New model of explanation for phase angle pattern of online partial discharge measurement in winding insulation of turbine generator

On-line partial discharge (PD) measurement in electrical insulation of the turbine generator (TG) is an essential approach to control the quality of insulation and to avoid any undesired shutdowns of TG. Although in the last few decades the number of research in PD on-line monitoring methods in the stator winding of TG has increased significantly, it is still not clear yet why PD only appears at certain phase angles of the AC cycle in the phase resolved partial discharge (PRPD) pattern. Moreover, there is not yet any clarification on how the winding configuration of stator may affect PD phase angle pattern. For this reason, this work examines detailed study of the impact of the winding diagram on PD occurrence in the stator winding of real turbine generator class TVF-60-2 (60 MW, 10.5 KV). In addition, a computer simulation of various sizes of ellipsoidal cavities from 0.1 to 2.5 mm between conductor bar and stator core were carried out by using Laplaces equation in Finite Element Analysis (FEA) software ComSol to investigate the relation between the cavity size and coefficient of electric field. As a result of that, the phase angle of PD occurrence in the stator winding will highly depend on the configuration of bars connection, and the computer simulation has helped identifying the PD inception voltage and applied voltage of various ellipsoidal defects size. These two results have helped proposing an initial new theoretical model of explanation the relationship between voltage distributions and phase angle of PD occurrence to determine the degradation level of insulation caused by ellipsoidal defects in the insulation bars of the stator winding which can be used for stator windings of TG rated 6 kV and higher. This model is the initial step to develop a further comprehensive model of explanation for PRPD patterns which will consider all other types of defects as well as the space charge effects from the previous PD.