Electric Field Analysis of Stress Grading System Delamination in the Stator End-winding of High Voltage Rotating Machine

The effective stress grading (SG) system can suppress the partial discharge activity on stator end-winding of high voltage (HV) rotating machine. In order to evaluate the damage of stress grading system delamination to the insulation of high voltage rotating machine, several three-dimensional finite element models, including two adjacent stator end-windings of a rated 24 kV hydro-generator, were modeled. In these models, various delamination positions between the stress grading system and the groundwall insulation were taken into account. Then the transient electric field analyses of these models were carried out. The electric field distributions in stress grading system delamination were obtained and evaluated. The results show that there is no obvious change in the surface tangential electric field of the stress grading system if the overlapping state of stress grading system is not changed. On the other hand, the electric field maximum in most areas of the air gap caused by delamination between the medium resistance coating and the groundwall insulation exceeds the threshold in a cycle, which means that there will be the partial discharge in air gap. The partial discharge in air gap will cause damage to the groundwall insulation and the stress grading system of the stator end-winding.