Investigation Regarding Corona Free Endwinding Design of Form Wound High Voltage Stator Windings

High voltage rotating machines are supposed to be optimized not only in terms of performance but -among others also with respect to space and material utilization. When designing machines there has been considerable effort made to reduce the overall size of generators particularly in the axial direction. The winding is one of the more demanding parts of the generator but the end winding is an obvious place where axial space could be saved. For an optimized design, the distance between adjacent insulated bars or coils is a very important factor. Inadequate end winding spacing can lead to corona activity between adjacent coils or bars. Exact prediction of the needed space is difficult as prediction is dependent upon several contributing factors. Dielectric properties of the insulation material, location of the bracing system, imperfection of insulation surface as well as of the corona inception and extinction voltage in the air gap between insulated electrodes are important factors to understand for an optimized end winding design. In this paper the results of experimental investigations on air for various electrode configurations are presented. Metallic electrodes were covered with standard insulation material which is used in generators. The corona inception and extinction voltages as well as the partial discharge rate and the amplitude were measured for different electrode configurations. Additionally, the corona discharges were monitored with a special UV camera. The distance and the pressure of the air were varied in a wide range. The main purpose of this study was to optimize and simplify the spacing requirements of the end winding design. Therefore the corona inception and extinction voltage in the air gap between insulated electrodes as a function of distance and pressure was measured. Subsequent to that, the translation between the classical Paschen's Law for breakdown and the corona inception and extinction voltage in the air gap between insulated electrodes was investigated.