Effect of long-term aging test under DC voltage on Roebel bars

Measurement of the charging and discharging current of stator winding insulation under DC high voltage is one of the tests used in assessing the condition of rotating machines. Since the DC high voltage may be applied for a relatively long period of time during the charging of the winding, the value of the applied voltage can be a matter of concern, and no aging of the insulation under test should occur while the condition assessment is performed. Previous experimentation compared the aging effect of AC and DC high voltage on the groundwall insulation of 13.8-kV Roebel bars. A Voltage Endurance Test (VET) was performed on 12 spare bars for this purpose. Half of the test specimens were subjected to a voltage of 35 kVac, and failures occurred on all bars in less than 150 h. A ratio of 1.7 was used between the direct voltage and power frequency voltage (rms) for these tests. The other half of the specimens were then tested at a voltage of 59.5 kVdc and no failure occurred after more than 2000 h. It was later decided to perform an AC voltage endurance test on bars previously tested under DC voltage, to try to evaluate the effect of DC VET on the integrity of the insulation. Another aspect of the aging process that was evaluated during these experiments was the effect of corona partial discharge activity at the junction of semi-conducting (S/C) and stress-grading coatings. Another six bars of the same production were modified in order to generate sustained corona PD activity to try to establish the associated risk. This paper presents the results of comparative VETs between AC and DC voltages performed on the epoxy-mica based insulation of hydro-generator Roebel bars, as well as results of AC VET performed on six bars previously subjected to a long-term DC VET and the effect of corona activity on the integrity of the insulation.