to this.

Straflo – a Design with Weaknesses
“With regard to its flexibility and its control characteristics, Straflo turbines have a clear disadvantage. As this hydroelectric set only features single regulation, its operational range is comparatively narrow: Below 70 percent admission, it shows a steep decline in efficiency and it should be detached from the grid“, explains DI Martin Reisser from ANDRITZ Hydro in Ravensburg, Germany, adding, “There has indeed been an attempt to build double regulation Straflo turbines. As far as I know, two of these hydroelectric sets were deployed. Unfortunately, though, they did not function properly. There is a simple reason for this: Nobody ever found a practicable way to keep the pitch of the runner blades and that of the guide vanes aligned. This resulted in uneven operations which invariably entails cavitations. This is why in the end this concept was abandoned. Generally speaking, the classic Straflo turbine will not prevail because in addition to its poor regulation characteristics, it comes with another drawback: In case of runners with big diameters like here at Lixhe where the diameter is 2.6 metres, the tips of the runner blades can reach an enormous tangential velocity. The immediate proximity of the rubber seal increases the chance of small leakages. In practice, related maintenance work will in most cases be unavoidable.”

Inflexible Operations accounts for Upsurge Issue
People in charge at EDF are quite aware of this disadvantage. “We already had to replace the sealing strip once, and that was far from simple,” says Anne-France Fontaine, who conducted the alteration activities at the Lixhe power station. It was a bit of a foretaste to the extremely laborious retrofit of the Lixhe power station that was to follow. In 2009, the operators were confronted with new guidelines. They were required to carry out studies with regard to the power station’s passability by fish, and they had to get a grip on its upsurge issue. As the latter was without a doubt quite obviously a direct result of the poor regulation characteristics of the four Straflo turbines, their replacement – at least in part – suggested itself. “In everyday operations, we were forced to turn off the hydroelectric sets whenever the upstream head dropped below the 70 percent mark. Consequently, the upstream head naturally began to rise again. After the turbines returned to service, it dropped again. This led to the undesired surges we were required to reduce or eliminate in guidelines from the public authorities,” says Anne-France Fontaine.

Reduction in Capacity
Subsequently, EDF Luminus, Belgium’s biggest operator of hydro-electric power stations, commissioned extensive variation studies. As the most economically viable solution, these studies suggested to replace two of the four Straflo turbines by double regulation Kaplan Bulb turbines. “The installation length of the installed Straflo hydroelectric sets is particularly short. This prevented the installation of equally-sized double regulation Bulb turbines at the same positions. It was only possible to install machines with a smaller diameter. In the end, two Bulb turbines with a 2.6 m diameter replaced pre-existing Straflo turbines 3.5 m in diameter. As a matter of course, this also means a reduction in capacity and consequently a performance reduction at full load. While the original hydroelectric sets had a power of 4.5 MW each, the new turbines are rated at 3.035 MW. Had we replaced all four generating sets, overall output would inevitably have dropped. By replacing only two of the turbines, we managed to keep production at the same level while at the same time achieving the operational flexibility required“, the project manager summarises. Along with the replacement of the electromechanical equipment, modernisation of the control and communication systems was on the agenda as well.

Design Challenges
Following the approval by EDF Luminus for the refurbishing project in 2011, the contract for the electromechanical equipment was awarded to ANDRITZ Hydro in Ravensburg, Germany, in the autumn of 2012. The main challenge for the experienced turbine makers was to design the two new turbines such that they can be fitted between the existing head and tail water stop logs. In preparation of this, an in-depth survey of the installation situation had been carried out earlier. For this, the ANDRITZ Hydro design engineers not least also elaborated a 3D analysis that allows various views and sections. Finally, the Bulb turbine was designed to match a 6.61 m net head and a nominal discharge of 51.0 m3/sec. The 4-blade runner turns at 176.47 rpm and has a nominal power output of 3.035 kW. Aside of the electrical equipment supporting the turbines, the two directly coupled generators were also included in the package supplied by ANDRITZ Hydro. They were made by the Austrian manufacturer ELIN Motoren who has a century-long tradition in building generators for hydro-electric power stations. Their nominal apparent output is 4.070 kW. They are cooled by the passing water flow which is another benefit they have over the strictly air-cooled Straflo turbines.

Conversion during Operation
“Due to the aforementioned dimensional differences, it is tremendously difficult to design a double regulation turbine to fit into the existing cavity of a Straflo turbine“, says Martin Reisser. His opinion is supported by the Belgian project manager: “It is invariably more difficult to convert an existing power station than to build a new one from scratch. But this project really was a great challenge for all involved. First, numerous requirements from the building authorities had to be fulfilled. Quite naturally there were issues in the field of statics and ultimately of construction logistics as well. After all, times were when numerous people from many companies were ‘roaming’ the construction site“. Implementation work commenced in June of 2013 with the removal of the two Straflo turbines. An important aspect of the project organisation was to keep the two remaining turbines in operation in parallel with the ongoing work. This was not possible during the dismantling. Particularly the demolition of the concrete structures caused vibrations that could have had detrimental effects on the two Straflo turbines. In the subsequent project phase during which the entire electrical equipment of these turbines was replaced, they were quite obviously also turned off. “Generally,  we have tried to keep the two Straflo turbines in operation whenever possible“, says Anne-France Fontaine.

Stumbling Blocks along the Course of the Project
In order to protect the two generating sets from the dust produced by the conversion work, an XXL-sized curtain was put up. Not with the desired success, though. Fontaine: “Unfortunately, this did not work quite as well as we had imagined. Owing to the fact that the old generators are air-cooled, we could not fully prevent a little dust from getting through to the generators. After a while, the dust settles in the stator and has adverse effects on the efficiency“. This was not the only issue the project team was confronted with. Another stumbling block was the dimensioning of the old overhead crane which proved unsuitable for the weight of the new machinery. It had not been designed to take on Bulb turbines weighing 40 tons. There was no choice but to replace the existing crane by a current model capable of bearing a heavier load. Another challenge was the construction of the two intake pipes. It was decided to build these using timber formwork. A decision that Anne-France Fontaine and her team would not make again: “This made it very laborious and excessively complicated. If I had to do it again, I would opt for a metal variant“, the project manager confirms.

Electricity Production as of August, 2014
In March of last year, preparatory construction work had finally reached such a state that the first of the two turbines could start its long journey to Belgium from the ANDRITZ production plant in the Southern Germany. Following its arrival on site, the experienced ANDRITZ Hydro installation team proved that the excellent reputation it enjoys is well justified. With a lot of sensitivity yet target-oriented and quick, they completed the installation of the first turbine. In retrospect, Anne-France Fontaine confirms this: “Co-operation with ANDRITZ Hydro was indeed excellent. We could rely on a high level of workmanship quality and between their people and ours, everything worked out just fine“. In August of 2014, the new generating sets in the converted Lixhe power station finally started producing electricity. This marked an historic milestone for Belgium’s biggest running-water hydro-electric facility, even though conversion work had not been completed yet. Currently, the last batches of work are in their final stages while extensive operational tests are carried out. “At this time, all operational conditions are being tested through their paces. Additionally, the optimal operating points are determined and fine-tuned“, says the project manager. She and her team are now drawing a positive balance of the conversion project that came with many challenges for all involved. The operators confirm that they are very satisfied with the efficiency of the turbines and with the high flexibility of the power station’s regulation that has been achieved at last. At a nominal water flow of 51.0 m3/sec. each, the generating sets can stay online at works water volumes as low as 15 m³/sec. with no problems. This constitutes the foundation on which the new Lixhe power station can now ensure constant head and tail water levels.

Eyes on the Next Project
The rich experience gained during this conversion project is expected to pay off in the near future. The Lixhe power station will not remain the only EDF Luminus plant to be converted, after all. In the next few years, conversion of the Andenne power station is top on the agenda of Belgium’s second biggest electricity supplier. The plant is situated some 30 kilometres upstream from Lixhe and is also still equipped with Straflo turbines, of which it has three. Before long, two of them are planned to be replaced by state-of-the-art double regulation Kaplan Bulb turbines. That the operators will again rely on the proven quality supplied by ANDRITZ Hydro is almost a matter of course. A lot of know-how gathered in the Lixhe project will in any case be reused for the upstream hydro-electric station in Andenne. And the waves near the Belgian border to the Netherlands will likely soon altogether be a thing of the past. Some ten percent of the overall electricity generation capacity installed in Belgium – slightly more than 2,000 MW – are contributed by EDF Luminus who like to view themselves as “first challenger“ of Belgium’s biggest electricity supplier. Renewable resources such as wind and water account for 200 MW or ten percent of this capacity. With 73 MW installed hydro-electric power, EDF is the biggest operator of hydro-electric power stations in the country, producing electricity in seven big running-water power stations on the rivers Meuse and Sambre. The Brussels-based power company currently employs some 1,000 people. Its main shareholder is the French electricity corporation, Électricité de France, holding 63.5 %.