Transformer Windings Ageing, diagnosis and asset management. Version 4

This report addresses state of the art in the field of transformer ageing and diagnostics, with emphasis on advising end-users on transformer asset management and on transformer specification. The basis is results from two projects on laboratory studies on ageing of mineral oil impregnated cellulose insulation, and also some studies on service aged transformers. Ageing is governed by thermally activated processes and accelerated from presence of contaminants like water, lighter carboxylic acids and oxygen. It is found that commercially available thermally upgraded papers used for limiting ageing rates may vary a lot in quality, mainly due to varying nitrogen content. Two schemes for assessing the present and future ageing condition are possible: - Monitoring by-products formed during ageing, using these as ageing indicators. The recommended indicators are furanes, methanol, water soluble acids, water and CO and CO2 - Calculation based on ageing kinetic algorithms using thermal conditions, contamination and materials used as input parameters. Schemes are given for calculation of ageing condition and expected remaining life estimation. Qualified thermal analysis of service history is necessary for carrying out qualified estimations. Three different rigs for studying the condition of transformers in service have been tested. An experimental rig has been built to study temperature dependent partitioning of water and other contaminants in order to facilitate condition of cellulose in the windings from oil samples. Partitioning curves for water, acids and furanes in aged transformer insulation is studied.