Synchronous generators were manufactured with asphalt bonded, mica flake insulation from 1917 to the 1950s. There is a large fleet, perhaps two thousand, of these machines still in service today. This paper presents the results of a well documented series of field tests, many to failure, on 20 asphalt-mica insulated fossil fuel and hydroelectric synchronous generators on the American Electric Power system and laboratory tests on over 30 spare asphalt-mica stator bars and coils at AEP's Dolan Laboratory. The paper clearly shows that a straightforward AC overpotential test and a simple graphical evaluation and analysis can be used to confidently determine the remaining service life of asphalt bonded, mica flake insulated synchronous generators that have been well maintained and operated within design limits.
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