31.08.2016 - Proof of safety requirements for the gravity dam of the new Limmern pumped storage plant on the Muttenalp have been fulfilled. Over a period of 60 days, Axpo filled the Muttsee reservoir to the maximum water of level of 2474 metres above sea level for the first time in order to test dam performance. Testing results fulfil all the requirements: The dam holds what it promises – in concrete terms, 23 million m3 of water. The multi-phase initial filling programme took place under close cooperation with the Swiss Federal Office of Energy (SFOE), which subsequently confirmed the safety of the plant. The Limmern pumped storage plant will make an important contribution to supply security thanks to its high pumping capacity, turbines with a performance of 1000 megawatts each, and its flexible scope of utilisation.
The initial filling of the Muttsee reservoir took place in phases from July to the end of August. The reservoir was filled to the maximum level of 2474 metres above sea level twice and then partially drained. Water was pumped up from the Limmern Lake, which is located 600 metres below the reservoir. During the initial filling, the condition of the over one kilometre-long dam was closely monitored based on various measurements and checks. The findings were analysed continuously and documented for the attention of the SFOE.
The dam’s extent of movement is safety-relevant. Slight deformations can occur in the concrete owing to the constant pressure of the water, temperature fluctuations and the impacts of geology. Nature and technology must be in harmony and the impacts precisely calculated based on models and available empirical data. Axpo extensively investigated the local geology before launching construction. The effort was worthwhile. Test results during filling and draining confirmed calculations on all points. The sealed underground is stable and the buoyancy force of the dam is lower than expected thanks to built-in drainage systems. No improvements will be necessary. Dam movement at the new crest is maximum 3-4mm at full water level, which corresponds to the norm. The SFOE has confirmed the safety of the dam.
The legally prescribed initial filling programme monitored by the SFOE is mandatory for newly constructed dams before they go into definite operation. The last such programme was carried out 26 years ago on the Panix gravity dam. At that time the programme was completed with a first initial filling to full capacity, which fulfilled proof of safety. At the Limmern pumped storage plant, the first large-scale pumped storage plant in Switzerland, the programme included a fully documented additional cycle with partial drainage, renewed filling to full capacity, followed by complete drainage of the Muttsee. During the first two years of operation, the dam will be closely monitored, followed by periodic checks according to the Swiss Federal Office of Energy requirements.
The Muttsee has now been filled to full capacity for the second time. From now until mid-September, the reservoir reservoir will once again be completely drained. The final report will be submitted to the SFOE, which will issue approval for operation. When the reservoir is empty the second pressure tunnel will be connected to the works water system and filled with water for the first time during the next reservoir filling. This is a prerequisite for commissioning of machine groups 3 and 4. Machine groups 1 and 2 are already in operation and have been successfully synchronised with the grid. The Limmern pumped storage plant will make an important contribution to future, environmentally friendly supply security thanks to its high pumping capacity, turbines with a performance of 1000 megawatts each, and its flexible scope of utilisation.