Principles and characteristics of vibration sparking in high voltage stator slots

Recently some stator discharge problems in northern America and Europe have greatly brought people¿s concerns to the safety of stator coils. Different from the traditional partial discharge (PD) mechanism, the stator damage problem was thought to be probably caused by a newly promoted theory, namely the vibration sparking (VS). It is well known that during the operation of generators there is come voltage on the surface of the conductive coating, usually less than 10 V, but for ageing generators the voltage can be much higher. During vibration of stator bars caused by electromagnetic force, the conductive coating incessantly gets in touch with and disconnects from stator core, which under some circumstances (usually much easier than PD) may result in vibration sparking (VS) in stator slots. According to recent researches about PD, usually a certain high voltage around 200 V or above is needed for most of the gaps in stator slots of which gap distances are between several micrometers to tens of micrometers to have electrical breakdown. However VS can emerge when the gap voltage is only several tens of volts according to the experiment in this paper. In this paper, first of all, by combining two models (promoted by Jackson and Liese) for the first time, principles of VS were studied integratedly, gap voltage and ground current were calculated and verified through experiments. In addition a model platform of VS was built up by which conditions needed for the emergence of VS and characteristics of VS were studied. Finally optimal resistance per square of conductive coating was calculated based on theoretical analysis and experimental results, which can provide advice for manufacturers to greatly restrain the occurrence of VS.