Repairing high-voltage steam turbine generators requires expertise at the best of times, but when the hybrid ‘hairpin’ coil design is far from the conventional diamond coil, it is essential that those entrusted with the repair have the knowledge and capability to complete the project accurately.
For a sugar refinery in Scandinavia, which was presented with just such a situation, the solution lay with Sulzer and its rewind facilities located at the company’s Birmingham Service Center, UK.
Nordic Sugar produces nearly 1 million tonnes of sugar annually from its plants in Scandinavia and Lithuania, with a history going back over 100 years. In one particular refinery, as part of the on-going maintenance programme, the main Brown Boveri 10.8kV, 9MW generator needed to be removed and rewound having reached the end of its effective service life. The project would need to be completed within six months in order to prevent delays in other maintenance work at the site.
The factory operates during the autumn and winter when the sugar beet are harvested and processed. Once the campaign has been finished, the factory closes and it is during this time that routine maintenance can be completed. However, the maintenance plan demands that individual projects are completed on time to ensure that the complete schedule of work is completed on time and to budget.
Expertise and quality required to build coils
With the repair of the generator awarded to Birmingham Service Center, the site engineers were dispatched to remove and transport the several tonnes of generator back to the Sulzer workshop. Using the original machine drawings as well as the dimensions obtained during the strip-down, the rewind shop passed on the details of the original coils to the coil shop. In fact, this generator uses a rare design of coil that is a hybrid of a diamond and a hairpin coil and would require the expertise and build quality that Sulzer is renowned for.
Keith Barbier, International Contract and Projects Manager for the Birmingham Service Center, commented: “One of the reasons that companies such as Nordic Sugar come to Sulzer, for their repair requirements, is the knowledge that they will receive the required quality and expertise, especially when compared to the OEM's. Sulzer has a reputation for quality and precise workmanship. Our Birmingham Service Center, with its own coil shop and rewind facilities for large machines, is fully equipped to deliver projects such as this.”
For this project, the coil shop was required to produce 72 hairpin coil sides which would be joined together once installed in the stator by the rewind shop. Fortunately, the depth of experience within the coil shop includes experienced engineers some of whom remember working on similar coil designs over 20 years ago. During this project these engineers showed some of the younger engineers how to construct this specialist type of coil for which the manufacturing process was developed years ago.
The production process
The process starts with the raw copper being rolled using the in-house rolling mill and annealing process, allowing Sulzer to start the manufacturing process very quickly, rather than having to wait for a third party supplier. Each strand is insulated, cut to length and formed by hand into a hairpin shape using a CAD-designed forming jig. This particular design does not employ a Roebel transposition which makes for a slightly easier construction.
The more regular diamond coils are shaped using automated shaping machines and while these are the most common design, there are regular occasions when a different approach is required. For a semi-enclosed slot design, a hairpin or push-through coil has to be manufactured and this has to be formed and shaped by hand. Using CAD-designed, wooden formers, every coil is produced with exactly the right shape and dimensions.
Once formed and taped using the latest insulation technology, the coils are consolidated in a heated, hydraulic press and checked for dimensional accuracy to ensure a secure fit in the stator slot. With the first coil manufactured from the dimensions provided by the rewind shop, it is checked for fit in the cleaned stator to ensure that the dimensions are exactly correct.
Having confirmed the dimensional accuracy of the new design, a full set of coils is manufactured and transferred to the rewind shop where the winders use their experience and skill to braze all of the copper strands, insulate, tape and form the connections to complete the job.
Customers receive the service they require
Keith Barbier continued: “The winders have a particular skill that is, fortunately, alive and well at Sulzer, something the company is keen to maintain and pass on the knowledge and experience to the next generation. In a project such as this, which has quite an unusual coil design, we will use our experienced winders assisted by the younger employees to help expand their knowledge and experience.
“The coil shop uses the same high standards in manufacturing quality and testing for every coil whether it is being used in-house by the neighbouring rewind shop or by a client on the other side of the world. It is this attention to detail coupled with the precision repair that Sulzer delivers that ensures our clients receive the service they require.”
Once the construction of the new hairpin coils has been completed, every coil will be tested using the in-house facilities, which are able to conduct inter-strand tests, Tan delta tests and a high voltage withstand test, which provides the client with the evidence of the quality of the build.
Meanwhile; in the rewind shop, the stator is stripped and cleaned, the core flux tests completed, ready for the new coils to be installed and connected.
Benny Hinchliffe, Head of International Sales at the Birmingham Service Center, concluded: “At 9MW, this is a medium-sized rebuild for Sulzer, but the hairpin construction means that there are some specialist skills required to ensure that the coils are manufactured to the standard required. In this case we supplied our own rewind shop to complete the project.
“Sulzer provides a worldwide service which is delivered to the client's exact specifications, be that a single set of coils or a complete turnkey project to refurbish a 200MW hydro generator. It is this flexibility coupled with the high quality products and professional approach that enables Sulzer to deliver on large rotating machine rebuilds, rewinds and refurbishments.”