Importance of cooling system optimization with regard to lifetime and reliability of power transformers

The paper presents different risk aspects of transformer insulation breakdown caused by ageing of transformer insulation. Evaluation of recent artificial ageing of paper-in-oil transformer insulation results explains the effect of winding temperature on the achievement of the targeted transformer lifetime. These results and findings of two studies made at the Milan Vidmar Electric Power Research Institute (EIMV) in 2004/2005 assessing the ageing and cooling system state of large power transformers provided the basis for highlighting the occurrence of unexpected early ageing of particular large transformers that had been installed in the Slovenian power grid around 1980. Issues of temperature measurement and cooling system control associated with oil-forced type of transformer cooling are discussed in detail. Oil-forced type cooling has been installed in majority of the 400 kV and 220 kV large power transformers operating in the Slovenian power grid. The paper also discusses the importance of cooling optimization in order to have transformer residual lifetime extended and its life-cycle cost minimized.