Partial discharge behaviors of a surface discharge problem of the stator insulation for a synchronous machine

This paper presents the partial discharge (PD) behaviors of the stator winding insulation focusing on a surface discharge problem of the11kV, 14.5 MVA synchronous machine. This synchronous machine has been installed in 2004 and still operated up to now. The offline PD measurement has been continuously performed during 2014-2017. The PD activities comprising of the magnitude of the highest PD pulse, Qm, at different test voltages and the PD patterns at the rated phase voltage were analyzed. The investigated synchronous machine was stopped for least 12 hours prior to running the off-line PD test in order to cool down so that the winding temperature did not exceed 40 °C during performing the experiment. In accordance with the industry practice guide the voltage applied to the tested stator winding insulation was increased in step with 0.5 kV per step for the duration of 1 minute from 0.5 to 6.3 kV (phase voltage) for monitoring the PD activities. The experiment was made with one phase to ground of the stator winding insulation at a time, with the other two phases grounded. Then two times re-tested as mentioned above were done to ascertain the PD activities measured. After that other two phases of the stator winding insulation were tested as the same manner. The test results show that the change in Q _m value recorded in 2014 _ 2016 were nearly constant respect to the applied voltage. The Q _m was a strong tendency increase in 2017. The sharp increase in Qm gave the sign of a significant degree of contamination on the insulation surface. Besides, the PD patterns measured were sensitive to the progress of discharges. After that this machine was inspected. The contamination and defects at the surfaces of the stator insulations were detected. From the investigation, the PD measurement is one of the effective ways to monitor and evaluate the insulation conditions of the stator insulation.