Experience with DC Ramp Test Performed on Asphalt-Mica insulated Hydraulic Generators

In order to prevent forced outages of old generators, condition-based maintenance and diagnostic tests are periodically conducted on their stator insulation systems. The DC ramp test is easy to perform, involves a light material and has proven to be an effective and nondestructive means for reducing in-service stator failure. During a DC ramp test, the I-V curve is continuously recorded. A recently developed method of modeling facilitates the separation of the measured current curve into its leakage, absorption and capacitive components. This modeling also helps to precisely point out the outset of any non linearity in the leakage current that could be related to a defect in the insulation system. This paper presents DC ramp test results conducted on old asphalt insulated machines. Modeling of the I-V curves is also performed and the results are compared with similar measurements conducted on a spare coil.