The paper concerns the case study of a group of oil power transformers in which, on the basis of the measurements of concentration of gases dissolved in oil, excessive generation of hydrogen was noticed. Analysis performed using the ratio method proposed by IEC 60599 standard, and the Duval triangle method, identified partial discharge (PD) generation in the transformer units considered. Deepened analysis indicated however that such a diagnosis may by false and excessive hydrogen generation is a result of stray gassing. Additional studies such as the measurements of partial discharge intensity using acoustic emission method as well as internal inspection and factory tests (including PD measurements using electrical method) of selected units seem to confirm this hypothesis. However, taking into account the recommendations described in CIGRE brochure 296, lack of compatibility of constructional materials with the oil used in the analyzed group of transformers is also probable. Finally, this may be concluded that assessment of the results of the DGA needs to be carried out carefully when an increase in the concentration of hydrogen in the transformers in service is observed. An unambiguous determination of the cause of the fault cannot be indicated only on the basis of basic analysis. Some other, more specific analyses and tests should be used.

The growing global demand for electricity is one of the crucial factors contributing to the increase in greenhouse gas (GHG) emissions. Power transformers, although essential components in electricity transmission and distribution systems, significantly impact the environment. This paper employs a Life Cycle Assessment (LCA) to evaluate the carbon footprint of two oil-immersed transformers with a capacity of 31.5 MVA and 25 MVA, manufactured in Poland in 2023, with an assumed lifetime of 40 years. The analysis follows a cradle-to-use approach and considers various scenarios, including differences in the average operating load levels, electricity generation from different sources, and a time frame spanning 2024–2063. After 2–3 years of transformer operation, even at low loads, the CO₂ equivalent (CO₂-eq) emissions associated with energy losses exceed those generated during the transformer production. These results underscore the critical importance of utilizing advanced construction materials and design solutions. Moreover, this analysis highlights the need to implement systemic actions aimed at restructuring electricity generation, especially in regions that heavily dependent on fossil fuels.

This article presents a description of four independent case studies concerning situations when power transformers were directed to internal inspection. This inspection was the result of a specific case of a routine diagnostic procedure that was carried out and, where the transformer was switched off by a Buchholz gas relay. The case studies described were selected such that they represented situations when availability of historical data on the previous measurements was limited and a quick diagnosis had to be made on the basis of the results from the last measurement. In all of the cases presented here, the analysis of the gases dissolved in oil had played an important role in the detection of the defects that turned out to be dangerous for further exploitation of the transformers considered. The first signal about a possible developing defect was elicited solely from the measurements of the oil samples taken from the transformer in service. However, more detailed recognition and initial localization of the defect was possible after additional supplementary measurements (winding resistance, sweep frequency response analysis, etc.), which required the transformer to be switched off. The conducted sequence of actions, based on the developed diagnostic procedure, indicated the possibility of effective and early withdrawal of the transformer from operation, before it underwent a serious failure.

A Duval triangle is a diagram used for fault type identification in dissolved-gas analysis of oil-filled high-voltage transformers and other electrical apparatus. The proportional concentrations of three fault gases (such as methane, ethylene, and acetylene) are used as coordinates to plot a point in an equilateral triangle and identify the fault zone in which it is located. Each point in the triangle corresponds to a unique combination of gas proportions. Diagnostic pentagons published by Duval and others seek to emulate the triangles while incorporating five fault gases instead of three. Unfortunately the mapping of five gas proportions to a point inside a two-dimensional pentagon is many-to-one; consequently, dissimilar combinations of gas proportions are mapped to the same point in the pentagon, resulting in mis-diagnosis. One solution is to replace the pentagon with a four-dimensional simplex, a direct generalization of the Duval triangle. In a comparison using cases confirmed by inspection, the simplex outperformed three ratio methods, Duval triangle 1, and two pentagons.

The article describes a case study when the voltage collapse during lightning impulse tests of new power transformers was noticed and when the repeated tests finished with a positive result. The step-by-step process of reaching the conclusion on the basis of dissolved gas analysis (DGA) as a key method of the investigations was presented. The considerations on the possible source of the analysis showed that the Duval triangle method, used in the analysis of the concentration of gases dissolved in oil samples taken from bushings, more reliably and unambiguously than the ratio method recommended in the IEC 60599 Standard, indicated a phenomenon which was identified in the insulation structure of bushings analyzed. Additionally, the results from DGA were found to be consistent with an internal inspection of bushings, which showed a visible trace of discharge on the inside part of the epoxy housing, as a result of the lightning induced breakdown.

This article deals with the influence of temperature on the lightning impulse breakdown voltage (LI BDV) of mineral oil in a non-uniform electric field. The experiments were performed in point–plane electrode arrangements under a standard lightning impulse voltage of negative polarity. The LI BDV for the temperatures of 20, 40, 60, 80, 100 and 110 °C was measured using a step method with registration of the light signals emitted during pre-breakdown and breakdown phases. The average propagation velocity was also determined. The results confirm that the mineral oil tested under LI voltage behaves similarly to AC voltage-based stress. The LI BDV increases with temperature up to 100 °C and then tends to decrease. The results demonstrated that viscosity dependent processes are responsible for increasing the LI BDV with temperature up to the 100 °C; and liquid vapor formation processes are responsible for decreasing the LI BDV at temperatures equal to and higher than 100 °C.