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Sulzer: high voltage coils

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World Pipelines,

Danny Fox, Coil Shop Manager at Sulzer’s Birmingham Service Center, looks at the key aspects of replacing coils in HV equipment and argues that in an ideal scenario, every repair would be carefully planned and scheduled to minimise the effects of any downtime on the business. However, it is sometimes necessary to react swiftly to unexpected events. The key to better uptime and improved efficiency, however, is to minimise the likelihood of such an event occurring and to reduce the effects it can have on the business when it does – it is at this point that a ‘cost versus quality’ decision can have far reaching implications.

For large rotating machines such as HV and MV motors and generators, testing the replacement coils and balancing the rotor is key data that evidences to the client the quality of the finished and repaired product.

The efficiency of high voltage equipment can be considered in a number of ways; cost efficiency is influenced by reliability, service lifetime and maintenance costs, while power efficiency depends on the intrinsic design as well as component reliability.

The product level requires precision engineering in the design of any new coils, high standards of quality control in selecting and processing the raw materials as well as consistent verification of the quality of insulation being installed. The process aspect concentrates on the continuous design improvement and rigorous quality control with thorough component testing completed throughout the production processes.

Sulzer’s Birmingham Service Center has a long history of producing high quality, high voltage coils for use in motors and generators across the world. This reputation has been earned through its ability to meet and surpass the latest build quality standards including those used in the hydro-electric and nuclear industries.

The Service Center concentrates on B-stage resin rich coils that are typically used on larger multi-megawatt machines. Resin rich coils are manufactured using a hot pressed slot section and an uncured end-winding section, which requires coating and curing after the stator rewind has been completed. This produces a very tough coil which retains some flexibility at the ends that allows it to be transported safely and installed into the stator of a machine much more easily.

The use of B-stage resin rich coils is predominant in the rewinding of larger machines but this method also delivers benefits for smaller machines. The construction of the B-stage coils allows the resin to flow in the end of the winding and bond the layers of insulating tape to produce a more homogenous insulation and sealing of the winding. In most cases, the use of modern insulating materials will improve both the dielectric and thermal performance when compared to those used by the OEM during original manufacture.

The process begins with an initial assessment of the components due for repair and is followed by the design stage, which involves using the latest 3D CAD packages to produce a series of manufacturing drawings for the coil shop. Precision and accuracy of the drawings, coupled with the latest insulation technology allows for improvements in coil design to be achieved.

Sulzer purchases continuously cast copper rod and uses its own in-house rolling and annealing process, combined with automatic shaping machines and computer controlled heated presses.

By using the latest CAD software, the bars or coils can be precisely designed to ensure an exact fit in the stator slot, making the installation process more efficient. Comprehensive testing techniques guarantee the reliability of the new coils, which is especially important for coils being used in marine, nuclear and hydro generators, where the coils can be required to pass a 400 hour voltage endurance test to provide evidence to support their build quality.

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

Additionally, many clients require potential suppliers to demonstrate their professional credentials and standards, often in the form of compliance with various international standards. The Birmingham Service Center has invested considerably in improving both its production equipment as well as its management systems. As a result it now holds certifications for compliance with ISO 9001, ISO 14001, and BS OHSAS 18001.

Edited from press release by

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