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New coils boost output of geothermal generator

Published: 4 June 2014 - Marianne Evans

The Palinpinon geothermal power plant is located in Valencia, Negros Oriental, in the Philippines, where it produces a combined output of 192MW. Plant 1 was originally commissioned in 1983 and having reached 30 years in service, one of the generators was scheduled for a rebuild with new coils. The contract to rewind the generator was won by Sulzer, with the task of designing and constructing the new coils going to the Service Centre in Birmingham, UK.

The site comprises two power plants, the first containing 3 x 37.5MW generators and the second containing 4 x 20MW generators. Demand for electricity in the local area is high, with the site operating at full capacity and therefore it is essential that the generators are properly maintained and operating at their maximum potential.

The project to overhaul one of the Fuji 37.5MW generators required careful planning to ensure the minimum of disruption. Using the large company database, combined with the OEM drawings for the generator, the Sulzer Service Centre was able to design the new coils as well as a set of wooden formers to replicate the stator, which would be used later to check the shape of the coils during the manufacturing process.

Using the latest CAD software, the coils can be precisely formed to ensure an exact fit in the stator slot, making the installation process more efficient. The use of precise quality control, extensive testing and advanced coil production techniques, ensures the reliability of the new coils is guaranteed. With its in-house copper rolling mill, the Birmingham Service Centre is able to maintain round-the-clock coil production to meet even the tightest deadline, using air freight when necessary to minimise the repair timetable.

Every coil is tested in the dedicated test cell with the results recorded and kept in the engineering archives. Testing includes tan d, which relates to the power factor of the coils as well as strand-to-strand short circuit testing and the outer corona protection (OCP) surface resistance measurement.

Back in the Philippines, the dismantling process of the generator began and it was clear that 30 years of continuous operation had taken its toll. The coil support blocks had started to fail and would need to be replaced with new components, manufactured from G11 material, a fibreglass epoxy laminate with excellent electrical insulating properties.

As the process continued and the stator was removed, it became apparent that there had been a high degree of partial discharge. Damage was also found to the middle separator strip and the semi-conductive side packing. All of the evidence relating to the historic damage was documented and photographed and would form part of the final report which is presented to the client at the end of the project.

The stator core was completely stripped, cleaned and tested before the new coils could start to be inserted, blocked and braced. With all the new coils in place and the new caps fitted, the stator was sprayed with anti-track varnish. Meanwhile, the rotor was also stripped and cleaned before being tested along with the retaining rings. Non-destructive testing (NDT) methods were used to check the integrity of all of the components before the cleaned rotor coils were replaced and the finished rotor was again sprayed in anti-track varnish.

The redesigned stator coils used the latest in insulation technology, which, compared with 30 years ago, allows a reduced thickness of insulation to be used while still providing the required safety margin. This meant that the volume of copper used to make the new coil could be increased, which enabled the output of the generator to be increased from 37.5MW to 40MW - an improvement of over 6%, which will contribute toward helping to satisfy the local demand for electricity.

Modern insulation technology allows for thinner layers which can withstand greater dielectric stress and higher temperatures, which also allows more space for copper within the same slot area. This reduces the resistance of the stator winding, which also runs cooler, allowing a small increase in output.

By purchasing raw copper and using its own in-house annealing process combined with automatic shaping machines and computer controlled heated presses, all of which can operate 24 hours per day at its Birmingham Service Centre; the high-voltage coil production facility can deliver consistently high quality coils with very short delivery times, which can then be dispatched to any location worldwide for delivery to site, ready for installation.

Dave Hawley, general manager for the Formed Coil business at the Sulzer Service Centre in Birmingham, commented "Our engineers are frequently involved in the development of new technical standards as well as new materials technologies, allowing us to develop new designs in line with the latest standards to provide our customers with improved efficiency and reliability. We work with a wide range of clients from small, local rewind shops to global OEMs and for each one we are able to tailor the level of engineering and technical support to ensure that every job, however big or small will be completed on time and to the highest quality standards."

Sulzer is a provider of engineering solutions for the power generation sector, from on-going condition monitoring to complete turnkey projects. The company has developed a large network of repair centres capable of completing turbine repairs as well as employing experienced engineers, equipped to complete projects on site if necessary.



 
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