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Deterioration of dielectric insulation system of turbine generator windings in power plants is continuous process with service time. Mica composite serves the machine as potential barrier for asphalt bonded insulation of 1940s, resin rich epoxy system of 1960s and existing global vacuum pressure impregnation with varied electrical field strengths in hostile atmospheres in power plants. Operational stresses disintegrate the micaceous insulation due to ionization of micro voids. A prominent diagnosis dielectric test for ages is Tan δ to assess over all integrity of winding's insulation. Emphasis is made in this paper on characteristic behavior of dielectric loss factor on various insulation systems of generator stator windings. Tests had been performed on machines of as old as half a century to latest half a decade operating life in power plants. Results of tan delta at increased high voltages and tan delta tip up are evaluated against threshold values. Reasoning of varied magnitudes of the parameters are discussed. Recommendations to revamp a generator winding of 1945 vintage are presented to customer for enhancing life of the machine.

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