DESIGN AND MANUFACTURING PROCESS

A. Influence of the esters’ properties on the design of the thermal and dielectric design of the transformer

Although numerous studies on the thermal and dielectric properties of ester fluids have been published since the 90s, the subject remains active nowadays. At present, the focus is mainly put on the influence of temperature and other agents on the fluids’ performance and the influence of aging byproducts on the dielectric strength of the fluids. An aspect that has not been sufficiently studied is the difference between the natural ester fluids of different origins. It must be considered that within the term “natural ester” fluids with different basis and different properties are included (i.e. sunflower oil, rapeseed oil, soya oil, or hazelnut oil).

Regarding the dielectric design of transformers, the main efforts have been made in comparing the breakdown of esters and mineral oils in the case of inhomogeneous fields and different voltage waveforms and the influence of these aspects in the dielectric design of the transformers. As the permittivity of the esters is comparable to that of the paper, another important design aspect studied is the probability of creep breakdown. Some works have been presented to understand creepage under a variety of conditions. However, more works are necessary to increase the level of confidence of ester filled transformer of high power, as the oil volumes, in that case, are larger and the streamer propagation of those volumes of ester liquids can compromise the dielectric design of the transformers. Streamer formation and partial discharges in gas bubbles present in oil have received attention. Analyzing the streamer behavior is important because it allows an understanding of the breakdown phenomena in liquid dielectrics. The streamer behavior of esters and mineral oil is qualitatively similar, but streamers in both cases are very different in the propagation process.

A comparison of the impulse (1.2/50 μs) breakdown voltage of a natural ester and mineral oil has been carried out by the authors of this work. Both fluids were dried and regasified before the measures. The electrode configuration was needle-sphere and the electrode distances were 10 mm for positive impulse and 3 mm for negative impulse. Fig. 1 shows the obtained results fitted to a Weibull distribution. As can be seen, the ester behaves slightly better under positive impulse although the performance is poorer than that of mineral oil for negative impulse. It must be noted that the positive impulse supposes a higher risk for the transformer integrity, as the breakdown voltage values are lower. The main thermal properties of different biodegradable liquids have been also been compared with those of mineral oil by the authors of the paper. Table II summarizes some of the obtained results.

B. Impregnation process

During the impregnation process, the solid insulation of the transformer must be adequately dried and subsequently impregnated with the dielectric liquid. The impregnation of the solid insulation with dielectric oil is carried out to eliminate the air that might remain inside the solid insulation. The presence of air weakens the insulation level and favors the appearance of partial discharges that degrade the insulation. By removing most of the gas from the transformer and from the oil the hazard of the small bubbles of gas is highly reduced.

Due to their higher viscosity, natural and synthetic esters might require a much longer time to achieve impregnation of the cellulosic materials, used in insulation systems of power transformers. There are very few works which have studied impregnation processes in different solid components of transformers’ insulation systems even though it is a critical process to obtain a suitable insulation system.

In 1984, researchers from Toshiba Corporation studied the impregnation of transformer boards in mineral oil carrying out a theoretical study of the equations that govern the phenomenon. These authors included capillarity as a variable that influences on impregnation process. Other researchers have carried out studies of board impregnation in mineral oil analyzing the effect of temperature on impregnation time or modeling and validating the impregnation process through the measure of breakdown voltage.

 

Table 1. Properties of the Fluids

In the last decade, the impregnation process of alternative liquids-based transformers has been studied by several authors. These authors compared the degree of impregnation in mineral oil, a natural and a synthetic ester finding that the required time to get an adequate impregnation is a function of the viscosity and capillary effect of oil inside cellulose material. It was also found that high temperature can help to overcome the influence of a higher dynamic viscosity of vegetable insulating oil in comparison with mineral oil. Additionally, it was obtained that the time duration for impregnation significantly affects the dielectric properties of the natural ester-impregnated pressboard such as the capacitance and dielectric dissipation factor.

In a recent work [1], the authors of this paper carried out an analysis of the impregnation process of a natural ester in different kinds of paper, Fig 2.

Four different cellulose-based insulation materials were used in the experiments: Crepe, Diamond Dotted Paper (DDP), Kraft, pressboard (PSP 3055), and three different fluids: mineral oil, natural ester, and synthetic ester. The impregnation was carried out in the vacuum oven at 5mbar and different temperatures to determine its effect, given that both viscosity and surface tension are temperature dependent. For these tests, the papers were cut into 1.5 cm wide strips and held vertically, with the bottoms immersed in the oil samples. The capillary action occurs when the cohesive intermolecular forces of the liquid are smaller than the interfacial tension between the liquid and surrounding solid surfaces. This physical principle helps us to compare the impregnation speed in the previously listed materials. Fig 2 represents the impregnation slope of a natural ester in the different kinds of papers considered. These models allow predicting the impregnation slope associated with any of these materials for a given temperature. One useful application would be to approximate the temperature at which the esters offer similar impregnation performance as the mineral oil at room temperature.

REFERENCE
[1]. O. Sancibrián et al, “Analysis of the Impregnation Process of Cellulosic Materials by Ester-Based Insulating Fluids”, Advanced Research Workshop on Transformers, ARWtr 2019, Cordoba