The Dolgarrog plant is one of the most important hydropower plants in Wales, not only from a historical point of view but also because of its significance for the Welsh electricity industry.

The plant, which was put into operation in 1924 and is now operated by RWE INNOGY UK, was partially renovated in the last four years at a cost of about 15 million GBP – around 21 million euros. The key elements of the remodeling project were the replacement of parts of the old steel penstock and the exchange of machine unit 4, a 10-MW turbine-generator unit from 1950. For the exchange of unit 4 the operators relied on the competency of Austrian hydropower expert Kössler, for whom the project marked the entry to the United Kingdom. This premier was abundant with technical challenges and thus was one of the most technically demanding projects that the experienced turbine manufacturer from Lower Austria had ever encountered.

Dolgarrog, a small village in Conwy County Borough in the northern part of Wales, was an important site during the early industrial development in the 18^th century. In the beginning flour mills were established in Dolgarrog and after a while textile manufacturing companies and saw mills popped up as well. They all used water from the River Conwy, passing near Dolgarrog. The abundance of water as well as the mountainous terrain of the picturesque surroundings also met the requirements for an electrical utilization of hydropower. The construction works on the Llyn Eigau Dam, which was to serve as a reservoir for the planned Dolgarrog hydropower plant, began as early as 1911. However, this project ended in a tragedy due to its poor construction. After five days of heavy rainfall disaster hit the area. On November 2, 1925 the dam broke and all ponded water poured into the neighboring Coedty Reservoir, whose dam walls burst as well. The flood struck the small village, killing 17 people. It would have been more had many of the residents not been at a film performance at the safe local theater building at the time of the incident. A subsequent examination found that the work on the foundation had been insufficiently executed and concrete of poor quality had been used. In 1930 the British parliament passed the “Reservoirs Safety Provision Act” as a consequence of this tragedy. It was introduced to guarantee the safety of dams and is still in effect today.

Two Power Plants Under One Roof
The Dolgarrog power plant was put into operation in the same year as the incident happened. It was designed to provide electricity for the local aluminum works, which it did until the 1940s, when the smelting works were stopped and the factory became less and less important. In the early 2000s the factory was closed and there are now plans to turn the area into a leisure park. While the traces of the aluminum industry slowly fade, the Dolgarrog power plant is still operating. Today the plant, with an installed capacity of 32 MW, represents one of the most important top-performance electricity suppliers of Northern Wales. However, Dolgarrog actually comprises two independently operating plants, whose turbine-generator units are set up under one roof, in one powerhouse. There are basically two process water channels. One is fed from the Cowlyd Reservoir and is considered a “high head” scheme, meaning a high-pressure plant. The other channel takes its process water from the Coedty Reservoir and is a “low head” scheme, a low-pressure plant. However, this description should not be taken too literally, as the “low head” scheme still uses a head of about 264 meters. The improvements were made on the “low head” section only.

Safety From Pressure Surges
In 2009 RWE INNOGY decided to replace the above-ground section of the original penstock from the 1920s and to restore machine unit 4 at the same time. Not only safety issues played an important role in the planning, but also landscape aesthetics, resulting in an underground installation of the penstock. Safety was the main reason for the exchange of the machine unit. “The previous plant was designed and equipped with a pressure regulator to manage transient conditions – i.e. any possible pressure surge situations. The risk associated with such a valve was too high and so we decided on a modern and less complex unit that meets safety concerns with improved technology,” Karl Henninger, project manager of Kössler, explains. RWE INNOGY spent approximately 15 million GBP on the project. After the preliminary design had been carried out in the fall of 2010, the project managers took time in 2011 and the beginning of 2012 to develop the most effective technical solutions with regard to the life of the machine unit and environmental protection. The contract for the construction project was obtained in 2012. The order received by Kössler included the installation of the turbine and the generator, the ball valve, the corresponding hydraulic power unit, lubricating oil, the cooling system for the mechanical segment, the automation as well as the low-voltage element for the electricity. In addition Kössler was commissioned to dismantle the old and install the new equipment.

Working in a Narrow Space
In April 2013 RWE INNOGY took the “low-head” system off the electrical grid. During this time the old above-ground penstock was removed and machine unit 4 was dismantled. This marked the start of Kössler’s construction job in Dolgarrog, which in hindsight turned out to be one of the most challenging projects. “The project was from planning to commissioning one of the biggest technical challenges we had encountered until then. The assistance of our parent company Voith was a great help,” says Karl Henninger. In order to meet the technical requirements of the local conditions, a new hydraulic turbine design had to be developed in only a short time: a design that would satisfy all demands, achieve the necessary cavitation protection and attain the highest possible degree of efficiency. “This task was perfectly performed by our colleagues from Voith Development Center in York (US). They laid the basis for further construction and assembly works. In addition Voith experts from Heidenheim, Germany, accompanied all these activities,” says Mr Henninger. The plan was to replace the old Boving turbine from 1950 with a contemporary Francis spiral turbine, which had to meet many different criteria. One aspect was to maintain the specified turbine axial height. Another one was to build a turbine of comparable cubage that fitted with the previous penstock diameter and the predetermined axial position. To make matters worse the space available was extremely narrow. “Although only four machine units, out of the five originally installed units, were set up in the power house, the space was barely sufficient. Furthermore the old power house crane was not constructed for such masses. For this reason we had to dismantle the generator completely before delivery and assemble it again on the construction site with special gadgets,” adds the technical expert.

Large Centrifugal Masses
Another key aspect of the project was controlling the transient conditions in the penstock in various operating scenarios. Based on the fact that the gross head was 263.81 meters the maximum pressure surge must not exceed 292 meters in any worst case scenario, this correlates with a pressure surge of about 10 percent. “No equalizing valves were allowed, as is usually customary. The reason was the 90-year-old remaining part of the penstock, which would not tolerate any pressure surges beyond that. We had to meet these demands with mechanical means, i.e. large centrifugal masses,” Mr Henninger explains. Another requirement was minimizing and simplifying maintenance. This aspect was important during the designing phase of the machine unit. Of course, robustness was a major factor as well, since the power plant is a top-performing plant. It is switched on and shut down quite often during the day. The output, however, will have to remain steady because the license allows no deviation. The advantage of the new Kössler Francis spiral turbine is that it takes less water to attain the nominal output of 10.18 MW.

Exciting New Challenges
“The project involved another challenge – to get all the appropriate licenses and to take all needed precautions for the installation. In the UK you have to get the approval of your client for the construction process as well as that of HSE. There are special conditions in the UK that you have to consider,” states Karl Henninger. He adds that the experts had to pick up certain peculiarities about how work is done on the British Isles. Having a reliable and competent partner in Wales to take over the project management and handle all of the client’s requests directly was an invaluable assistance to Kössler and essential to the success of the project. In the fall of 2014 the installation works on machine unit 4 were nearly completed. The commissioning and first tests turned out successful. Today both “low-head” machine units in Dolgarrog are operating again. It was a pleasant completion of a challenging project, which marked the company’s entry to the hydropower market on the British Isles.

Local Investments
RWE INNOGY focused not only on the technical improvement of one of its most important hydropower plants, but also on guaranteeing the most environmentally-friendly procedure possible. 300 trees had to make way for the installation of the penstock along its route, but the operator planted 4,000 trees in that area as an ecologically balancing and accompanying measure. About 100,000 GBP were invested in preservation measures for nature and archaeological sites nearby. Furthermore streets and bridges for an improved infrastructure, a garden as a commemoration site for the dam accident of 1925 and a children’s playground were built. RWE INNOGY is one of the leading operators, project developers and investors in the field of renewable energy in Wales. The hydropower plant houses units with an installed capacity of 44 MW. Thus the Dolgarrog power plant represents the most significant plant and is the central control station for the other power plants in the region. The successfully completed renovation will guarantee a reliable operation of the power plant and a clean electricity supply for Northern Wales for decades.