Enhancing the Critical Characteristics of Natural Esters with Antioxidants for Power Transformer Applications

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 20, No. 3; June 2013 899

1070-9878/13/$25.00 © 2013 IEEE

Enhancing the Critical Characteristics of Natural Esters with Antioxidants for Power Transformer Applications

A. Raymon1, P. Samuel Pakianathan2, M. P. E. Rajamani3 and R. Karthik4

1, 2, 3Department of EEE, National Engineering College, Kovilpatti, Tamilnadu, India 628 503 4Liquid Dielectrics Laboratory, Department of EEE, National Engineering College, Kovilpatti, Tamilnadu, India 628 503

ABSTRACT The energy demand of the whole world is dominated by petroleum products for centuries in the field of critical application like electricity. The application of mineral oil in power system equipment like transformer is potentially hazardous to environment. The exploitation of petroleum oil is running out of demand and in the near future, oil scarcity exists. This work focuses in identifying alternate dielectric for replacing traditional mineral oil. India is a tropical country, which produces vegetable oils like sunflower oil, palm oil, coconut oil, mustard oil, corn oil, soybean oil and rice bran oil profusely and it can be used as dielectric fluids in transformer. To be more proficient in the operation of transformer, these vegetable oils should be converted in to superior dielectric fluids. This work also embraces the individual and combinatorial effect of selected natural and synthetic antioxidants in natural ester. For investigation sunflower oil, rice bran oil, soybean oil and corn oil are chosen. The critical parameters like breakdown voltage, viscosity, flash point and fire point are calculated according to IEC and ASTM standards before and after addition of antioxidants with vegetable oils. Astoundingly it is found that the use of natural and synthetic antioxidants can enhance the critical parameters of natural ester for transformer application and the antioxidant activity in the natural ester is discussed. The interpolation functions are calculated based on the results obtained in minimum and maximum concentration of antioxidant added. These functions are used to predict the intermediate output variations caused for each antioxidant concentrations (in gram).

Index Terms — Power transformers, natural esters, antioxidants, green fluids, interpolation functions. 1 INTRODUCTION

THE first oil-cooled or oil-insulated transformer was constructed by Brown of Germany in the year 1890 [1]. Almost every field transformer becomes a vital part of power system [2]. The critical part of a modern power system is the transformer life management and the transformer life primarily depends on the insulating mediums used. The basic insulations used in transformer system are categorized into solid, liquid and gas. Among them solid (press board, kraft paper) and liquid (mineral oil) insulations are indispensably used. The primary functions of insulation are mechanical support between windings and removal of thermal stress by cooling. The principle of operation and the oil-filled technology had not been changed for years.

Cooling of transformers is implied more than a century by mineral oil derived from crude petroleum. Such oils possess high dielectric strength, but poor in flash point and fire point characteristics. Moreover the major limitations of mineral oil

are low biodegradation and panic threats to human beings and ecosystem [3]. Due to their poor performance at high temperature the use of mineral oil is restricted in ecologically sensitive locations (indoor type applications). For many special applications, synthetic liquids like silicone and perchloroethylene are used in transformer. The characteristics of these oils are different; hence for in-service units this direct substitution is not possible. Due to their very high cost, the use of such oil is limited to special transformer application only [4]. The increased number of transformers from world war-II use of mineral oil as insulating fluid and this proficient use makes diminution of petroleum resource. Furthermore, the increased number of accidents including transformer explosion, hazardous disposal and oil spill make descending statements in the use of transformer oil. The conventional mineral oil disposal method like incineration leads to the formation of by-products, commonly known as dioxin [5].

However the reported availability of natural petroleum resource and demand forecast cannot maintain the equilibrium condition. In addition to the energy demand, future expansion of transformer’s capacity and superior Manuscript received on 19 February 2013, in final form 17 April 2013.

900 A. Raymon et al.: Enhancing the Critical Characteristics of Natural Esters with Antioxidants for Power Transformer Applications

design [6] deserves the need for alternative insulating fluid with non fossil origin [7]. Since 1880 vegetable oils are experimented and the use of natural ester came into existence in 1990’s due to their significant properties better than traditional mineral oil, silicone oil [8] and synthetic ester [9]. In recent years, the advent of clean and functional technologies is of high demand to use natural ester as eco-friendly installations. The newest versions of natural esters are derived by compressing plants and animal tissues. Such natural ester report properties like slower ageing rate, less total operating cost (initial cost and operation cost), recyclability, and low noise level. Moreover vegetable oil could be an appropriated response to environmental, safety and health problems, and could reduce the exploitation and end-life costs of transformers.

The natural esters like sunflower oil, coconut oil and soy bean oil are used successfully in transformer systems [10, 11]. Even though coconut oil accounts low conductivity at higher temperature [12], it is not used for many reasons. Apart from the advantages in using vegetable oil as transformer fluids, it holds few disadvantages such as higher cost than mineral oil, high viscosity, poor oxidative stability [13] and poor dielectric constants at high temperature [14]. The examined study of vegetable oil in distribution transformer shows high thermal rise compared to mineral oil, but it did not surpass the international standards [15]. The shortcoming of vegetable oils in the application of industry is due to their natural forms (lack oxidative stability).

The performance of natural ester depends on the composition of fatty acids present in oil. The natural esters like sunflower, soybean oil, rape seed oil and corn oil contain high levels of such element [16]. The decrease in oxidative stability and shelf life extension is due to the removal of antioxidants [17, 18]. The work also evaluates the effectiveness of less toxic new antioxidants derived from natural extracts. Several synthetic antioxidants which act as oxidative inhibitors are investigated. The study reveals that the performance of antioxidants depends on the natural ester and its production technology [19, 20]. The study conducted in soybean oil reveal that, 0.7 wt. % and 0.8 wt. % of synthetic antioxidants like BHA (Butylated Hydroxy Anisole), BHT (Butylated Hydroxy Toluene) and TBHQ (Tert Butyl Hydroxy Quinone) can increase the induction time of oil and much relatively increases oil stability [21].

Addition of synthetic antioxidants and phenol derivative can increase the oxidative stability of oil in the presence of metal contaminants [22]. The application of antioxidant in static and dynamic conditions is tested and improvement in oxidative stability is observed. α-Tocopherol shows better oxidative stability at higher composition than BHA (Butylated Hydroxy Anisole), BHT (Butylated Hydroxy Toluene) and TBHQ (Tert Butyl Hydroxy Quinone), whereas at lower composition PG (Propyl Gallate) and BHA (Butylated Hydroxy Anisole) shows better oxidative stability [23]. In order to improve the performance of natural ester as alternative to transformer systems, chemical alteration is required to begin green fluid revolution. The natural and

synthetic antioxidants are used to expand the performance of next generation safer, environmentally friendly transformer fluids.

To achieve the above requirement, a new form of dielectric with environmentally acceptable standards should be developed. The natural esters being abundantly available are chosen carefully for investigation. The natural esters are selected based on three environmental factors; they are feasibility of resource, less tendency to release toxic gases and biodegradability. The formulation of natural ester by food grade additives improves the performance of oil which surely paves the way for green fluid transformation in high voltage applications.

2 CURRENT EFFORTS In account of the problems stated above, there is a need to

derive an alternative fluid with superior critical properties or transform the natural ester into high performance green fluids. For many decades the vegetable oils are an accent being used in food applications. But the gaining importance of using vegetable oil in transformer industry and focus laid to improve the properties of biodegradable oil is answerable. The use of high performance food additives like BHT is used in mineral oil at 0.2 % weight to ensure oxidative stability. Such food grade additives, in other name called as antioxidants have gained importance in this research work. The vegetable oils obtained from production industries are treated with high performance additives like natural and synthetic antioxidants to guarantee their performance more than traditional mineral oil and equivalent to natural ester standards.

3 ESTER INSULATING FLUIDS

The molecular structure of oil and fat is common and both belong to the biological substance called lipid. Fats are solid at room temperature and whereas oils are liquid at room temperature. In general, fats and oils are esters of the tri-alcohol and glycerol (or glycerine). Therefore, fats and oils are commonly called triglycerides (commonly called as triacylglycerols) [24].

3.1 NATURAL ESTERS

Vegetable oils possess a triglyceride structure. The functional classification of natural ester is categorized in to four types; they are saturated, single unsaturated, double unsaturated and triple unsaturated. The performance of natural ester depends on the composition of fatty acids present in vegetable oil. It determines the critical characteristics of natural ester under static and dynamic loading.

For oils with saturated fatty acid exhibit higher viscosity since they are of chemically stable, oils with triple-unsaturated fatty acids exhibit lower viscosity due to their unstable property [24]. Oils like sunflower oil, rice bran oil, soybean oil and corn oil are plentiful produced in India, which are 100 % environmental friendly.


These oils are used in transformers worldwide for

special purposes and left out for certain reasons. Oxidative instability increases with the number of unsaturated linkages (double bonds). The oxidation stability of natural esters can be improved by adding suitable antioxidants. Mineral oil oxidation inhibitor like DBPC is used in the range of 0.25 to 0.35 % [25], which acts as quenchers of free radicals that initiate chain oxidative reaction. The critical characteristics of vegetable oil are superior to mineral oil except in viscosity. The viscosity of vegetable oil is three to four times higher than mineral oil.

3.2 SYNTHETIC ESTERS

Synthetic ester is formed by combining acid and alcohol. The resultant polyol structure is bonded with several acid and alcohol groups giving them a very stable structure [26]. The critical characteristics of synthetic ester are attributed due to its high performance, oxidative stability and significantly more biodegradable than mineral oil. Due to their higher cost, its application is limited in fields of traction transformers in railways and modulators.

4 COMPOUND DESCRIPTION

4.1 ANTIOXIDANTS

In general antioxidants are compounds that delays or slows down oxidation process [27]. Antioxidants interferes the function of chain reaction involved in oils. By eliminating or keeping the amount of free radicals to minimum quantity, it can avoid oxidation of oil and peroxide formation. The main functional properties of antioxidant in oil are free electron scavenging, metal chelating and synergism [28, 29]. At normal temperature the performance of antioxidant is vivid and one of the important concerns of using antioxidant at high temperature is its percentage diminishes as it was before [30, 31]. Antioxidants are frequently used in combinatorial modes in two or three compositions for being advantageous of their parallel mechanisms.

Based on the source of existence and functional mechanism, the antioxidants are categorized into three types they are natural antioxidants, secondary antioxidants and synergists. The different types of antioxidants related to their occurrence are displayed in Table 1. All antioxidants used in this work have points of potency and limitation.

Table 1. Types of antioxidants.

SYNTHETIC ANTIOXIDANT

Butylated Hydroxy Toluene

Butylated Hydroxy Anisole

Tert Butyl Hydro

Quinone

Propyl Gallate

Pyrogallol Lauryl

Tert Butyl Hydro Quinone

2,4,5- trihydroxybutyrophenone

NATURAL ANTIOXIDANT

VITAMIN-E (TOCOPHEROL) VITAMIN-E (TOCOTRIENOL)

alpha (α) beta (β) gamma (γ) delta (δ) alpha (α) beta (β) gamma (γ) delta (δ)

SYNERGISTS Ascorbic

acid Carsonic

acid Citric acid

Phosphoric acid

Ethylene Diamene

tetra-acetic acidCarotenes Oryzanol Rosemary Extracts

Table 2. Description of antioxidants.

Origin Class Abbreviated as

Chemical name and formula Function

Natural

Primary α-T α-Tocopherol

C31H52O3 Quenchers of singlet oxygen/

Synergist

Secondary CA Citric Acid (anhydrous)

C6H8O7 Singlet oxygen scavengers/

Synergist

Secondary AA L-Ascorbic acid

C6H8O6 Oxygen Scavengers /

Synergist

Synthetic

Primary BHA Butylated Hydroxy Anisole

C11H16O2 Free radical scavengers

Primary BHT Butylated Hydroxy Toluene

C15H24O Free radical scavengers

Primary PG Propyl Gallate

C10H12O5 Free radical scavengers /

Metal chelators


Therefore, certain points like effective concentration, and synergism, are taken into consideration when selecting antioxidants. The Table 2 gives details only about the selected antioxidants under careful study. Among the synthetic, natural and synergists categories, the chosen antioxidants are used for preparing samples. The class, origin and functional mechanisms are given in Table 2.

5 SAMPLE DESCRIPTIONS

5.1 PROPERTIES OF BASE FLUIDS (SUNFLOWER OIL, RICE BRAN OIL, SOYBEAN OIL AND CORN

OIL)

The vegetable oils like sunflower oil, rice bran oil, soybean oil and corn oil are used for investigation. These refined oils are purchased from domestic oil companies in required quantity. The oil thus obtained is heated to a temperature of 100 OC to remove moisture. Then the oil is brought to room temperature and critical characteristics are measured according to standards, which are displayed in Table 3. The measurement shows that, further enhancement is required for the optimized characteristics of oil.

5.2 GREEN FLUIDS

The natural ester fluids (base fluids 1, 2, 3 and 4) are mixed with high performance natural and synthetic antioxidants. For measuring the efficiency of antioxidants in base oils, two compositions are used, one in 1 g composition (lower order) and other in 5 g (higher order). For samples involving multiple antioxidants mixture, 0.5 g (1:1 ratio) is taken as

minimum range and 1 g (1:1 ratio) is taken as maximum range. The critical parameters of the samples are measured as per standards.

Therefore the breakdown voltages of the samples are measured at room temperature and elevated temperature (at 70 OC). The measurements before and after adding antioxidants are compared using recorded data, the interpretation of enhancement and possible mechanisms and individual properties of antioxidants are discussed.

5.3 METHODOLOGY

A 500 ml of oil is taken in a conical flash and heated using heating chamber to a temperature required to dissolve the antioxidant (from Table 2). Then the different quantity of antioxidant in individual and combinations are added to the heated oil.

Table 3. Critical properties of base fluids.

Critical Parameters

Base Fluid 1

Base Fluid 2

Base Fluid3

Base Fluid 4

Sunflower Oil

Rice Bran Oil

Soya bean Oil

Corn Oil

Breakdown Voltage (kV)

34 39 27 32

Flash Point (OC) 260 260 310 300

Fire Point (OC) 270 280 320 310

Viscosity (cSt) 132 154 140 134

Table 4. Samples prepared from base fluids and antioxidants.

Samples prepared using base fluid 1 Samples prepared using base fluid 3 Sample 1 Base Fluid 1 + 1 g AA Sample 25 Base Fluid 3 + 1 g AA Sample 2 Base Fluid 1 + 5 g AA Sample 26 Base Fluid 3 + 5 g AA Sample 3 Base Fluid 1 + 1 g BHA Sample 27 Base Fluid 3 + 1 g BHA Sample 4 Base Fluid 1 +5 g BHA Sample 28 Base Fluid 3 +5 g BHA Sample 5 Base Fluid 1 + 1 g BHT Sample 29 Base Fluid 3 + 1 g BHT Sample 6 Base Fluid 1 + 5 g BHT Sample 30 Base Fluid 3 + 5 g BHT Sample 7 Base Fluid 1 + 0.5 g of BHT + 0.5 g of CA Sample 31 Base Fluid 3 + 0.5 g of BHT + 0.5 g of CA Sample 8 Base Fluid 1 + 1 g of BHT + 1 g of CA Sample 32 Base Fluid 3 + 1 g of BHT + 1 g of CA Sample 9 Base Fluid 1 + 0.5 g of α-T + 0.5 g of CA Sample 33 Base Fluid 3 + 0.5 g of α-T + 0.5 g of CA

Sample 10 Base Fluid 1 + 1 g of α-T + 1 g of CA Sample 34 Base Fluid 3 + 1 g of α-T + 1 g of CA

Sample 11 Base Fluid 1 + 1 g of PG Sample 35 Base Fluid 3 + 1 g of PG Sample 12 Base Fluid 1+ 5 g of PG Sample 36 Base Fluid 3 + 5 g of PG

Samples prepared using base fluid 2 Samples prepared using base fluid 4

Sample 13 Base Fluid 2 + 1 g AA Sample 37 Base Fluid 4 + 1 g AA

Sample 14 Base Fluid 2 + 5 g AA Sample 38 Base Fluid 4 + 5 g AA

Sample 15 Base Fluid 2+ 1 g BHA Sample 39 Base Fluid 4 + 1 g BHA

Sample 16 Base Fluid 2 +5 g BHA Sample 40 Base Fluid 4 +5 g BHA

Sample 17 Base Fluid 2 + 1 g BHT Sample 41 Base Fluid 4 + 1 g BHT

Sample 18 Base Fluid 2 + 5 g BHT Sample 42 Base Fluid 4 + 5 g BHT

Sample 19 Base Fluid 2 + 0.5 g of BHT + 0.5 g of CA Sample 43 Base Fluid 4 + 0.5 g of BHT + 0.5 g of CA

Sample 20 Base Fluid 2 + 1 g of BHT + 1 g of CA Sample 44 Base Fluid 4 + 1 g of BHT + 1 g of CA

Sample 21 Base Fluid 2 + 0.5 g of α-T + 0.5 g of CA Sample 45 Base Fluid 4 + 0.5 g of α-T + 0.5 g of CA

Sample 22 Base Fluid 2 + 1 g of α-T + 1 g of CA Sample 46 Base Fluid 4 + 1 g of α-T + 1 g of CA

Sample 23 Base Fluid 2 + 1 g of PG Sample 47 Base Fluid 4 + 1 g of PG

Sample 24 Base Fluid 2 + 5 g of PG Sample 48 Base Fluid 4 + 5 g of PG


The temperature of the oil is maintained to the melting point of antioxidant until the solute dissolves completely in the solvent. The conical flask is transferred into the magnetic stirred unit and 750 to 800 rotations per minute (rpm) is maintained for 20 minutes. Then the oil is transferred to storage vessel for measurements.

5.4 PREPARATION OF SAMPLES

For the investigation refined oils such as sunflower oil is taken as base fluid 1, rice bran oil as base fluid 2, soya bean oil as base fluid 3 and corn oil as base fluid 4. The samples are prepared by adding different proportion of antioxidant into base fluids and mixed using magnetic stirrer unit. The temperature of the oil and speed of the stirrer is maintained constant throughout the mixing process. The samples prepared using the above process is given in Table 4 and its snapshots are displayed in Figures 1, 2, 3 and 4.

6 SAMPLE DESCRIPTIONS 6.1 BREAKDOWN VOLTAGE MEASUREMENT

The breakdown voltage of the samples are measured using breakdown voltage test kit with measuring capacity of 60 kV as recommended by IEC 60156. The test kit contains two spherical electrodes of standard diameter and inter-spacing of 2.5 mm. The oil is filled in the test cup to a height of 40 mm above the surface of electrode. Application of voltage is

started at least five minutes after pouring the oil. The test voltage is varied linearly at the rate of 2 kV/s using the control knob provided in the test kit. Five successive measurements of breakdown voltages are taken by giving time delay of one minute between each measurement. The time delay is given in order to disperse the byproducts to expel before next consecutive measurements are conducted [32]. The average of five values is taken as breakdown voltage of the sample [33]. The four factors that determine the breakdown voltage of the oil are moisture, air bubbles, suspended solid particles and fluid acidity [34, 35].

6.2 FLASH POINT AND FIRE POINT MEASUREMENT

The flash points of the samples are measured using Pensky Martin Flash point apparatus at room temperature and pressure as recommended by ASTM D 93 [36]. The Pensky Martin Flash point apparatus contains a closed brass test cup where the oil sample is filled in test cup and the temperature of oil sample is amplified by energy regulator. The flash point is identified by introducing a test flame in the opening provided on the surface. The state at which the vapour thus formed inside the test cup mixes with air to kindle a temporary fire on the oil surface less than one second. Similarly, the fire point temperature is marked during continual fire on the oil’s surface; when a small test flame is directed to the sample [34, 35].

6.3 VISCOSITY MEASUREMENT

The viscosities of the samples are measured using Redwood Viscometer at room temperature and pressure as recommended by ASTM D 445 [37]. The viscometer contains silver plated oil cup with opening called orifice of standard diameter. A quantity of 50 ml oil sample is filled in the test cup and by opening the orifice the time required for collecting the sample is noted to find the kinematic viscosity of the sample. The factors that determine the viscosity of the sample are temperature and fluid resistance offered by the oil [34, 35].

7 RESULTS AND DISCUSSION

7.1 BREAKDOWN VOLTAGE

The breakdown strength of all the samples recorded in Table 5 shows, the antioxidant potential to enhance natural esters. The base fluids 1 and 2 are highly unsaturated, and base fluid 3 is moderately unsaturated and base fluid 4 is less unsaturated. The breakdown strength of natural ester depends upon the nature of antioxidant used and the unsaturated elements present in base fluid. Antioxidants like Ascorbic Acid (AA), Butylated Hydroxyl Anisole (BHA), Butylated Hydroxyl Toluene (BHT), Citric Acid (CA), α–Tocopherol (αT) and Propyl Gallate (PG) are investigated at 1 g and 5 g concentrations with base fluids 1, 2, 3 and 4. From the results of samples prepared out of base fluids and antioxidant, it is inferred that, the base fluids show good dielectric strength after transformation. The enhancement percentage as given in

Figure 1. Samples prepared using base fluid 1.





Table 6 shows the individual and combinatorial effect of antioxidants used.

The recorded breakdown voltages of base fluids at room temperature (RTP), base fluid 1 and 2 show decrement in breakdown strength with increasing concentration. The individual concentration effect of antioxidants added with base fluids are publicized in Figures 5-10.

At elevated temperature (70 OC), the performances of base fluids are excellent except for the base fluid 1 with L-Ascorbic Acid, Butylated Hydroxy Anisole and base fluid 2 with Propyl Gallate.

In addition to that, the natural esters treated with antioxidants regain its dielectric property quick after breakdown.

The formation of carbon is less in natural esters before and after treatment with antioxidants. More concisely, the reported less carbon formation during breakdown enables less tendency to gas formation.

The investigation airs, the natural esters transformed using antioxidants would be an appropriate choice to power transformer applications.

The percentage enhancement of breakdown voltage for various antioxidants is illustrated in Table 6.

Table 5. Breakdown voltage of base fluid 1, 2, 3 and 4 treated with antioxidants.

Antioxidant Added quantity

in gram

Base fluid 1 Base fluid 2 Base fluid 3 Base fluid 4

Breakdown voltage (kV) Breakdown voltage (kV) Breakdown voltage (kV) Breakdown voltage (kV)

AT RTP

AT 70OC

AT RTP

AT 70OC

AT RTP

AT 70OC

AT RTP

AT 70OC

AA 1 42 45 46 60 42 47 40 53 5 20 40 40 46 53 55 50 60

BHA 1 33 48 44 39 40 45 45 50 5 42 52 55 40 42 55 50 60

BHT 1 34 52 44 40 39 48 50 55 5 44 47 46 50 38 40 50 55

BHT + CA 0.5:0.5 38 44 45 57 39 43 43 45

1:1 45 48 38 44 40 50 49 51

α-T + CA 0.5:0.5 54 60 39 48 37 52 47 60

1:1 49 60 48 55 38 55 48 55

PG 1 51 60 32 41 40 60 55 55 5 54 60 25 35 43 60 55 60

Table 6. Percentage Enhancement in Breakdown Voltage of base fluid 1, 2, 3 and 4 with antioxidants.


in gram


Breakdown voltage (kV) Breakdown voltage (kV)Breakdown voltage

(kV) Breakdown voltage (kV)

AT

RT

P

%

EN

HA

NC

ED

AT

RT

P

%

EN

HA

NC

ED

AT

RT

P

%

EN

HA

NC

ED

AT

RT

P

%

EN

HA

NC

ED

AA 1 42 24 46 18 42 56 40 25 5 20 -41 40 3 53 96 50 56

BHA 1 33 -3 44 13 40 48 45 41 5 42 24 55 41 42 56 50 56

BHT 1 34 0 44 13 39 44 50 56 5 44 29 46 18 38 41 50 56

BHT + CA 0.5:0.5 38 12 45 15 39 44 43 34

1:1 45 32 38 -3 40 48 49 53

α-T + CA 0.5:0.5 54 59 39 0 37 37 47 47

1:1 49 44 48 23 38 41 48 50

PG 1 51 50 32 -18 40 48 55 72 5 54 59 25 -36 43 59 55 72


Figure 5. Breakdown voltage characteristics of base fluid 1, 2, 3 and 4 with Ascorbic acid.

Figure 6. Breakdown voltage characteristics of base fluid 1, 2, 3 and 4 with Butylated Hydroxy Anisole.

Figure 7. Breakdown voltage characteristics of base fluid 1, 2, 3 and 4 with Butylated Hydroxy Toluene.

Figure 8. Breakdown voltage characteristics of base fluid 1, 2, 3 and 4 with Butylated Hydroxy Toluene and citric acid anhydrous.

Figure 9. Breakdown voltage characteristics of base fluid 1, 2, 3 and 4 with α –Tocopherol and citric acid anhydrous.

Figure 10. Breakdown voltage characteristics of base fluid 1, 2, 3 and 4 with Propyl Gallate.


7.2 FLASH POINT AND FIRE POINT

The enhancement percentage of flash points and fire points of transformed base fluids are given in Tables 8 and 9. The flash point and fire point characteristics of base fluids treated with antioxidants are publicized in Figures 11 and 12. The investigation infers the thermal stability of antioxidants is high even at higher temperature; the reduction in flash point and fire point are recorded for base fluid 2, 3 and 4 added with BHT, AT + CA and PG which are given in Table 7. Though this very less percentage reduction did not affects the dielectric performances of base fluids. All the antioxidant helps to protect the oil from the effect of heat and oxygen during heating. Further the antioxidant reacts with free radicals and retards the formation of peroxides, which are prone to chain oxidation. The potency of antioxidants to protect oil depends on the amount of fatty acids present in base fluids. Upon careful investigation, the following problems are identified in samples prepared from base fluids and antioxidants at elevated temperature.

Various inferences that obtained during measuring flash

point and fire point are of listed below

Samples prepared with base fluid 2, 3 and 4 and BHT is thermally unstable with increasing concentration due to higher rate of ignition mixtures formation during experiment. This enables the fluid get ignited on external fire source and readily decreases the thermal characteristics.

Samples prepared with base fluid 3 and CA is thermally unstable with increasing concentration. Because CA reacted with base fluid 3 shows higher rate of ignition mixture formation compared to base fluids 1, 2 and 4 during experiment.

Samples prepared out of base fluid 1 and antioxidants readily shows good enhancement due to the rich natural tocopherol content. This presence can increase the thermal stability of fluid during operation.

Table 7. Flash point and fire point of base fluid 1, 2, 3 and 4 treated with antioxidants.


in gram


Flash Point (OC)

Fire Point (OC)

Flash Point (OC)

Fire Point (OC)

Flash Point (OC)

Fire Point (OC)

Flash Point (OC)

Fire Point (OC)

AA 1 270 295 290 300 310 320 310 320 5 280 320 287 295 300 320 310 320

BHA 1 275 280 270 280 330 350 310 320 5 280 290 265 275 300 310 315 325

BHT 1 280 300 265 275 310 325 310 320 5 270 285 250 260 270 280 290 300

BHT + CA 0.5:0.5 260 300 265 275 310 320 310 315

1:1 270 295 270 280 300 320 315 320

α-T + CA 0.5:0.5 280 295 270 285 280 290 310 315

1:1 290 300 270 280 280 290 300 310

PG 1 275 285 270 290 300 310 315 320 5 270 280 275 280 310 320 320 330

Table 8. Percentage enhancement in flash point of base fluid 1, 2, 3 and 4 with antioxidants.


in gram


Flash point (OC) Flash point (OC) Flash point (OC) Flash pont (OC)

AF

TE

R

AD

DIN

G

%

EN

HA

NC

ED

AF

TE

R

AD

DIN

G

%

EN

HA

NC

ED

AF

TE

R

AD

DIN

G

%

EN

HA

NC

ED

AF

TE

R

AD

DIN

G

%

EN

HA

NC

ED

AA 1 270 4 290 12 310 0 310 3 5 280 8 287 10 300 -3 310 3

BHA 1 275 6 270 4 330 6 310 3 5 280 8 265 2 300 -3 315 5

BHT 1 280 8 265 2 310 0 310 3 5 270 4 250 -4 270 -13 290 -3

BHT + CA 0.5:0.5 260 0 265 2 310 0 310 3

1:1 270 4 270 4 300 -3 315 5

α-T + CA 0.5:0.5 280 8 270 4 280 -10 310 3

1:1 290 12 270 4 280 -10 300 0

PG 1 275 6 270 4 300 -3 315 5 5 270 4 275 6 310 0 320 7


7.3 VISCOSITY

Viscosity plays a significant role in determining the heat conventional rate of fluids. In general the viscosity of fluid changes with temperature, increasing as the temperature is decreased, and decreasing as the temperature is increased. The limiting factor that controls the amount of antioxidant is the oil type. The base fluid with saturated fatty acid exhibit higher viscosity and base fluid with unsaturated fatty acids exhibit lower viscosity.

The viscosities of base fluids 1, 2, 3 and 4 are four to five times higher than mineral oil. Viscosity values for various test samples are depicted in the Table 10. The viscosities of samples prepared using base fluids and antioxidants are noticed in Figures 13 and 14. The percentage decrement in viscosities of samples is displayed in Table 11. From investigation it is inferred that,

All the samples prepared out of base fluids and antioxidant shows optimized viscosities.

Figure 11. Flash point and fire point characteristics of base fluid 1 and 2 with antioxidants.

Figure 12. Flash point and fire point characteristics of base fluid 3 and 4 with antioxidants.

Table 9. Percentage enhancement in fire point of base fluid 1, 2, 3 and 4 with antioxidants.


in gram


Fire point (OC) Fire point (OC) Fire point (OC) Fire point (OC)

AF

TE

R

AD

DIN

G

%

EN

HA

NC

ED

AF

TE

R

AD

DIN

G

%

EN

HA

NC

ED

AF

TE

R

AD

DIN

G

%

EN

HA

NC

ED

AF

TE

R

AD

DIN

G

%

EN

HA

NC

ED

AA 1 295 9 300 7 320 0 320 3 5 320 19 295 5 320 0 320 3

BHA 1 280 4 280 0 350 9 320 3 5 290 7 275 -2 310 -3 325 5

BHT 1 300 11 275 -2 325 2 320 3 5 285 6 260 -7 280 -13 300 -3

BHT + CA 0.5:0.5 300 11 275 -2 320 0 315 2

1:1 295 9 280 0 320 0 320 3

α-T + CA 0.5:0.5 295 9 285 2 290 -9 315 2

1:1 300 11 280 0 290 -9 310 0

PG 1 285 6 290 4 310 -3 320 3 5 280 4 280 0 320 0 330 6


The viscosities of transformed base fluid 1, 2, 3 and 4 shows relatively good decrement values due to the effect of the antioxidant.

The temperament of this decrement is due to high dispersion of additives in base fluids.

At higher temperature the viscosity of transformed fluid will decrease further. Such elevated temperature could cause peroxide formation, and results in higher sludge deposit in transferred fluids, but the effect will be less evident in the presence of antioxidant.

Table 10. Viscosity of base fluid 1, 2, 3 and 4 treated with antioxidants.


in gram

Base fluid 1

Base fluid 2

Base fluid 3

Base fluid 4

Viscosity (cSt)

Viscosity (cSt)

Viscosity (cSt)

Viscosity (cSt)

AT 30OC AT 30OC AT 30OC AT 30OC

AA 1 121 145 127 127

5 127 147 135 125

BHA 1 128 143 123 124

5 125 143 145 124

BHT 1 109 136 120 120

5 127 140 103 125

BHT + CA 0.5:0.5 109 151 103 124

1:1 119 151 115 127

α-T + CA 0.5:0.5 110 152 103 105

1:1 115 147 110 110

PG 1 112 150 120 108

5 119 154 125 115

Table 11. Percentage enhancement in viscosity of base fluid 1, 2, 3 and 4 with antioxidants.


in gram

Base fluid 1

Base fluid 2

Base fluid 3

Base fluid 4

Viscosity (cSt) Viscosity (cSt) Viscosity (cSt) Viscosity (cSt)

AF

TE

R

AD

DIN

G

%

DE

CR

EM

EN

T

AF

TE

R

AD

DIN

G

%

DE

CR

EM

EN

T

AF

TE

R

AD

DIN

G

%

DE

CR

EM

EN

T

AF

TE

R

AD

DIN

G

%

DE

CR

EM

EN

T

AA 1 121 -8 145 -6 127 -9 127 -5

5 127 -4 147 -5 135 -4 125 -7

BHA 1 128 -3 143 -7 123 -12 124 -7 5 125 -5 143 -7 145 4 124 -7

BHT 1 109 -17 136 -12 120 -14 120 -10 5 127 -4 140 -9 103 -26 125 -7

BHT + CA 0.5:0.5 109 -17 151 -2 103 -26 124 -7

1:1 119 -10 151 -2 115 -18 127 -5

α-T + CA 0.5:0.5 110 -17 152 -1 103 -26 105 -22

1:1 115 -13 147 -5 110 -21 110 -18

PG 1 112 -15 150 -3 120 -14 108 -19 5 119 -10 154 0 125 -11 115 -14

Figure 13. Viscosity characteristics of base fluids 1 and 2 with antioxidants.


8 INTERPOLATION FUNCTION

The mathematical functions associated with the critical properties of transformed base fluids 1, 2, 3 and 4 are calculated by using Lagrange’s Interpolation Formula. This formula is used to calculate the intermediate variation of parameters for varying antioxidant concentration within the intervals. Here the output

function is calculated as a function of input function, where ‘X’ as concentration variable in Null (0 g), minimum (1 g) and maximum (5 g) level for each antioxidant added with base fluids, volume of oil (500 ml) used and temperature (RTP) are assumed as constants. The input factors (X0->NULL, X1->MINIMUM and X3->MAXIMUM) are calculated from the quantity of antioxidant added (in gram) to base fluid.

Figure 14. Viscosity characteristics of base fluids 3 and 4 with antioxidants.

Table 12. Interpolation function representation of critical properties for base fluid 1.

Antioxidant added

Base fluid 1

Breakdown Voltage (kV) Flash Point (OC) Fire Point (OC) Viscosity (cSt)

AA V0 = [-0.07939X2 + 0.3145X

+ 1] V FL0 = [-5e-3X2 + 0,04422X

+ 1] FL FR0 = [-0.01389X2 + 0.1064X

+ 1] FR VI0 = [0.01895X2 – 0.1023X

+ 1] VI

BHA V0 = [0.01917X2 – 0.0486X

+ 1] V FL0 = [-0.01057X2 + 0.0682X

+ 1] FL FR0 = [-5.55e-3X2 + 0.04259X

+ 1] FR VI0 = [5e-3X2 – 0.036106X

+ 1 ] VI

BHT V0 = [0.014705X2 – 0.0147X

+ 1] V FL0 = [-0.0173X2 + 0.0942X

+ 1] FL FR0 = [-0.01202X2 + 0.1361X

+ 1] FR VI0 = [0.041681X2 – 0.1989X

+ 1] VI

BHT + CA V0 = [-0.09559X2 + 0.5955X

+ 1] V FL0 = [-0.0124X2 + 0.0701X

+ 1] FL FR0 = [-0.01202X2 + 0.0675X

+ 1] FR VI0 = [0.03295X2 – 0.1844X

+ 1] VI

α-T + CA V0 = [-0.3443X2 + 0.9282X

+ 0.73955] V FL0 = [-0.04441X2 + 0.7429X

+ 0.73955] FL FR0 = [-0.7803X2 + 1.1332X

+ 0.73955] FR VI0 = [-0.02068X2 + 0.1826X

+ 0.73955] VI

PG V0 = [-1.99147X2 + 2.6930X

+ 0.73955] V FL0 = [-0.5978X2 + 0.9736X

+ 0.73955] FL FR0 = [-0.66904X2 + 1.0406X

+ 0.73955] FR VI0 = [-1.0253X2 + 1.4335X

+ 0.73955] VI


Antioxidant Added

Base fluid 2


AA V0 = [-0.043588X2 + 0.22306X

+ 1] V FL0 = [-0.023653X2 + 0.1390X

+ 1] FL FR0 = [-0.015176X2 + 0.0865X

+ 1] FR VI0 = [0.01233X2 – 0.07078X

+ 1] VI

BHA V0 = [-0.0115X2 + 0.1397X

+ 1] V FL0 = [-0.02365X2 + 0.1390X

+ 1] FL FR0 = [-8.94e-4X2 + 8.93 e-4X

+ 1] FR VI0 = [0.0123X2 – 0.07078X

+ 1 ] VI

BHT V0 = [-0.0230X2 + 0.1513X

+ 1] V FL0 = [-6.731 e-3X2 + 0.0259X

+ 1] FL FR0 = [9e-4X2 - 0.0188X

+ 1] FR VI0 = [0.02467X2 – 0.1415X

+ 1] VI

BHT + CA V0 = [0.026952X2 - 0.2064X

+ 1] V FL0 = [-6.6155 e-3X2 + 0.0446X

+ 1] FL FR0 = [-8.925e-3X2 + 0.04462X

+ 1] FR VI0 = [6. 5e-3X2 – 0.0325X

+ 1] VI

α-T + CA V0 = [-0.0595X2 + 0.5506X

+ 0.73955] V FL0 = [-0.5977X2 + 0.8965X

+ 0.73955] FL FR0 = [-0.5923X2 + 0.8527X

+ 0.73955] FR VI0 = [-0.5599X2 + 0.7748X

+ 0.73955] VI

PG V0 = [-1.18756X2 + 1.4223X

+ 0.73955] V FL0 = [-0.5209X2 + 0.8198X

+ 0.73955] FL FR0 = [-0.44946X2 + 0.7099X

+ 0.73955] FR VI0 = [-0.48192X2 + 0.7228X

+ 0.73955] VI


Similarly the output factors (y0, y1 and y3) are calculated by dividing the output critical parameters of each input factors with base critical parameter (without antioxidant addition). For each base transformed fluid, the critical parameters like breakdown voltage, flash point, fire point and viscosity are represented as mathematical function in Tables 12, 13, 14 and 15. Thus by varying the concentration (x in gram) of antioxidant, the corresponding output (V0, FL0, FR0 and VI0) (with constant Input (V, FL, FR and VI)) can be obtained.

8 CONCLUSIONS The investigation is used to determine the effectiveness of

antioxidant with natural esters. The addition of antioxidant with natural esters (base fluid 1, base fluid 2, base fluid 3 and base fluid 4) enhances the critical properties of oil. Moreover the performance of transformed natural ester fluids with antioxidant does not decelerate in efficiency at elevated temperature and less formation of carbon is observed during experiment. By keeping individual focus on each antioxidant

composition, the cooperative mechanism is observed when combinations of antioxidants are used. With effect of the results it can be said that, more number of high performance antioxidants in combinatorial mode will result in constructive outcomes. The study imparts the use of green fluids formed by combining natural esters and antioxidants in transformer applications. Results provide insight view in the performance of natural esters like sunflower oil, rice bran oil, soybean oil and corn oil. The use of natural and synthetic antioxidants in natural ester paves a new dimension in the research of power transformers. The operating cost (initial cost and maintenance cost) is high compared to mineral oil, but such drawback is overcome by factors like slow ageing rate, high oxidative stability and less tendency towards formation of gases. This transformation not only helps in keeping environment green but leads to hazard free and zero rate of accidents. Hence this approach is vivid in technical, economic and environmental aspects. Overall investigation concludes that, the natural ester transformed using antioxidant is an appropriate substitute of mineral oil for power transformer.


Antioxidant Added

Base fluid 4


AA V0 = [-0.0345 X2 + 0.2845X

+ 1 ] V FL0 = [-0.006X2 + 0.036X

+ 1] FL FR0 = [-0.006X2 + 0.036X

+ 1 ] FR VI0 = [0.0115X2 – 0.2845X

+ 1] VI

BHA V0 = [-0.0720 X2 + 0.4720X

+ 1 ] V FL0 = [-0.005X2 + 0.35X

+ 1] FL FR0 = [-0.0055X2 + 0.0355X

+ 1 ] FR VI0 = [0.016X2 – 0.96X

+ 1] VI

BHT V0 = [-0.112 X2 + 0.672X

+ 1 ] V FL0 = [-0.0095X2 + 0.0395X

+ 1] FL FR0 = [-0.0095X2 + 0.0395X

+ 1 ] FR VI0 = [0.024X2 – 0.134X

+ 1] VI

BHT + CA V0 = [-0.0585 X2 + 0.3985X

+ 1 ] V FL0 = [-0.005X2 + 0.035X

+ 1] FL FR0 = [-e-3X2 + 0.0110X

+ 1 ] FR VI0 = [0.017X2 – 0.097X

+ 1] VI

α-T + CA V0 = [-0.09 X2 + 0.55X

+ 1 ] V FL0 = [-0.0075X2 + 0.0375X

+ 1] FL FR0 = [-0.0025X2 + 0.0125X

+ 1 ] FR VI0 = [0.046X2 – 0.266X

+ 1] VI

PG V0 = [-0.142 X2 + 0.852X

+ 1 ] V FL0 = [-0.0095X2 - 0.0595X

+ 1] FL FR0 = [-0.0045X2 + 0.0345X

+ 1 ] FR VI0 = [0.0425X2 – 0.2425X

+ 1] VI


Antioxidant Added

Base fluid 3


AA V0 = [-0.0855 X2 + 0.6395X

+ 1 ] V FL0 = [-0.002X2 + 0.002X

+ 1] FL FR0 = [0.008X2 – 0.048X

+ 1 ] FR VI0 = [0.023X2 – 0.123X

+ 1] VI

BHA V0 = [-0.0925 X2 + 0.6395X

+ 1 ] V FL0 = [-0.017X2 + 0.077X

+ 1] FL FR0 = [-0.0245X2 + 0.1145X

+ 1 ] FR VI0 = [0.034X2 – 0.164X

+ 1] VI

BHT V0 = [-0.09 X2 + 0.53X

+ 1 ] V FL0 = [-0.0065X2 + 0.0065X

+ 1] FL FR0 = [-0.009X2 + 0.019X

+ 1 ] FR VI0 = [0.024X2 – 0.1740X

+ 1] VI

BHT + CA V0 = [-0.0860 X2 + 0.526X

+ 1 ] V FL0 = [-0.011X2 + 0.011X

+ 1] FL FR0 = [1.38e-17X2 + 4.16 e-17X

+ 1 ] FR VI0 = [0.0585X2 – 0.3285X

+ 1] VI

α-T + CA V0 = [-0.0725 X2 + 0.4425X

+ 1 ] V FL0 = [0.02X2 - 0.12X

+ 1] FL FR0 = [0.008X2 - 0.048X

+ 1 ] FR VI0 = [0.0565X2 – 0.3285X

+ 1] VI

PG V0 = [-0.0905 X2 + 0.5705X

+ 1 ] V FL0 = [0.01X2 - 0.05X

+ 1] FL FR0 = [-0.01X2 - 0.05X

+ 1 ] FR VI0 = [0.032X2 – 0.1820X

+ 1] VI


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A. Raymon was born in Vilathikulam, India in 1987. He obtained his B.E (Electrical) degree from P.S.R. Engineering College, Sivakasi, Tamilnadu, India. He is presently pursuing his M.E. (High Voltage Engineering) from National Engineering College, Kovilpatti, Tamilnadu, India. His area of

interest includes Power System, High Voltage and Insulation Engineering.

P. Samuel Pakianathan was born in Palayamkottai, India in 1987. He obtained his B.E (Electrical) degree from Government College of Engineering, Tirunelveli, Tamilnadu, India. He is presently pursuing his M.E. (High voltage engineering) from National Engineering College, Kovilpatti, Tamilnadu,

India. His area of interest includes Power System, High Voltage and Insulation Engineering.

M.P.E. Rajamani is Assistant Professor (senior grade) in the Department of Electrical Engineering, National Engineering College, Kovilpatti, Tamilnadu, India. He obtained his B.E (Electrical) degree from University of Madras, M.E. (power electronics) from Anna University, Chennai, India. His area of interest includes power electronics and high voltage and insulation engineering.

R. Karthik is an Assistant Professor (senior grade) in the Department of Electrical Engineering, Liquid Dielectrics laboratory, National Engineering College, Kovilpatti, Tamilnadu, India. He obtained his B.E (electrical) degree from the University of Madras, M.Tech (high voltage engineering) from SASTRA University, Thanjore and presently completed the Ph.D. degree at Anna University,

Chennai, India. He is a recipient of young scientist fellowship from Department of Science and technology. He has published more than 30 papers in Conference and Journals. His area of interest includes Power system, High Voltage and Insulation Engineering.

Documents

Enhancing the Critical Characteristics of Natural Esters with Antioxidants for Power Transformer Applications