Transformers play a vital role in providing reliable power supply to the consumers. Without the use of transformers, the power supply network cannot be operated as much effectively. Hence the maintenance of power transformers also have become more important. The oil insulation used in the power transformers act as electrical insulating medium as well as a coolant for the transformer. Most of the transformers use mineral oil as liquid insulation due to its extended dielectric properties and withstanding capabilities. It protects the transformer windings from electrical and mechanical stress. However, the oil used in transformer has its own limitations and thereby necessitates the research to improve the characteristics of the transformer insulating oil to serve better.
Factors Affecting The Quality Of The Oil :
The quality of the insulating oil can be determined by considering the factors like heat transfer, breakdown voltage, electric charging ability etc. Heat transfer is the key function to be performed by the transformer oil as the power transformers broadly suffer from thermal heating. The ability of the oil to transfer the heat can be analyzed by studying the variations in the viscosity of the transformer oil in accordance with variations in temperature. The breakdown voltage of the transformer oil decides its withstanding capability. Oil with greater breakdown voltage is desirable. However ageing process tends to decrease the breakdown voltage. The grade of the insulating oil depends on the ingredients present in it. The degradation of the oil takes place due to high temperature, oxidation by exposure to atmospheric and presence of water particles. All these lead to failure of the equipment which in turn create economic loss.
Mineral Oil as Liquid Insulation :
In high rated power transformers, the insulating oil is made to pass through the windings and core of the transformer to transfer the heat and hence ensures coolness around the windings. But this process leads to accumulation of charges around the surface of the oil and creates partial discharge which further cause breakdown of the transformer. In order to eliminate all the above demerits researching and analyzing the transformer oil becomes inevitable and therefore various mixture of oil are tested for efficient operation of transformer. It is suggested that addition of 20% of synthetic Ester with mineral oil provides efficient characteristics as compared with use of mineral oil alone as insulating oil.
Mineral oil is the one which perfectly suits for heat transfer aspect of transformer at normal operating temperatures as it has high viscosity gradient whereas silicon oil has proved to be ineffective for the dissipation of heat. But adding 20% Ester oil does not affect the margin of viscosity index of the mineral oil. While comparing oil mixtures for breakdown voltage it is found that silicon oil stands at the least and the Ester oil have been promoted to have high dielectric strength and breakdown voltage value. The breakdown voltage of mineral oil alone was an average value. Hence proper mixing of 20% of Ester oil will increase the withstanding capability of the transformer oil. The addition of this small amount of Ester oil will protect the oil from getting contaminated by water and other degradation impurities due to their great water-soluble properties.
Problems with Mineral oil:
In the case of oxidation parameters, the silicon oil is found to be extremely suitable as it prevents the formation of acids compounds and decrease the ageing process by a considerable amount. The mineral oil has proved to be more susceptible to the ageing impurities and the Ester oil to some extent. However, the transformer oil cannot be prevented from oxidation by appending silicon oil. The mineral oil has efficient heat transfer properties as compared with other oils but it has the demerits of average breakdown voltage value and more reactive to oxidation process. Each oil has its own limitations and merits. After testing Various oil samples, it is noted that a mixture of mineral oil with 20% Ester oil tends to be more reliable than use of mineral oil alone and hence prove to be cost effective. Mineral oil is not the favorable one when considering the safety aspect as it produces hazardous waste materials after a long period of service and cannot be disposed easily without affecting the environment.
Advantages Of Vegetable oil :
Now a days many researches are performed for using pure vegetable oil as insulating oil in transformer which is eco-friendly. vegetable oil is the naturally occurring esters, oil extracted from seeds like rapeseed etc. It has the potential of high operating voltage and is biodegradable. Unfortunately, if any leakage occurs the mineral oil can cause severe hazards to the environment while the plant extracted vegetable oil will not affect and which is also non toxic when exposed to the surroundings. Also, these oils have desirable flash points and does not react heavily during any fault in the equipment as was the case in mineral oil. It possesses the advantage of high viscosity there by act as an effective coolant and reduces the heat developed by thermal stress. The breakdown voltage value of these vegetable oils is not any way lower than that of mineral oil and is considered to be a compromising factor. Oxidation stability is the factor by which the natural Ester oil grades down as it has the tendency of getting oxidized more easily. During oxidation the oil tends to form the by products such as acids and sludges which will leads to faster ageing of transformer. Hence special care has to be taken while handling the natural vegetable oils by tightly sealing the device from exposure to atmospheric conditions.