Impact of spacers regarding endwinding design of large rotating electrical machines

Today large rotating electrical machines are supposed to operate at optimised conditions and the highest degree of efficiency. Often the overall size of a generator has a significant impact on transport restrictions as well as cost efficiency. A key market requirement is a combination of high efficiency, very compact design and cost efficiency of acquisition and operation. One of the main elements to meet these demands is an optimised endwinding design of high voltage stator windings. A compact design requires among other things minimised distances between adjacent insulated bars or coils in the endwinding which consequently leads to a continuously increasing stress (thermal, electric) on spacers located in the endwinding area. Therefore the influence of spacers on the corona behaviour of air and hydrogen is the main focus for high voltage insulation stator winding design, especially in relation to the endwinding. The end winding area has to be designed in such a way that no corona activity can occur during the operation of the generator. On the one hand the distances between adjacent bars or coils should be as small as possible while on the other hand any corona activity should be avoided. In this paper the results of experimental investigations in air and hydrogen for electrode-spacer configurations are presented. Metallic electrodes were covered with standard insulation material which is used in generators. The test arrangement allowed changing the gas pressure whereas the gap distance between the electrodes was defined by the used spacer. To determine the corona inception and extinction voltage we used a conventional PD measurement system according to the IEC 60270 standard. Additionally for verification of our results we used acoustical (ultrasonic microphone) and optical (UV camera) PD detection systems. The purpose of this study was to determine the influence of spacers on partial discharge behaviour (corona inception and extinction voltage) of insulating systems used in endwinding areas. On the basis of gas physics and our previous studies we expected a decreasing corona inception and extinction voltage by increasing pressure and gap distance for spacer configurations. This is in comparison with configurations without spacers. In our experiments we were able to corroborate this. However, to formulate a conclusive relationship between the influences of spacers on PD inception level, further work needs to be done.