Date: Tuesday 28 February 2012
Location: Luxembourg City, Luxembourg
Extension of the Vianden Pumped Storage Power Plant in Luxembourg with the new Unit 11
To meet the increasing demand for power plant capacity that operates flexibly, the operator of the existing pumped hydro storage power plant at Vianden, the Société Electrique de l'Our (SEO) is expanding the plant with an additional pump-turbine of 200 MW. The new unit will be housed in a separate cavern and will have its own waterways. In parallel, the working storage volume in the upper and lower storage reservoirs is being increased by 0.5 million m 3. Construction started in January 2010.
The 11th unit in the planning phase Lahmeyer International (LI) carried out a preliminary study on behalf of SEO in 2006, which was initially a project concept for the plant expansion that was optimised from a technical, economic and environmental perspective. In 2007 and 2008, LI then developed the design in cooperation with TREngineering, a Luxembourg-based engineering company, taking it to the planning stage of a construction solution. Preliminary geotechnical  and hydraulic model tests  were carried out at the same time.
The resulting project of the 11th unit is set up as follows (Figure 1):
- The new pump-turbine, which has a rated speed of 333.3 rpm, will be accommodated in a powerhouse cavern that is 52 metres long, 25 meters wide and has maximum height of 46 metres. In order to ensure cavitation-free operation, the pump-turbine must be located at 36 metres below the lowest level of the lower storage reservoir. A ball valve will be used as a shut-off device on the upstream water, while an underwater suction pipe flap is to be used on the downstream side.
- The generator transformer will be housed in an extension of the powerhouse. The area is separated from the cavern for fire protection reasons. The existing, western entrance tunnel to the cavern of units 1 to 9 is to be used to access the new cavern. A new tunnel segment, 140 meter-long with a maximum gradient of 10% on the longitudinal side will branch off from it and lead to the cavern of the 11th unit at the level of the powerhouse floor. The new route was cut as an exploratory gallery during the preliminary investigation in early 2008, which consisted of exploration drillings in the cavern area and in-situ rock mechanics experiments for measuring the primary geological tensions.
- A second, completely new access gallery will be constructed from the Our Valley at the front of the cavern and at powerhouse floor level. The tunnel ventilation, smoke extraction, energy dissipation systems, as well as the irrigation and drainage pipes, will run through this gallery. Approximately 240 metres after the entrance, a utility tunnel will branch off from the main gallery, which leads 160 metres towards the powerhouse and transformer, and will run to the surface waterways of the 11th unit. During construction, it will be used to excavate the upstream water gallery. During subsequent operation it will provide access for inspections and repairs.
- The waterways of the 11th unit consist of individual galleries that connect the upper storage reservoir via a short link to the lower storage reservoir. In the powerhouse area, the gallery axis bends away from the general direction almost at right angles to feed the pump-turbine water in the powerhouse diagonally to the longitudinal axis of the cavern. At the end of the suction pipe, the waterway turns back to the previous general direction. The upper waterway begins at the new inlet/outlet tower in the existing northern upper storage reservoir. A perpendicular pressure shaft, which travels down to the powerhouse level, is connected directly below the tower. After passing through a bend, a 240-metre long, flat, inclined pressure tunnel leads to the powerhouse. The entire upper waterway will have a steel lining fitted to it (Fig. 2). A concrete-lined lower waterway tunnel, which is approximately 500 metres in length, will connect the pump-turbine suction pipe to the inlet/outlet structure in the lower storage reservoir, which is a concrete structure with inlet trumpets, rakes and closures. In order to achieve good hydraulic flow conditions to and from the lower storage reservoir, limited deepening measures will be necessary in the Our river bed.
- The energy transfer between motor/generator and transformer will be achieved using single-pole encapsulated generator leads. Three 220-kV cables will run from these, underground through the new tunnel entrance and then outside to the existing switchyard. The existing overhead line will then transport the power further towards and away from the new unit.
In parallel to the construction of the 11th unit, the working storage volume of the plant will be increased from the existing 6.84 million m 3 by 0.50 million m 3 to 7.34 million m 3 . This volume is equivalent to 4 full-load hours of the entire plant in turbine operation. The water level will be raised by 100 cm in the upper storage reservoir. The crown area of the ring dam will be adapted for this purpose and a floating wave protection wall will be installed to provide the necessary freeboard. The alternative of a lateral storage reservoir extension, that has been examined using a 3-D terrain model, has proven to be technically complex and uneconomical. The lower storage reservoir will also be given a greater storage volume by increasing the water level. Thanks to the large water surface area it is possible to achieve the same capacity with an increase of only 50 cm. The Lohmühle lock was designed in this way when a dam of the same type was constructed. New calculations using the finite element method have once again demonstrated the design stability. The water level increase in the lower storage reservoir also requires adjustment measures to be taken in order to ensure adequate freeboard levels on the Lohmühle dam, the renovation of another lock at the Stolzembourg site, the raising of riverside paths and additional protective measures in two affected areas.
Construction work began in January 2010. It started with the set up of the construction yard, which had to be set up using the rather limited available space in the our valley. It was firstly necessary to re-route National Road 10 in the area of the new tunnel entrance. Because of the harsh winter, it was not possible to begin work on the upper storage reservoir until mid-February 2010
Excavation of galleries and caverns
For the construction of the 11th unit, a total of approximately 150,000 m 3 of rock must be removed from the galleries and fly caverns. The supply and sewage lines that are necessary for underground excavation work to be carried out and the DN 1800 ducts used to supply fresh air and water were installed in the existing access tunnel 2. The first blast took place on 12/04/2010 at the face of the exploratory gallery that had been bored previously. A crown access gallery was driven from the exploratory gallery into the crown of the machine cavern (Fig. 3). It was then possible to tackle the enlargement cutting of the cavern.
The excavation work in the tunnels and caverns was completed on 31/05/2011. The geological conditions encountered fully matched the expectations.
Construction of the new inlet tower and work on the upper storage reservoir
The dam crown raising works on both upper storage reservoirs began in mid-February 2010. The dam crown was first removed in sections of 0.5 m. The open air side and storage reservoir side foundations for the wall protection elements were then constructed using in-situ cast concrete. Then, in a final step, the trough that had been created was filled and a new road surface was constructed on top of it.
The scheduled decommissioning and emptying of upper storage reservoir I took place on 15/06/2010 in order to create the new inlet structure (Fig. 3) for Unit 11 and to carry out extensive asphalt refurbishment work. A time window of only 4.5 months was scheduled for these measures. After the earthworks in the (Ø 70.00 m) inlet funnel were completed in mid-July 2010, the concrete work began with the construction of the tower base. The base wall has eight radial openings, which were equipped with rakes and dam boards after concreting. Slipform was used owing to the short time that was available for constructing the tower shaft and head. The slip height was 36.53 m. The slipform was created in a rolling shift operation and completed in nine days. In parallel to the work on the inlet and outlet tower, the abutment and central support were concreted as supports for the dam crown-inlet tower connecting bridge. After the connectingbridge was installed in late October, it was possible to carry out the damming work on the upper storage reservoir on 01/11/2010.
Units and electrical equipment
Planning for the mechanical and electrical equipment began in late 2009 immediately after the main work packages had been awarded. After the basic design had been developed for the main components, pressure piping, pump-turbine, generator, turbine building crane and transformer, the manufacture of the first parts began.
Focus of the work in 2011
The main activities in 2011 have continued to focus on the construction and the hydraulic engineering as well as on the assembly of the first components for the pump-turbine mechanical equipment. Assembly of the suction pipe started on 16/05/2011. The assembly work of the steel reinforced vertical elbows of the pressure piping also started on the same date. The installation of the cofferdam to secure the excavation for the inlet/outlet construction in the lower storage reservoir outlet construction was a major milestone. In addition, adjustments to the floor of the lower storage reservoir were necessary in the outlet structure area to improve the flow. This required the temporary decommissioning of the whole Vianden Pumped Storage Power Plant. Due to the fact of the high systemic importance of the plant for the European power grid, a time window of only 28 days was available for this particular work.
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