Comparative AC and DC voltage endurance tests performed 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 the condition assessment of rotating machines. Since the high voltage may be applied for a relatively long period of time during the charging of the winding, its value can be a matter of concern. No aging of the insulation under test should occur while the condition assessment is performed. In order to be able to compare the aging effect of AC and DC high voltage on the groundwall insulation of a stator winding, the so-called Voltage Endurance Test (VET) was performed on 12 spare 13.8-kV Roebel bars. Half the test specimens were subjected to a voltage of 35 kVac; for this test, a ratio of 1.7 between direct voltage and power frequency voltage (rms) was used. The other half were then tested at a voltage of 59.5 kVdc. This paper presents the results of this comparative VET between AC and DC voltages performed on epoxy-mica based insulation of hydro-generator Roebel bars.