On Thermal Performance Analysis of Oil-Immersed Power Transformers: Winding Cooling Curve

IEC 60076-2 is one of the standards providing comprehensive information as guidance for evaluating the thermal performance by measuring the top and bottom oil temperature rises and winding resistance and deriving the rest of the required temperature rises using simple calculations. The standard suggests, however, different mathematical formulae to analyse the winding cooling curve to be able to evaluate the winding average temperature rise by extrapolation. Each of the winding cooling curve formulae has a physical interpretation which holds certain information about the winding and oil thermal properties resulting from the heat run test. The current study presents a close investigation of these mathematical formulae as a source of information for power transformer thermal analysis. The focus is on the two-exponential function, which expresses the most accurate interpretation of the cooling process, including time constants of winding and oil. Additionally, a novel algorithm is introduced based on the cooling curve analysis to estimate the winding hot spot in comparison to IEC-Model and fibre optic sensor measurements.