Comparative AC VET on new bars and on bars previously subjected to DC VET

A previous experiment was performed to compare the aging effect of AC and DC high-voltage on the groundwall insulation of 13.8 kV Roebel bars [1]. For this purpose, Voltage Endurance Test (VET) was performed on twelve spare bars. Half of the test specimens were submitted to a voltage of 35 kVac, and failures occurred on all bars in less than 150 hours. For this experiment, a ratio of 1.7 between direct voltage and power frequency voltage (RMS) was used. The other half of the test specimens were then tested at a voltage of 59.5 kVdc. Further in this experiment, since none of the bars subjected to the DC VET had failed during a test period of more than 2000 hours, it was decided to perform an AC voltage endurance test on these bars. This paper presents the test results of a comparative AC voltage endurance test performed on new bars and on bars previously subjected to a long term DC VET.