Located near Phoenix, Arizona, construction of the Roosevelt Dam on the Salt River started in 1906 and was completed in 1911. In the 1970s, the hydroelectric facilities were upgraded, replacing eight 25 Hz generators with a single Westinghouse Electric Corporation 36 MW, 60 Hz unit, which went online in 1974.

In order to extend the operation life of the generator, the operator commissioned a complete refurbishment program which included the complete overhaul of the high voltage generator. Measuring over 8m in diameter, when it came to dismantling this generator, some expert heavy engineering was going to be required.

Extended lifetime

This project was awarded to Sulzer based on its expertise in managing and delivering complete turnkey solutions through its worldwide service network. With a reputation for quality, speed and precision the Birmingham coil shop and service center would also play a crucial role in delivering this project on time.

Work on site started with dismantling the non-drive end of the generator, which involved the manufacture of a bespoke lifting frame to allow the stator to be lifted clear of the rotor. The frame consisted of four heavy duty stands which housed hydraulic cylinders which acted like giant bottle jacks. Large box-section lifting beams were secured across each pair of stands and then connected to the stator.

Gradually, the hydraulic cylinders were pressurized and the stator was slowly lifted until there was sufficient clearance between the two to allow a scaffold floor to be installed in between the rotor and the stator. Once the scaffold was in place, the engineers could start work in this elevated position to remove the old windings and allow the core to be inspected.

The iron core of the stator is made up of thin laminated steel sheets and these laminations can loosen and deteriorate with age, causing core losses. It is essential to check the condition of the core when carrying out maintenance on the stator, especially in older installations, so that repairs can be carried out if necessary at the same time as the repairs to the coils.

The rebuild starts

In this case, the core was tested using an electromagnetic core imperfection detection (ELCID) technique and then thermal imaging, which together determined that the core was in an unsatisfactory state, with hot spots and shorts within the laminations. Accordingly, it was decided to replace the core, otherwise there would be a serious risk of stator burn out.

Once the original core had been removed, the stator frame was checked dimensionally and found to be distorted and uneven, presenting another concern that would have to be resolved before and reinstatement could begin. Specialist machining equipment was brought to site and the Sulzer engineers managed to bring the bottom flange back into specification, allowing the parallel build of the new core to begin.

Once the machining had been completed and the equipment removed, an enclosure was constructed around the whole stator, rotor and the lifting beams in order to provide a clean environment for the installation of the new core and the new coils. The new laminations were manufactured in the UK at Sulzer's Birmingham Service Center and shipped in batches to the dam site, where the engineers started the rebuild.

The process of installing the new laminations involved the construction of a ring of plates around the stator frame to allow 48 single stage hydraulic cylinders to be used to pressurize each set of laminations and to remove any air pockets. The cylinders were connected to multi-port manifolds to ensure that the same pressure was applied equally around the lamination packs. Although a time consuming process, it was essential that the construction of the new core was of the highest quality to ensure the reliability of the generator when it was returned to service.

Once the new core had been built it was tested using the methods mentioned earlier to check for any imperfections. Once these tests had been passed, the stator bore was painted with capillary epoxy as additional protection against vibration, before the new coils started to arrive from the coil shop at Sulzer's Birmingham Service Center to be installed. Once again staged shipments were made to allow the coil manufacturing process and the coil installation process to be as efficient as possible.

Turnkey solutions for the customer

The new coils were manufactured using designs that had been created by Sulzer's in-house design engineers. In all, 432 diamond coils were manufactured for the generator and the engineers on site worked 24 hours per day to ensure the coils were installed as quickly as possible. Once that was completed the coil to coil and phase connections could be brazed together, insulated and blocked in position.

Danny Fox, Business Manager of the coil shop at the Birmingham Service Center comments: "We are unusual at Sulzer as we offer both a turnkey repair service as well as supporting many other OEMs, rewind and repair customers with high quality coils. In this instance the project required a dedicated workforce to produce a great many coils in order to meet a tight deadline as part of a complete project management and implementation package. The continuous quality control, combined with our ability to deliver such a large order in a short timeframe, enabled us to play our part in the refurbishment of an iconic hydro generator."

With the project progressing well, the coils were ready for curing, which involved the construction of the curing oven around the stator. The enclosure, which had originally been constructed to provide a clean working environment, had been cleverly designed to allow enough space inside to build an oven around the stator. Once complete, hot air fans were installed with temperature probes to maintain the correct oven temperature.

he design called for two coats of epoxy resin, followed by a top layer of class "F" (155°C) anti-track varnish to be applied to the coils, after which the oven could be removed and the static testing could be completed. This testing included the ELCID core test, assessment of wedge tightness, HV (High Voltage) phase–to-phase as well as phase-to-ground testing, all of which were completed satisfactorily.

The final task was to remove the scaffolding as well as the enclosure and to lower the stator back to its original position. Once the lifting equipment had been removed, the whole area was cleaned ready for handover of the generator back to the customer.

Mike Stanley, General Manager at the Birmingham Service Center, concludes: "Projects such as this can deliver significant benefits apart from just extending the operational life of the generator. Improved coil and insulation technology has enabled us to improve the reliability and efficiency of high voltage generators across the world."