Abstract: Power transformers are electrical machines that allow us to transport electric energy, with reduced losses, from generation stations to consumption points. This definition gives us an idea of the number of machines of this type that are used in power distribution systems worldwide. The lifespan of this equipment often exceeds 30 years of operation. Therefore, they can be described as very robust equipment. The main problem that may affect power transformers is the operation at high temperature. This paper summarizes the results of a post mortem study carried out on an 800 kVA distribution transformer. The methodology that is considered for estimating the temperature distribution in the windings of the machine is based on the calculation of the degree of polymerization of the dielectric paper. This parameter is associated with the state of the paper insulation that protects the conductors of the windings. By knowing the value of this magnitude for a new and for an aged paper, and the period of operation of a transformer, the temperature distribution along the height of the windings can be estimated. With these results and the loading regime that a transformer has endured throughout his life, one can draw conclusions for future designs or for similar transformers still in operation.

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