Autor: Roland Gruber , 18.07.2016

The reservoir Pian Palù lies at approx. 1,800 m above sea level and was constructed by the ENEL in the 1950s. The  reservoir supplies the hydroelectric power plant in Cogolo built in 1929.

The Trentino municipality of Pejo, which owns the water concession, decided in 2014 to reuse the water turbined there further down in the valley to generate power. With a maximum design capacity of 10,000 l/s, a decision was made in favour of the construction of three downstream power plants. The South Tyrolean Troyer SpA was commissioned with the delivery of a total of six Francis turbine units, medium voltage equipment and the control engineering for the two upper of the total of three power plants. With a planned schedule of only 11 months, the turbine pros from Sterzing had to design, produce, install and complete the commissioning of all units– a challenge that could only be met with teamwork.

The municipality of Pejo has around 1,800 residents and lies at an average elevation of 1,173 m in the Val di Pejo, a tributary valley of the Val di Sole. It is also located in the well-known National Park Stilfserjoch and is surrounded by a majestic mountain world with the peaks of the Cevedale, Palon de la Mare, San Matteo and the Vioz. The 105 km long River Noce also has ist origins there in the national park at 3,360 m above sea level. On its way through the Pejo valley it is dammed at around 1,800 m above sea level in the Lago Pian Palù. From there the water travels underground to the power plant in Cogolo. Up to 10,000 l/s are used there for the generation of peak energy. From here, the water is directed back into the Noce river through an outflow channel. However, there was a considerable head still unexploited by the original ENEL project. The municipality of Pejo, as the holder of the water concession, decided in 2014 to fully exploit this hydro potential and tendered the construction of three downstream power plants.

One specification was that each power plant must be capable of operating independently if the municipality wanted to sell one of the power plants. In addition, each power plant must be able to pass on the entire design flow downstream in the event of a malfunction. The invitation to tender also separated the two upper downstream power plants, christened as Contra and Castra, from the third downstream plant. This resulted in two separate construction projects that were realised separately. „For the two upper power plants Contra and Castra we received the order for the design, delivery, installation and commissioning of a total of six machines including the technical equipment on 20 June 2014,“ says Martin Windisch, Project Manager of Troyer SpA.

The same mechanical equipment was ordered both for Contra and for the downstream Castra. To shorten the time necessary for engineering, the same hydraulic profiles were used on the two power plants, which differ only slightly in the gross head. The latter amounts for Contra to 84.13 m and for Castra to 77.68 m. The construction of a water catchment wasn‘t necessary, as, in the case of the Contra power plant, the water from the outflow channel of the upstream plant is guided directly via a surge tank with a connected penstock to the machines. The Castra power plant is supplied by a 1,275 m long penstock by the Contra power plant. Two horizontal Francis turbines with a rotational speed of 600 RPM and a smaller vertical Francis turbine with a rotational speed of 750 RPM ensure power generation in each of the two power plants. In this way the power plants achieve good efficiency even with low flows of water. The Contra power plant is able to reach a total output of 6,815 kW while Castra is able to produce 6,308 kW. In the case of the generators, a choice was made in favour of synchronous machines from WKV for the horizontal and from TES for the vertical turbines. „In order that the maximum design quantity may be forwarded to the next plant in the event of a malfunction, we also delivered and installed three dissipation valves,“ according to Windisch.

The exact amount of energy produced by the three downstream plants is difficult to estimate. The problem, which also played a role for the designing of the power plant control system, lies in the unpredictable peak production of the storage power plant. If nothing is produced upstream, the machines of the downstream power plants don‘t receive a single drop of water for the turbine  lades. „The production of the storage power plant is determined by the prices obtained on the energy market for the day ahead, which closes at 10 PM. But the downstream power plants, especially Contra as the first stage, receive no information with regard to this,“ according to Martin Windisch. The only way to gauge the water flow coming out from the upstream power plant is by means of a water level reagulation in the compensation basin. If the water level rises, then the Castra power plant knows that the upstream power plant is working again and the automation system will start up the machines at Castra. However, the Castra power plant of course has a seamless communication channel with Contra and its automation system is able to prepare for the precise quantity of water. A communications interface with the storage power plant would be desirable, but at the moment it does not seem that a direct communication channel will be opened any time soon.

The biggest challenge in this project, however, was the tight schedule of only 11 months from the issuing of the order to commissioning. Together with the condition of having to deliver six machines, this is very ambitious. „On 22 August 2014 we received approval of the design drawings and the delivery of the first components had to take place five months later. A project like this can only be successfully completed on time if the equipment is delivered by one supplier, whose departments have excellent teamwork and comunicate through extremely short channels, such as in our company“, reports Project Manager Windisch. Troyer was able to comply with the delivery deadline set for January 19th, 2015. However, during the course of the project, the South Tyrolean supplier was informed that the deadline for the end of commissioning had been anticipated by 14 days. Given the already tight schedule, this was a real challenge. „We had to act very quickly and decided to set up a second mounting and commissioning team who worked simultaneously on the second plant“, according to Windisch. Thanks to perfect and simple communication between all technicians involved,, this challenge could also be mastered, and the two plants were able to go online with the national grid on time on May 14th, 2015. A total of 15,000 man hours were necessary on the part of Troyer AG for the realisation of this project. With 6,000 man hours, nearly half of these were dedicated to installation. „It was definitely a very challenging year for our company, and sometimes we all wonder how we were able to manage it all,“ summarises Martin Windisch.

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The „masi di contra“ power plant is the first out of three power plants using the turbined water of the storage power plant in Cogolo.

pictures: Troyer AG



One of the two horizontal Francis turbines built by Troyer SpA and installed at the Maso Castra plant.

pictures: Troyer AG



An additional vertical turbine ensures good efficiency even with low flows of water. One of the two horizontal Francis turbines built by Troyer SpA and installed at the Maso Castra plant.

photo credits: Troyer AG