Method for lifetime estimation of power transformer mineral oil

Abstract Large amounts of mineral oil are used in electrical equipments as insulation and cooling medium. To avoid damages and cut-off of power electricity supply it is necessary to evaluate the mineral oil condition. Lifetime estimation of mineral oil which is based on accelerated thermal ageing at three temperatures (according to IEC 60216-1/2001) requires a long experimental time. This standardized method permits the lifetime calculation using a model based on the equation ln D ( T ) = a + b T , where a is the intercept and the slope of the plot ln τx versus 1/T, b is a material constant, T is the accelerated ageing temperature and τx is the time to reach the acceptable limit value of a chosen degradation parameter. In the present paper, mineral oil lifetime at 80 °C (D1 = 1.53 × 105 h) has been obtained on the basis of accelerated ageing tests at three different temperatures (115 °C, 135 °C and 155 °C) and choosing the oil electrical resistivity (ρ) as degradation parameter. The activation energy of the ageing process determined in these conditions was 101.8 kJ/mol. To reduce the test duration, this paper proposes a simpler method based on a single ageing test at a higher temperature and the activation energy of the oxidation reaction being determined from non-isothermal Differential Scanning Calorimetry (DSC) measurements. According to the proposed method, the plot ρ versus τ (ageing time) was drawn only for the accelerated ageing temperature T = 155 °C and the lifetime value (τx) for this temperature was experimentally obtained. In this case, the activation energy value Ea corresponding to oxidation reaction was determined (Ea = 102.4 kJ/mol). Based on both Ea and the τx at 155 °C, the lifetime at 80 °C (D2 = 1.86 × 105 h) was determined and compared with D1. The differences between D1 and D2 values are acceptable and the proposed method is effective in terms of costs and experimental time (the experimental time is reduced by a factor of 12–14) as compared to the present standardized method.

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