In general, there are many types of electrical rotating machines and transformers within various classes of power range and duty cycles. These machines are operated under either industrial or civil conditions. It is a matter of fact that every machine can eventually fail and both damage and losses can occur. Therefore it is necessary to evaluate failure rates and their distribution over certain types of electrical machines and their subsystems. This paper presents an analysis based on the publicly available data, summarizes and compares results obtained from the studies which have been carried out in the past years. In this paper, important machines' subsystems in terms of failure rate are identified. The failure rate is presented in examples. The cause and location of the faults are discussed. Furthermore, this paper suggests general possible procedures for failure elimination or mitigation of the risks. Outcomes of this paper may be important in the field of maintenance, diagnostics and testing, project management and asset management.

Currently, a significant number of transformers are designed and used in the electricity distribution network. Since these machines are commonly based on an oil-paper electrical insulation system, it is crucial to know how certain important parameters of an oil behave within a wide range of ambient temperatures. Therefore, the range of temperatures should also include low temperature region as these conditions might occur in real application. The aim of this paper is to focus on the temperature dependent viscosity, density, thermal conductivity and specific heat of an oil in order to develop a strongly coupled thermal model of a transformer which is filled with biodegradable natural ester insulation fluid. Obtained results can be well applied within the design, operation and diagnostics phase of these types of electrical machines.

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Inseparable part of the high voltage transformers is electro insulating liquid. As known, for one hundred years are used mainly mineral oil based insulating fluids since having good electrical and physical properties and being relatively cheap. Over the years other liquids has been considered/used as replacement to mineral oils such as polyfluorobiphenyls (PFB), poly-chlorobiphenyls (PCB), silicones, synthetic esters, gas to liquid (GTL), etc. This paper and research is dealing with testing of modern biodegradable and sustainable oil Envitrafol and its improvements by nanoparticles. As known natural ester-based electro insulating fluids are polar substances with relatively high viscosity and corresponding properties. This paper is focusing on the study of the impact of the modification of natural ester-based fluid by nanoparticles on breakdown voltage (BDV).

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