For over 100 years mineral oil has been used as liquid insulation for power and distribution transformers. Transformer liquid insulation accomplishes a double mission acting as dielectric material, providing electrical insulation between regions of the transformer at different voltage levels, and as a cooling agent that dissipates the heat losses generated in the transformer windings to the ambient. Moreover, the insulating fluid impregnates the solid insulation of the transformer improving the dielectric properties of cellulose. It is also used for diagnosis purposes since its periodic sampling and analysis is nowadays regarded as a powerful tool for transformer fault detection. Despite the excellent performance and low cost of mineral oils, a recent increase in the environmental and safety requirements has driven the electricity transmission and distribution industry to look for alternatives to this material. Two types of biodegradable liquids are mainly used nowadays as an alternative to mineral oil: Natural Esters, obtained from different vegetable seeds, and Synthetic Ester Fluids.

Ester fluids have three main advantages with respect to mineral oils:

• Esters are biodegradable materials, and then the environmental risk and impact, and the disposal cost of the liquids are minimized. For this reason, these fluids are used in off-shore wind power transformers, urban substations, transformers operating on ships, and railway traction transformers.
• The flash point of esters is much higher than that of mineral oils and then the fire risk of ester-filled transformers is dramatically lower. This fact makes ester-filled transformers more suitable for areas near to population or to critical infrastructures. It should be considered that fires in transformers cause every year severe accidents around the world with human losses and the destruction of infrastructures.
• Esters are capable to adsorb greater amounts of water than mineral oils allowing the transformer dielectric paper to operate the dryer. It has been proven that this has a positive impact in the loss-of-life curve and in consequence on the maximum admissible hottest spot temperature.

On the other hand, esters have some properties that slow down their expansion as an alternative to mineral oils. The viscosity of esters is higher than that of mineral oil and therefore it circulates worse through the cooling channels of the transformer and the temperatures reached by the transformer would be higher. It has been estimated that ester-filled transformers operate between 1 and 3ºC hotter than similar mineral-oil-based units. Additionally, the oxidation rate of esters is higher, especially in the case of natural esters, which makes them more suitable for sealed units. Finally, natural esters have a high pour point and then they are not suitable for outdoor transformers in cool zones unless certain precautions are taken.

FIELD OF APPLICATION OF ESTER-BASED TRANSFORMERS

The use of ester fluids for transformer insulation purposes was proposed in the 90s and the first ester-filled distribution and power transformers were installed in 1996 and 2003 respectively. Since then, the use of esters has increased sharply, although the number of units operating around the world is still limited at present.

In [1] a study performed by one of the main suppliers of natural esters for electrotechnical purposes is presented which analyses the number of ester-filled transformers operating around the world, classifying them by their power and voltage level. Table 1 summarizes the data provided in that survey distinguishing between new units, which were originally designed to work with an ester as liquid insulation, and retro-filled units. Retro-filled units are transformers that had been originally designed and operated with mineral oil filling their tank and impregnating its solid insulation, in which the insulating fluid is replaced with an ester. As can be seen in Table 1, the number of ester-immersed transformers with high powers and voltages is still very low.

Table 1. Number Of Transformers With Medium To High Voltage And Power Filled Ester Fluids Around The World (2014). Data Taken From [2].

One of the factors that hinder the implantation of esters, especially for bigger units, is that there is still a lack of knowledge on the operation and maintenance practices that must be applied to these activities. Up to date, no IEC (International Electrotechnical Commission) Standards have been published in relation to the maintenance and operation of biodegradable filled transformers. The physical and chemical differences between alternative liquids and mineral oil make compulsory the assessment of these new fluids in comparison with mineral oil before they are widely applied.

REFERANS / REFERENCE
[1]. K. J. Rapp and J. Luksichn., “Application of Natural Ester Insulating Liquids in Power Transformers.” Proceedings of My Transfo 2014, November 2014.