Determination of copper in liquid and solid insulation for large electrical equipment by ICP-OES. Application to copper contamination assessment in power transformers

Copper is one of the main constituents of the components in power transformers and its presence both in liquid (mineral oil) and in solid (Kraft paper) insulators can lead to enhanced dielectric losses and to the subsequent deterioration of their insulating properties. Recently the latter have been correlated to plant failures which in turn may have severe impact on the environment. This paper describes the direct analysis of copper in insulating mineral oil by ICP-OES and how it was first optimized compared to the official American Society for Testing and Materials (ASTM) D7151 method. Detection and quantification limits of 8.8 μg kg−1 and 29.3 μg kg−1 were obtained. Secondly, copper determination was improved by coupling a microwave assisted dissolution procedure of the mineral oil which avoided the problems, in the real samples, due to the presence of solid species of copper which cannot be nebulized following traditional methods described in literature. Sixteen mineral insulating oils sampled from transformers in service were analyzed before and after dissolution. In order to evaluate copper speciation, size fractionation was performed by filtration on PTFE filters (0.45, 1 and 5 μm). This test was performed on all the oil samples. Finally, because of the key role of the solid insulator in failed transformers, the Authors applied the developed method to study the copper deposition tendency onto the insulating Kraft paper tapes exerted by two unused oils (a corrosive and a non-corrosive one) under defined ageing conditions.