During the operation of electric power transformers,their insulation often made from paper and mineral oil is subjected to thermal, mechanical, and electrical stresses. Therefore, the lifetime of power transformers can be assimilated with their insulation system lifetime. The aim of this paper is to explore and present main degradation aspects for mineral oil, subjected to the action of high electrical field and partial discharge and exposure to thermal stress exclusively, analyzing and pointing out changes in the dielectric properties. Thus, the obtained results indicate that both the thermal and, especially, the electrical stresses produce important variations of the values of the real and imaginary part of the complex relative permittivity, the electrical conductivity, and the loss factor which indicates a severe oil degradation.

Electrical transformers are some of the most important equipment in the entire electricity distribution chain. Their operation with optimal values of the parameters (electrical, thermal and mechanical) ensures the continuous supply of consumers. The modern electrical loads of power distribution transformers are often nonlinear and generate several power quality problems, especially the distortion of the waveform of the current that flows through the windings of the transformer. This generates additional stresses (electrical and thermal) of the various components of the transformer (originally designed to operate in pure sinusoidal mode) which can cause abnormal (faulty) operation of the transformer and ultimately reducing its life (estimated by the manufacturer for permanent sinusoidal regime). In order to prevent or diminish the negative effects of the nonsinusoidal regime on the transformer, a deliberate limitation (reduction) of its maximum load is performed. The procedure is known as transformer derating. Its main aim is to establish the most appropriate declassification factors (denomination), resulting from the correlation of the nominal and constructive data of the transformer with the power quality parameters, measured in its secondary part (usually, these are the current distortion level and the corresponding spectrum harmonic). This paper analyzes qualitatively and quantitatively these aspects and proposes a downgrading procedure for in-service transformers which it illustrates in a case study.

Electrical contacts may include various sub-systems or wiring harness connected via detachable connectors which depend on physical contacts for the electrical connectivity. Electrical contacts range from high, medium to low current depending on their usage. However, in the real-life condition, electrical contacts characteristics, especially at the interface, undergoes a gradual change which can be due to corrosion, temperature variation, aging, strained harnesses, discontinuities induced by vibration etc. These changes introduce additional parasitic circuits in the system. Moreover, in some cases where the contact resistance increases due to electrical loses, the local temperature may increase, thereby accelerating contact degradation. This paper presents a numerical analysis on the variation of temperature of a simple low current contact model having a thin oxide film layer at the interface which serves as the ageing factor using finite element method (FEM).

It has been suggested in earlier literature that the accuracy of the thermal simulation of electrical connectors is closely related to contact resistance. Contact resistance in electrical connectors occurs due to both constriction resistance (caused by narrow paths in which the current flows through the electrical connector) and film resistance (oxidized metals caused by the high resistivity of materials and impurities from the atmosphere etc.). This paper reviews the oxidation and wear affecting electrical connectors by proposing a thermal-electrical coupled finite element simulation (FEM) of the contact temperature rise of a simple contact model in COMSOL Multiphysics.