Effect of temperature and thermal expansion on slot partial discharge activity

Abstract

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) wiD 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 electricaOy stressed at 8.0 kV and thermally stressed by a circulating current. For each gap dimension, PD tests were carried out at room temperature (22°C), and with circulating current at 53, 70, 90 and 132°C. 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. l INTRODUCTIONOn-line partial discharge signals can show significant variation in pulse amplitudes and numbers depending on the load condition of a generator. Generally, this PD activity will remain fairly constant for generators in good condition and yearly measurement will be similar with only slow changes associated with the normal degradation by internal PD of the insulation system. However, when specific problems are present, such as slot discharges or degradation of the stress grading coating, the detected PD activity from one year to the next could change significantly due to degradation, but also due to variation in environmental/operating conditions.The intent of the CUtTent study is to evaluate the contribution oftwo of the main parameters affecting slot discharge activity: temperature and gap size. Bar vibration, which is another key factor influencing slot PD intensity, is not covered in this paper. As the load of a generator increases, the thermal expansion of the copper and the insulation could partially fill the gap between the bare insulation surface and the grOWlded stator core lamination, also in expansion. The extent of this expansion is obviously dependant on the nature of the insulation system. It is believed that the reduction of gap should reduce slot PD activity. Even if this phenomenon is straightforward and generally accepted, there is very little documentation comparing the specific contribution of the temperature and the gap size on the detected slot PD activity. The experiment presented herein will show which one of these two parameters is ...

Effect of surface degradation on slot partial discharge activity

“…This confirms that temperature is a major contributor in the slot PD process. This behavior is consistent with results of previous findings where the slot PD activity always increased proportionally with temperature [27,31]. Thus, both temperature and surface degradation play a role in the evolution of slot PDs over time.…”

“…Because of the heat, the insulation surfaces showed signs of thermal aging with changes in the color of the insulation. Previous work has revealed that an increase in temperature always leads to an increase in slot PD activity for short-term exposure [27]. Here, neglecting the influence of surface deposit, the sustained high energy of the PDs combined with the formation of ozone, which is a strong oxidizing agent, contributed to erode the insulation surfaces by degrading the organic compound of the epoxy resin and laterally damage the semi-conductive coating, extending the bare insulation area exposed to high voltage.…”

De las Conferencias de París al proyecto del Sistema: las últimas presentaciones husserlianas de la fenomenología

“…Esta irrupción vino de la mano de un corte de nudo gordiano respecto de los proyectos previos, los cuales quedaron de nuevo pospuestos en virtud de la sistematización de los manuscritos, en los que colaboraron Fink y Langrebe. 71 Husserl comentó a D. Cairns en 1931 que había pensado erróneamente que componer una introducción a la fenomenología para una audiencia amplia era algo simple, pero luego vio que sus intentos de los últimos diez años, en el arco que va desde las lecciones de Londres a las de Paris y sus rediseños, carecieron todos de resultados satisfactorios. 72 Husserl nunca concluyó el proyecto de la versión alemana, al que se refirió en 1930 como "el trabajo principal de mi vida", 73 pero sabemos que no mantuvo las energías en ese proyecto y, por el contrario, se orientó hacia los trabajos también inconclusos que conformaron luego el texto de la Crisis.…”

Measurements of slot partial discharges with an antenna during accelerated aging

“…In the first case, purely electrical slot PD are expected to progress much more slowly than when the degradation is accelerated by mechanical vibrations. Furthermore, degradation is expected to be faster at temperatures higher than room temperature, since an increase in temperature is usually expected to lead to an increase in the slot PD activity [3]. While the influence of these stresses is generally accepted, the resulting differences in the evolution of the PD patterns are much less understood.…”