The Need to Establish Accurate Detailed Documentation

Trending indicates that there has been a steady increase in the premature failure of stator and rotors associated with HV machines over the last decade.  

The root cause of these premature failures can be associated with industrial practices that have changed following de-regulation and the increased pressures of a competitive market. These issues include, but are not limited to:

• Changes in operational practices
• Loss of Technical expertise
• Manufacturing procedures
• Selection of materials
• Changes in design

Failures may be due to a combination of elements rather than any one individual factor:

• Factors affecting the electrical stresses imposed between adjacent phases, and also to ground, such as end winding clearances, decrease in main wall insulation thickness, and the use of cheaper alternative insulating materials
• Increase in copper section to improve heat transfer, resulting in an increase in IR² losses and thermal deterioration and reduced insulation thickness.
• Global VPI systems on air-cooled machines with long core lengths, resulting in problems with impregnation throughout the slot section and potential thermal cycling problems.
• Reduction in end winding overhang to reduce manufacturing material costs (copper and insulation materials) and also produce smaller more compact machines, resulting in a compressed end winding area with insufficient coil-to-coil clearances
• Quality control, inadequate quality acceptance tests and attention to detail on manufactured coil dimensions
• Changes in plant and design, including introduction of vacuum circuit breakers and thyristor controls, resulting in high rise time surges on the circuit stressing interturn insulation.

A detailed technical specification, coupled with quality inspection / record procedures can address the majority of these issues. This technical specification needs to address the particular machine operational requirements, conditions and maintenance regime and not just reference the relevant IEEE standards and NEMA MG1specifications.

An effective, detailed technical specification should:

• Be applied to all aspects of the machine assembly - (insulation systems for the rotor or stator, stator core materials, forgings etc…). Manufacturers are striving to remain competitive and offer the most economic system to meet the anticipated needs of their customers.
• Give strict guidelines - Often potential tenders are not presented on a level playing field and may be limited to a basic system in order to compete, rather than offering the system that will best meet the required operational needs.
• Confirm that tenders are offering a similar system, and that any divergences are apparent at the tender stage and not on completion of machine manufacture, when any contractual agreements and extended warranties that may be required to accept a substandard machine often have to be negotiated.
• NOT take the design aspect away from the supplier - by stipulating non-standard insulation specifications, materials or procedures.

Before preparing a technical specification, the vendor should, among other requirements, establish how prescriptive they wish to be with regard to technical details. One pitfall in making a specification too comprehensive is that the design could be established by the vendor and not the tenderer.

For more information

Transmission & Distribution Technologies

rotating.machines@kinectrics.com