The hydrogen brazing technology is said to have saved six days on the rebuild time while also being less demanding in terms of health and safety requirements.
Traditional oxy-acetylene brazing uses lower temperatures, requires a set of gas bottles for each brazing station, and the process produces fumes that need to be extracted away from the working area. In contrast, hydrogen brazing uses increased temperatures and produces water as the only byproduct.
The process uses a hydrogen generator, which converts water to hydrogen gas that is supplied to a number of brazing stations. The higher heat concentration is claimed to speed up the brazing process and means that there is no need to reinstate the insulation after it has been completed, saving time for the project. In this case, three brazing stations were set up in the centre of the stator, allowing the coil connection process to be completed quicker.
Adrian Larmour, UK Sales Manager at Sulzer, says: “For our customers, it means we can optimise the project times and keep downtime to a minimum.”
Operated by Statkraft, the generator is a 15 MVA, 12.1 kV, 13.5 MW (18’100 hp) unit that is driven by a Francis turbine. The original stator was built in two halves that were assembled during the 1960s. Since then, the generator has delivered power to the local grid. However, some of the winding insulation started to breakdown and an earth fault was detected, prompting a decision to rewind and rebuild the generator.
Engineers from Statkraft disassembled the stator and discovered damage to the core as well as a coil failure.
During initial site visits, Sulzer assessed the building as well as the equipment that was available at the facility to help complete the repair. The first task was to install a new 3-phase distribution board and improve the electrical system so that it could safely support the equipment required during the project. The installation of a fire alarm, spill kits and first aid equipment was routine, but there was also a need for heaters and dehumidifiers because the atmosphere inside the turbine building was said to be around 95% humidity.
One of the first challenges was the rotor, which contained asbestos from the original manufacturing process. Sulzer provided a solution to recover the copper rotor poles for Statkraft safely.
The stator core also needed to be replaced, leaving Sulzer with the generator frame as a starting point. The new core was built up using sectioned laminations to create a complete assembly without any gaps. Specifically, Sulzer rebuilt the core concentric to within 0.15 mm and circular from top to bottom to within 0.05 mm.
Two teams consisting of three experienced winders and two apprentices were assigned to ensure the generator repair was completed within the customer’s expectations.
“There are many different designs of stator and larger hydro-electric equipment needs to be rewound onsite so our younger team members can learn a lot from a project like this one,” Larmour added.
Once the windings had been installed, the team built a tent structure over the stator and installed a series of heaters to bring the ambient temperature up to 180°C, which was then maintained for three days to cure the windings.
In all, 227 new coils were designed, manufactured and tested in Sulzer’s Birmingham Service Centre. Of these, 216 were destined for the stator, with two spares for the build team and five spares for the customer. The remaining four coils were tested to destruction according to the specifications for hydro-electric applications and subjected to some of the most arduous test routines.