A proposed type test for interturn insulation in multi-turn coils

Stator insulation failures are a significant cause of forced outages of electrical machines. Accelerated aging tests or type tests have been developed and are now applied routinely to groundwall insulation in coils and bars used in machines to ensure satisfactory life time performance. It is generally accepted that many groundwall insulation failures in multi-turn coils are initiated by a premature breakdown of turn insulation. However, so far there is no type test available to examine the long term behaviour of the turn insulation. The authors have been striving to develop such a type test for turn insulation for more than ten years. Electrical, thermal and mechanical stresses have been investigated for the accelerated aging of turn insulation. A combination of thermal and mechanical stresses applied simultaneously has been demonstrated to produce a significant aging of the turn insulation. This paper summarizes the experimental results from tests on a large number of coils aged under electrical, thermal and mechanical stresses. The results from factorial tests are analyzed to select the optimum test parameters which are then used to define a possible functional type test for the turn insulation