Experience with accelerated aging and related diagnostic tests for stator coils and bars of rotating machines

Since the 1990s Manitoba Hydro has used accelerated aging tests in the form of thermal cycling and voltage endurance to qualify batches of stator coils or bars for installation in new or rewound generators and synchronous compensators. In the late 1980s IEEE Std 1043 had been developed as a test method for voltage endurance of entire coils or bars. Previously it had been common to limit voltage endurance tests to short sections cut from the slot portion, but this excluded the end arms which can be the location of many voltage endurance failures. It became apparent that thermal cycling prior to voltage endurance could reveal coils or bars whose insulation is subject to delamination. IEEE Std 1310 was developed in the 1990s to recommend the procedure for thermal cycling. The use of a partial discharge screening test in a frequency range preferentially sensitive to internal voids and delaminations can be useful as a means of identifying those coils or bars which may perform poorly on voltage endurance. This allows coils or bars considered to be marginal to be installed in the neutral third of a star connected stator winding. The PD screening test method is documented in IEEE Std 1434 Table 3 Case 3.