ABSTRACT

 This work investigates and evaluates local liquid dielectric for insulation of power transformers. Considering the importance of transformer insulation the investigation is borne out of the need to develop the capacity for local production of transformer dielectric. Natural esters of vegetables origin have been found to have the suitable dielectric properties for them to be used as replacement dielectric fluids for mineral oil. However vegetable oil has poor resistance to oxidation, hence the need for inhibiting the vegetable oil for use as insulation fluid. In this work inhibited locally extracted soybean oil (soybean oil treated with antioxidant) is subjected to accelerated ageing in order to determine its suitability as dielectric fluid. The properties of the soybean oil is investigated against standard and compared with those of mineral oil. The investigated soybean meets acceptability specifications since the measured flash point is 257⁰C, the pour point is -15⁰C, the acidity is low at 0.0027mgkoH/mg, its dissipation factor after ageing is 12.11 x 10^-3 (which is within recommended units) and showed a high dielectric breakdown voltage (59.08kV). Results obtained showed that the inhabited soybean oil has properties comparable to those of mineral oil and is suitable for use as transformer dielectric fluid.

CHAPTER ONE INTRODUCTION

1.0. Background of the Study

 

Electrical faults that occur in power transfers accounts for over fifty percent of transformer failure’s expenditures . Fifty percent of transformer failure’s expenditure is caused by insulation, dielectric, and oil-related faults [1]. The need for better dielectrics and transformer oils for insulation is unequivocal. In a liquid-filter transformer, the insulator liquid plays an important function by providing both the electrical insulation and the means of transforming the thermal losses to the cooling system. Insulating oil in a transformer must ensure the transfer of heat. This function is realized both by thermal conductivity and convection [2].

Nowadays, transformers can be filled with three basic types of insulating liquids: (i) mineral oils,

• synthetic oils or (iii) natural esters [3]. The use of each type is fortified by the

However, in the face of increasing demand for the use of environmentally friendly products in the industry, more and more companies are working towards developing the use of esters and specifically natural esters for use in the majority of their products.

Transformers have been filled with mineral oil for more than one century. This type of oil is a petroleum based product, essentially composed of hydrogen and carbon atoms. Carbon and hydrogen are assembled in different structures: napthenic (C_nH_2n), paraffinic (C_nH_2n+2) and aromatic (C_nH_2n-6)[3]. The distribution of carbons in napthenic and paraffinic structures define the type of mineral oil. This distribution is controlled by the crude oil and the refining processes used. Because of its wide availability, good properties and low cost, mineral oil is the fluid most used in the electric power transformer industry. New mineral oils have to be in accordance with the IEC 60296 or ASTM D3487. As mineral oil has been used for such a long time, a large data base of information is available to enable interpretation of changes to its characteristics and thereby predict the possible malfunction of a transformer filled with it. IEC 60422 is a good tool to evaluate the quality of insulating oils in operational transformers.

There are two main synthetic oils that can be used in transformers: silicone oil and ester of pentaerythritol (Synthetic ester). These oils were developed in the 1970’s to replace askarels (Polychlorinated Biphenyls) (PCB), which became outlawed because of their toxicity [3]. As PCB was a non-flammable liquid (no fire point), the main peculiarity of these new oils was their high fire resistance (higher fire point than mineral oil). Until now, their use was essentially restricted to distribution and traction transformers, partly because of their price (three to eight times more expensive than mineral oil) but also because a better fire resistance was required for these applications. Their better thermal stability was also a positive point for their use and especially for traction transformers. Esters of pentaerythritol also called tetra-ester are obtained in an esterification reaction between a tetra alcohol (pentaerythritol) and mono-carboxylic acids. These oils are composed of hydrogen, carbon and oxygen. New synthetic esters have to be in accordance with the IEC 61099 and a maintenance guide that is published in the IEC 61203. Silicon oil is a polymer based on silicon, which also includes carbon, oxygen and hydrogen atoms. Specifically, the final product is obtained by the polymerization of polydimethylsiloxane (PDMS). This oil presents the advantage to have a high thermal stability but on the other hand is not biodegradable at all.

The third insulating liquid is vegetable oils also known as natural esters (tri-ester) as opposed to synthetic esters. This work is channeled towards the investigation and evaluation of these natural esters as a transformer dielectric fluid. These oils are naturally synthesized from living organisms and come in particular from soybean, palm, groundnuts, sun flower, rape seed etc. Specifically, natural esters are created from an esterification reaction between a tri-alcohol and fatty acids. Other processes allow the final product to be obtained by the trans-esterification reaction (mono- ester) or mixture of mono and tri-esters [4].

On the other hand, the application of vegetable oils and animal fats for industrial purposes and especially application has been exploited. Inherent disadvantages and the availability of inexpensive options have however brought about low utilization of vegetable oils for industrial lubrication[5]. When applied in the science of tribology, vegetable oils fall order the class known as fixed oils[6]. They are so named because they do not volatilize without decomposing. Natural ester (vegetable oil) dielectric fluids have advantages over mineral oil in terms of fire safety, environmental risk, and thermal performance [7]. These high fire points fluids are self- extinguishing (non-propagating) and quality in many applications as an equivalent safeguard to space separation, fire barriers, and extinguishment systems, all of which mitigate a fire but do not prevent it. The thermal characteristics and interactions with cellulose insulation (i.e. transformer paper insulation) give longer transformer paper insulation life or allow higher or extended overheads without abnormal loss of insulation life.

However the major drawback in the application of vegetable oils as transformer oil or for industry uses in general is the fact that in their natural forms, they lack sufficient oxidative stability. By definition, the oxidative stability of oil is a measure of the length of time taken for oxidation, deterioration to commence on a general level, the rates of reactions in auto-oxidation schemes are dependent on the hydro carbon structure, oxygen concentration, and temperature  [8]. The untreated (i.e. uninhibited) oils from vegetable origin oxidize during use and polymerize to a plastic life consistency [5]. Even when they are not subjected to the intense conditions of industrial applications, fats and oils are liable to rancidity [9, 10]. Oil oxidation is an undesirable series of chemical reactions involving oxygen that degrades the quality of the oil. Oxidation is not one single reaction, but a complex series of reactions. In general terms, oxidative rancidities in oil occurs when heat, metals or other catalysts cause unsaturated oil molecules to convert to free radicals. These free radicals are easily oxidized to yield hydroperoxides and organic compounds, such as aldehydes, ketones, or acids which give rise to the undesirable odors and flavor characteristic of rancid fats [9]. The oxidative rancidity eventually affects the transformer it reduces its thermal efficiency and weakens its dielectric strength.

Combating the issue of oxidative instability in vegetable oils for industrial use is a continuing research area [11]. Three avenues are being pursued. These are:

1. Genetic modifications of oils to give higher mono unsaturated compounds

 

1. Chemical medication

 

• The use of antioxidants (i.e. additives) and property enhancers[12]

 

Genetic modification has been made possible by recent advances in biotechnology. DuPont technology has developed a soybean seed that presents 83% oleic acid as against having the  more unsaturated linolenic acid as the major constituent. This new seed provides oils that show about 30 times the oxidative stability and viscosity stability of the conventional oil.

Chemical modifications involve the partial hydrogenation of the vegetable oil and a shifting of its fatty acids.

The use of additives known as antioxidants to control the development of oxidative rancidity has been applied in the US since 1947[13]. They still remain one of the most efficient and cost effective ways to improve the oxidative stability of oils in both domestic and industrial conditions. Hence the use of antioxidants to combat oxidative rancidity is adopted in this work.

 

 

This work investigates and evaluates local liquid dielectric for power transformer insulation. Considering the issue of local availability and renewability, in this study the use of natural esters is considered in place of local mineral liquid dielectric for power transformer insulation.

1.1.   Statement of the Problem

 

This dissertation focuses on the evaluation of locally extracted soybean oil (a natural ester) for the production of liquid dielectric for power transformer insulation. However, whether applied for lubrication purposes or as transformer dielectric, one of the major challenges in the utilization of the more environmentally friendly soy bean oil is its poor oxidative stability. Hence the main problem to be addressed by this research is the setup and measurement of the dielectric losses and breakdown voltage of soybean oil under accelerated thermal ageing.

1.2.   Aim/Objectives of the Study

 

The aim of this study is to investigate and evaluate local liquid dielectric for power transformer insulation. To this end, this work will realize the following specific objectives:

1. To obtain a dielectric fluid for use in the insulation of power transformers by the oxidative inhibition of locally extracted soybean
2. To setup and carryout accelerated aging as per the IEC 61125 standard in which the dielectric losses is carried out.

• To increase the inhibited soybean oil breakdown voltage. Evaluation based on the IEC 60156 standard employing the 2.5mm gap. Furthermore other properties of the inhibited soybean oil are measured against the IEC 60247 for possible use as a dielectric fluid for power transformers.

1.3.   Scope of the Study

 

This work covers the evaluation of soy bean oil as a transformer liquid dielectric. The chemical procedures covered include antioxidation. Test covered include acidity test, viscosity test,

 

 

dielectric withstand test, flash point and pour point test. However the test does not include the use of the popular Dissolved Gas Analysis (DGA) for the evaluation of the liquid dielectric for partial discharge.

1.4.   Significance of the Study

Transformers are important component or equipment in power systems. Huge amount of money is spent every year on maintenance of transformer. This has big impact on the electrical industry and on the economy of the nation, since fifty percent of the transformer failure expenditure are said to be caused by insulation, dielectric and oil related faults. This project which is focused on investigation and evaluation of local liquid dielectric for power transformer insulation then has significance for the Nigerian electricity industry and to the country’s economy.

There has been the push by international community to erect legislation for the safety of the environment. The issue relating to the use of more environmentally friendly substances that are biodegradable and pose little or no negative impact to the environment is becoming a strong one. This is reinforced by the gathering of (this year, 30th November 2015) world leaders for the COP 21 conference in France. Hence, a study such as this that focused on exploring more environmentally friendly alternative to mineral oil for transformer becomes significant. The significance of using the soybean oil in transformer insulation are as follows:

1. Nigerian farmers will benefit from this study and will concentrate more on soybean oil production.
2. With respect to the use of soybean oil as alternative to mineral oil, the cost of running the transfers in Nigeria power system will be reduced since it is cheaper and locally available than the conventional oil. Thus, it will reduce the huge amount spent in running and maintaining power stations especially in developing country like Nigeria.
3. Due to the biodegradability nature of soybean oil, it will reduce the hazard posed by the conventional oil to the environment during oil linkage or
4. It offers a lower transformer circle cost and no hazardous-waste disposal cost required, it also has a potential for recycling value of used soybean
5. It is safer than mineral oil because of its higher flash point (ignition temperature) and reduces the impact of transformer
6. Its performance slow down the ageing of rate of insulation

 

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