Belgian experiences with testing using the Thermal Step Method on cables and stator winding insulation in rotating machines

摘要

As the Belgian technical competence centre for energy processes, Laborelec must deal with much demand from the energy production (Electrabel) and transport (Elia) industry regarding materials for electrotechnology. A frequent cause of failure in AC-supplied machines is the electrical breakdown of the generator insulation. Electrical power transmission also requires reliable high voltage AC power grids and there is currently a trend towards the increased use of buried links. Principal ageing factors have, however, been identified for dielectric material, mostly in composite material. These factors are space charges (electrical field), oxygen, water, mechanical factors and temperature. The knowledge and methodology acquired in a field, e.g. high voltage cable insulation ageing, can be applied in other fields covering materials used for electrotechnology (rotating machines, irradiated cables in nuclear power plants, medium voltage cables) and vice versa. Today, it seems increasingly probable that decreases in dielectric performance could be related to electrical charges, which tend to accumulate in insulating materials and which are commonly known as space charges. The experimental study of space charge effects has been made possible by the advent of several techniques, and one of these sensitive techniques is the thermal step method developed by the "Laboratoire d'Electrotechnique de Montpellier". The applications discussed principally concern the ageing assessment of in-service high voltage cables and the ageing of an air-cooled 150 MWatt generator for which white spots have been detected on the end windings.