Condition assessment of transformer core CRGO steels due to thermal ageing and their evaluation

Electrical steels with low core loss are prominently used as core material in the transformer due to their unique magnetic properties. The cold rolled grain oriented (CRGO) core steels are used in the transformer construction and their magnetic properties directly affect the efficiency of the transformer. As the transformer life increases there will be degradation in the properties of core material due to ageing and heat generated during in service conditions under load. The thermal ageing study was conducted for different CRGO materials of different grades having core loss of (a) 0.75 (b) 0.80 (c) 0.85 (d) 0.90 and (e) 1.00 expressed in Watts/kilogram and these are further evaluated by different analytical technique, such as XRD, Optical Emission Spectroscopy and magnetic properties for condition assessment of CRGO steel. The thermal ageing study revealed that minor change in chemical composition of major alloy components silicon and aluminum results in increased brittleness of CRGO material, XRD analysis of CRGO steel with insulating coating compound Mg2SiO4 shows deterioration in coating and the effect of thermal ageing for all the grades are noticed in magnetic and insulation properties namely core loss and surface insulation resistance respectively.