Different types of dielectric fluids used in transformers are becoming increasingly prevalent on the market. This article presents recent comparative study on negative lighting impulse breakdown voltage (LIBV) and acceleration voltage (V _a ) of six insulating liquids of different chemical composition namely naphthenic Uninhibited Mineral Oil (UMO), naphthenic Inhibited Mineral Oil (IMO), Natural Ester (NE), Synthetic Ester (SE), and new modern dielectric fluids such as Bio-Based Hydrocarbon (BIO) and inhibited liquid produced using Gas-to-Liquid technology (GTL). Tests are conducted for a point to sphere electrode setup based on IEC 60897 Standard and for two gap distances, 25 mm and 40 mm. When comparing the results obtained from the experiment the biggest differences between the liquids are noticed not for the LIBV evaluated using Weibull distribution but for V _a parameter; it is confirmed that both esters considered within the studies are characterized by lower values of V _a than other analysed fluids among which IMO, BIO and GTL are of comparative nature, especially for 40 mm gap. In turn, based on the obtained results of LIBDV and V _a , the maximum values of the electric field strength are determined by series of finite element method (FEM) simulations. Those simulations confirm stable electric field condition determining breakdown and discharge acceleration in a given liquid considered.

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.

An increasing number of dielectric fluids intended for use in transformers are appearing on the market. Due to the growing awareness of sustainable development and serious environmental risks posed by any mineral oil spills, transformer manufacturers are constantly working on alternative dielectric liquids. Therefore, it is crucial to test those liquids thoroughly. Commonly performed tests are also focused on dielectric strength under both lightning impulse breakdown voltages (IEC 60897) and breakdown voltages at power frequency (IEC 60156). In this paper authors propose extension of the standard list of tests by including the measurement of partial discharges at power frequency and acceleration voltage at lightning impulse of both polarities. In the first case a standard IEC TR 61294 is adopted while in the second one the methodology is enlargement of the provisions of IEC 60897 by the mentioned acceleration voltage measurements. In both cases, these additional tests enable a more comprehensive evaluation of dielectric liquids under consideration, especially in the case of newly ester liquids introduced in the market. The results show that proposed approach is worth to be further considered in terms of use a given liquid in power transformer.

In this paper, results of comparative studies on the positive lightning impulse breakdown voltage (LIBV) and accelerating voltage (V_a) of six insulating liquids of different chemical composition are presented. This paper discusses the behavior of uninhibited naphthenic mineral oil (UMO), inhibited naphthenic mineral oil (IMO), natural ester (NE), synthetic ester (SE), and two modern dielectric fluids: bio-based hydrocarbon (BIO) and inhibited liquid produced using Gas-to-Liquids (GTL) technology. Measurements are taken in a point-to-sphere electrode system for two selected gap distances: 25 mm (which is suggested by the IEC 60897 standard) and 40 mm. After analyzing the obtained results, it is noted that positive LIBV does not differ significantly between the tested liquids. Noticeable differences are observed, however, for V_a. The lowest values of this parameter characterize ester liquids, which is consistent with the common knowledge in this field. In addition, the obtained values of LIBV and V_a are used to evaluate the maximum values of electric field intensity through the application of simulations for each specific case based on the finite element method. These simulations confirm that, for a given parameter, maximum electric field stress is on similar level, regardless of the gap distance. This proves that the breakdown and appearance of fast discharges are determined by specific field conditions.

W artykule przedstawiono badania porównawcze elektroizolacyjnego oleju mineralnego typu Nytro 10XN i płynu izolacyjnego na bazie węglowodorów pochodzenia biologicznego typu Nytro BIO 300X. Porównano udarowe napięcie przebicia (w oparciu o normę PN-IEC 897) oraz parametr napięcia przyspieszenia badanych cieczy. Próby wykonano napięciem udarowym piorunowym normalnym (1,2/50 μs) o biegunowości dodatniej i ujemnej, w układzie elektrod ostrze–kula.

This study presents findings on the influence of gap length distance on the lightning impulse breakdown voltage of three dielectric liquids of different chemical origins. The liquids were tested in a model electrode system with a quasi-uniform electric field distribution and a pressboard plate placed on the grounded electrode. The experimental studies were supported using calculations and simulations to show the individual relationships between the lightning impulse breakdown voltage and gap distance, which represent the so-called volume effect of the most stressed liquid. The results of the experiment, which involved four considered gap distances of 2, 4, 6, and 8 mm, show that a dynamic increase in lightning impulse breakdown voltage with an increase in gap distance is associated with mineral oil and bio-based hydrocarbons. However, similar trends were not observed for synthetic ester. Calculations that allowed us to assess the impact of gap length distance on lightning impulse breakdown voltage support the observations from the experimental studies. The curves obtained in this field can be considered in the process of designing insulating systems for transformers.