Condition Assessment of Power Transformers Using Loss Tangent Measurements

Mineral oil, cellulose paper and pressboard are the main insulation materials in a power Transformer. During service, increase of moisture, as the by -products ageing and increases the risk of insulation failure. This paper presents the possibility to use loss tangent (or power factor) and capacitance (tanδ and C) measurements as an estimate of the 'wetness' of aged transformer insulation. Firstly, oil, paper and pressboard samples were aged under laboratory conditions and their tan S and dielectric constant (εr) values were measured. Applying the tanδ and εr obtained onto a transformer insulation model, the tanδ and C of transformer were calculated and the effects of insulation geometry, moisture content (MC) of pressboard, oil conductivity (σ oil) and insulation temperature under test were established. It was found that tanδ and C values of a transformer strongly dep ended on its insulation geometry and the temperature. If the temperature is fixed, the change of tanδ value of a transformer can be a direct measure of moisture in pressboard for 'wet' and 'extremely wet' conditions. For a 'dry ' transformer, the σoil oil value is needed as an additional known parameter in order to estimate accurately the moisture in pressboard.