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 [3] and hydraulic model tests [4] 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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