A new tool to study the ageing of the winding insulation of electrical machines

Abstract

Nowadays, a frequent cause of failure of AC-supplied machines is the breakdown of the stator insulation. In fact, these machines have to endure repeated mechanical, electrical and thermal constraints and, besides, when they are supplied by variable speed drives, the winding insulation is submitted to steep voltage fronts. In this paper, we study the setting up of the space charge as a consequence of the ageing of the stator insulation. The Thermal Step Method (TSM) was used to measure the space charge. We characterized samples taken from AC-machines stators windings which had been submitted to several ageing conditions (temperature, voltage fronts of different heights and steepness). In order to complete the investigations, we made also measurements of Thermally Stimulated Discharge Currents (TSDC) which give the energy levels of the traps. The results of this work prove the ability of the TSM for the composite insulating materials of a stator winding. They show that certain ageing parameters such as temperature and characteristics of voltage fronts act upon the level and the type of the space charge. By associating the two techniques (TSM and TSDC) we can perform a complete study of the apparition and of the development of the space charges, and therefore predict the stator insulation lifetime: it is a new diagnostic tool for these components.