Effect of temperature and thermal expansion on slot partial discharge activity

The influence of generator load changes on partial discharge (PD) activity can be significant especially when the semi-conductive coating of stator bars is eroded. It is common knowledge that thermal expansion (copper, core and insulation) will decrease the gap size in the slot between the bar and the core and this could lead to a reduction of slot PD activity. In the present study, a surface defect was made on a stator bar and a variable gap between the bare insulation surface and a ground plane was adjusted from close contact to fixed gaps of 0.25, 0.5 and 1.0 mm. Partial discharge measurements were recorded at different temperatures for each of these four gap sizes. The stator bar was both electrically stressed at 8.0 kV and thermally stressed by a circulating current. For each gap dimension, PD tests were carried out at room temperature (22degC), and with circulating current at 53,70,90 and 132degC. By comparing results from different gaps at a fixed temperature and results from different temperatures at a fixed gap, it was possible to determine which one of these two parameters had the greatest influence on the PD signals. The effect of these changes with respect to gap size was compared with an evaluation of the thermal expansion of a bar in its slot under normal operating conditions. This study leads to a better understanding of the change expected in slot PD activity with load variations when measurements will be performed on actual generators.