IEC 60034-18-31 addresses thermal endurance test procedures for classification of insulation systems used in AC or DC rotating electrical machines up to rated voltages of 15 kV with indirect air cooling and form-wound windings.
Whereas IEC 60034-18-1 describes in detail the general aspects and procedures for the functional evaluation of an insulation system as well as the impact of different ageing factors IEC 60034-18-31 provides more details concerning the test procedure and the sub-cycles.
IEC 60034-18-1 remains the leading document for the whole IEC 60034-18 series and provides as well testing principles applicable to thermal endurance testing of insulation systems used in rotating electrical machines. The principles of IEC 60034-18-1 are followed unless otherwise stated in IEC 60034-18-31.
The test procedures described in IEC 60034-18-31 are intended to compare the thermal endurance performance of the mainwall insulation between conductor(s) and ground and, where required by the design of the coil or bar, the insulation between the turns. The test performance of a candidate insulation system is compared to the test performance of a reference insulation system with proven service experience. In general, the purpose of this standard is to evaluate the thermal classification of a candidate system.
A pure thermal classification is typically not sufficient to prove the service life. Also, other load factors can have a larger influence on ageing than thermal ageing. Thus, instead of performing the test described in this standard, the concept of “service proven” in operation based on records and references should be preferably used by the manufacturer to demonstrate compliance of the reference insulation system and the product design. This is because the entire winding elements (bars or coils) must account for simultaneous stress and physical factors. These include the winding itself, the fixation of the winding system within the slot, end winding insulation, its support system, cooling and ventilation conditions during operation, and heat conductivity across all areas.
The outcome of the thermal ageing test is comparative and does not allow an estimate to be made, e.g. by extrapolation or calculation, of a definite lifetime in service because a combination of ageing stresses or additional factors can intervene. The test is not intended to simulate the in-service mechanical stresses experienced by the end-winding bracing or support materials. It does not include the evaluation of thermo-mechanical deterioration by expansion and contraction of insulation during temperature cycling.
The thermal class for the insulation system refers to its maximum allowed (“hot spot”) temperature. The average temperature reached in service should not exceed the allowed temperature rise according to IEC 60034-1.
The thermal class of an insulation system is not directly related to equipment operating or service temperatures or to service life. The limiting temperature at which an insulation system may be operated depends upon stator and/or rotor winding and machine design, the degree and intermittency of the loading, the degree of reliability required, and the length of the life desired.
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