OPERATION AND MAINTENANCE OF ESTER-FILLED TRANSFORMERS

The maximum admissible load of a transformer is related to the temperature distribution in the transformer for a certain loading profile, and also to the aging rate of the insulation. Using a natural or synthetic ester to fill the transformer tank may have an impact on both of these factors and in consequence new specific loading criteria must be stablished for ester-filled transformers. Work is still necessary on this point.

A. Temperature distribution in ester-filled transformers

In order to estimate the maximum overload of a power transformer it is critical to estimate the hot-spot temperature (hottest temperature of winding conductors in contact with solid insulation or insulating liquid) inside power transformers which is influenced by alternative fluids. The estimation of this temperature is essential because is one of the most critical parameters to estimate the remaining lifetime of solid insulation.

In the literature, some papers can be found where authors numerically predict hot spot temperature and temperature distributions in ester-immersed transformers. For instance, in 2010 Girgis et al. [14] compared the temperatures measured with fiber-optic sensors using alternatively a natural ester and a mineral oil as coolant in a 50 MVA commercial transformer. In 2015 Park et al. [15] employed a 2D Computational Fluid Dynamics (CFD) model in order to obtain temperature and velocity profiles of some alternative liquids used in a distribution transformer of 2.3 MVA and a power transformer of 16.5 MVA. In the same year, Lecuna et al. [16] carried out a 3D CFD simulation of an ONAN distribution transformer comparing a natural ester, a synthetic ester, a high kinematic viscosity silicone oil and a low kinematic viscosity silicone oil with a mineral oil. These works conclude that alternative liquids produce higher temperatures in the transformer windings designed for mineral oil. In [17] a 2D CFD simulation of two vegetal oils in the low voltage winding of a power transformer with zigzag cooling was performed, establishing a comparison of thermal capacity with a mineral oil. Figure 3 shows the average temperature of each disc of the winding for the three liquids studied, where the shape of the described curve for both vegetal oils is close to each other and different from mineral oil as a result of their higher density and viscosity.

B. Aging process of ester-impregnated solid insulation using oil analysis

The temperature has a critical impact on the ageing process of solid insulation. Consequently, it is needed to study the effect of these variables on the thermal degradation of solid insulation immersed in different dielectric liquids (mineral oil, natural and synthetic esters), as the condition of these components determines the lifetime of power transformers. In some research works, kinetic models have been proposed which are generally based on the data obtained from accelerated thermal ageing tests, carried out in the laboratory [18-20]. It is important to highlight that commonly laboratory tests are performed at higher temperatures than normal operating temperatures of power transformers, to reduce the time required to obtain representative results. Another main factor that influences the ageing process of solid insulation is the moisture content.

The authors of this work carried out an experimental study [22] aimed at analyzing the impact of the temperature and the moisture content of paper on the ageing rate of transformers insulated with natural esters. Figure 4 shows the results of accelerated ageing tests performed on natural-ester and mineral-oil paper insulation at different temperatures.

Fig. 5 compares the effect of moisture on both kinds of insulation. As can be seen, moisture has a lower influence in ester-based insulation than in mineral oil-based insulation. As is explained in [22] the reasons for that lower influence are the transesterification reaction of cellulose, which takes place in presence of an ester, and the hydrolysis reactions in esters.

On the other hand, although dielectric properties (breakdown voltage, partial discharge inception voltage, dielectric dissipation factor, relative permittivity, partial discharges…) have been also studied in some works to compare different commercial oils (mineral oil, natural and synthetic esters), most of these studies has been done without thermal aging [23], so there is still a lack of knowledge about dielectric behavior of biodegradable oils when they have suffered any kind of stress.

C. Estimation of the ageing condition of ester-impregnated solid insulation using oil analysis

The development of new liquids has required new studies of ageing due to the differences in the molecular structure between esters and mineral oil. The ageing mechanisms of the insulation systems and the ageing by-products are different from mineral ones as has been found by different authors [23-25].

The findings of these research works have mixed results in terms of the possibility of using these chemical markers (aging by-products) with ester-based fluids. The chemical markers were measured through either the relevant standard for mineral oil or through in-housed developed methods, due to the unavailability of standard techniques for ester-based fluids. The lack of systematic studies and standardized measurement techniques has slowed down the development of condition-monitoring techniques for ester-filled transformers.

CONCLUSIONS

The use of biodegradable liquids as liquid insulation of transformers has become more common in the last few years. Some properties of these liquids are better than those of mineral oils, such as their fire point and biodegradability. Some other aspects hinder their spread.

In this paper, a revision of the main aspects of ester fluids and their use in transformers is carried out by presenting some experimental data to compare their performance with that of mineral oils.

REFERENCES

[14] R. Girgis, M. Bernesjo and G.K: Frimpong, “Detailed performance of a 50 MVA transformer filled with a natural ester fluid versus mineral oil, CIGRE session Paris, A2-107, 2010

[15] T.-W. Park and S. H. Han, “Numerical analysis of local hot-spot temperatures in transformer windings by using alternative dielectric fluids,” Electr. Eng., Vol. 97, No. 4, pp. 261–268, 2015.

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[22] B. Garcia, T. Garcia, V. Primo, J. C. Burgos, D. Urquiza, “Studying the loss of life of natural-ester-filled transformer insulation: impact of moisture on the aging rate of paper”, IEEE Electrical Insulation Magazine, 33 (1), 15-23, 2017.

[23] U. M. Rao, Y. R. Sood, R. K. Jarial, “Performance analysis of alternate liquid dielectrics for power transformers”, IEEE Transactions on Dielectrics and Electrical Insulation, 23 (4), 2475-2484, 2016.

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