Multifactor stress considerations in the evaluation of turbogenerator insulation systems

International Standards for the introduction of major changes to the insulation system of motors and small generators are now established. These deal with thermal, electrical, and combined thermal and electrical, stress evaluations. The basis of these standards is the qualification of an alternative system by demonstrating that thermally and electrically it will perform at least as well as the existing system which has an established service record. The combined stress specification arose from arguments that while single stress tests involve multivariables it is the interplay of these variables in service which results in the development of an insulation breakdown, i.e. synergistic effects. The valid comparison of systems, it is asserted, should involve multifactor testing in which two or more parameters are varied to allow any interplay to develop. The author considers the relevance of multifactor testing to turbogenerator stator insulation development.