performance of coconut oil as an alternative transformer...

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 20, No. 3; June 2013 887

1070-9878/13/$25.00 © 2013 IEEE

Performance of Coconut Oil as an Alternative Transformer Liquid Insulation

B. S. H. M. S. Y. Matharage, M. A. R. M. Fernando

Department of Electrical and Electronic Engineering, University of Peradeniya,

Peradeniya, 20400, Sri Lanka.

M. A. A. P. Bandara, G. A. Jayantha Ceylon Electricity Board,

Sri Lanka.

and C. S. Kalpage Department of Chemical and Process Engineering,

University of Peradeniya, Peradeniya, 20400, Sri Lanka.

ABSTRACT

Transformer mineral oil has been replaced by alternative oils such as synthetic oil and natural esters due to their biodegradability and environmentally friendly nature. This paper presents performance of coconut oil as such an alternative. Generally, as the other alternative oils, coconut oil has high conductivity due to the presence of free fatty acids. In this study, three different types of coconut oil samples consisting of virgin, copra and RBD (refined, bleached and deodorized) were initially tested by frequency dielectric spectroscopy (FDS) measurements to see how the conductivity was improved by dehydration and neutralization. The FDS results were fitted by inverse power dependence and Cole-Cole models to estimate the conductivity and response functions. Afterwards, a set of new coconut oil samples extracted from copra were thermally aged at 120 °C under sealed conditions and compared with that of mineral oil. The performances of oil samples were evaluated using the test results of breakdown voltage, acidity, interfacial tension and FDS measurements under different aging periods such as 2, 5 and 7 weeks. Another 4 sets of new coconut and mineral oil samples were subjected to simulated thermal faults and electrical faults which include aging for 12 hours at 160 0C, one hour at 200 0C, exposing to partial discharges for four hours and subjecting to 20 low energy breakdowns. The performance comparisons were done by FDS measurements and dissolved gas analysis. In parallel, field-aged coconut oil samples collected from a sealed distribution transformer with 11 years of service were also tested by FDS measurements. In general, coconut oil shows its suitability as an alternative to the mineral oils for transformers, despite limitations found in some of their physical properties. It was found that the FDS results were in good agreement with chemical test results and with the estimated conductivity values.

Index Terms - Power transformers, insulation, mineral oil, coconut oil, aging, frequency dielectric spectroscopy, dissolved gas analysis, conductivity.

1 INTRODUCTION

POWER transformers consisting of oil/paper/pressboard insulation are considered as one of the main components in a power system. However during service, their insulation gets deteriorated due to applied stresses such as thermal, electrical, mechanical, etc., so that majority of failures are caused by the

failures of their insulations [1, 2]. In addition to the insulation, the oil is not only used as a liquid insulation but also as a coolant of the transformer.

The mineral oil extracted from crude oil, because of its better performance, has been used as the liquid insulation for more than 70 years [2, 3]. However, due to its poor biodegradability and future scarcity, attention has been paid to alternative natural esters including oil from sunflower, soya beans, vegetable, etc [4-12]. The studies found that those natural esters need to be purified to Manuscript received on 25 October 2012, in final form 7 March 2013.

Authorized licensed use limited to: Ramesh Fernando. Downloaded on June 15,2020 at 05:34:09 UTC from IEEE Xplore. Restrictions apply.

888 B. S. H. M. S. Y. Matharage et al.: Performance of Coconut Oil as an Alternative Transformer Liquid Insulation

improve their insulation properties [4]. For example, commercially available refined, bleached and deodorized type of natural esters had been purified to produce BIOTEMP and Envirotemp FR3 which are commercially available alternative oils [4, 7]. In general, the breakdown voltages of most of the vegetable oils are higher than mineral oils. Despite that, the properties such as viscosity, conductivity, acidity and water solubility are still to be improved [4]. As far as the aging is concerned it has been reported that the natural esters produced high content of dissolved gases such as CO and CO2. [6-8, 11]. To improve those properties methods such as partial hydrogenation, adding special antioxidants to reduce oxidation, minimizing the unstable tri-saturates, adding special clay to reduce conductivity levels, etc. have been proposed [4]. As far as overall performances are concerned, the investigations of alternative oils under laboratory conditions are at a satisfactory level [8, 9]. Transformers filled with new alternative oil have also been tested at different voltage levels in both transmission and distribution [10] and have verified better performance under sealed conditions [4-6].

The coconut oil is also a vegetable oil (natural ester) used for cooking purposes and is widely available in tropical countries such as India and Sri Lanka. However, most of its physical and chemical properties characterizing liquid insulation have been proved at satisfactory levels [13-15]. The coconut oil differs from some of the other types of vegetable oils as they are rich in saturated fats (about 90%) and out of that about 65% includes medium and short chains. Usually high amounts of saturated fat cause increases in the melting point, but for the positive side, due to increased oil-air stability the oxidation resistance is weaker [9] and the same applies to coconut oil as well. As for most natural esters, the coconut oil also has higher content of free fatty acids which increases the conductivity levels [4, 14, 16]. This paper presents the suitability of coconut oil as a transformer liquid insulation and its performance in comparison with mineral oil. Purification process such as dehydration and neutralization has been used to improve the oil conductivity.

In service, the transformer insulation deteriorates with time when subjected to single or combined stresses such as electrical, thermal, mechanical, etc. due to aging of the main insulation and mineral oil. Although field investigations provide realistic condition of the insulation aging, practically it takes a long time to see the real aging effects. In such situations, laboratory investigations are conducted with accelerated aging and simulated faults [6-8]. Electrical and chemical (non-electrical) tests are usually conducted for performance investigation for such situations. In non-electrical analysis, moisture content, acidity, interfacial tension and viscosity are some of the common test methods to see the insulation deterioration [6, 8, 17] whereas the content of dissolved gases are measured to see the effects under faulty conditions [7, 8, 18]. Out of electrical tests, frequency dielectric spectroscopy (FDS) measurements are widely used as a non-destructive test to assess the condition of the insulation [19-21]. In this work, the oil samples were aged as well as subjected to thermal and electrical faults. Their performances were investigated by chemical and electrical tests. FDS measurements were conducted in all cases and the results were modeled to estimate the conductivity and response function.

2 CHEMICAL AND PHYSICAL PROPERTIES

2.1 CHEMICAL PROPERTIES

Coconut oil as with most of the other natural esters consists of triglycerides. Figure 1 shows a typical reaction occurring between glycerol and acid groups to form triglycerides. These triglycerides contain different fatty acids (R1, R2 and R3) which might be saturated or unsaturated depending on the oil type. Table 1 shows the comparison of acid composition in coconut oil with other commonly used vegetable oils in transformers.

Figure 1. Chemical reaction between glycerol and fatty acids when forming triglyceride in coconut oil [9].

Table 1. Acid composition in coconut oil and other alternative oils [15].

Fatty acid name R1, R2, R3 Coconut

Oil Rapeseed

Oil Soya Oil

Sunflower (high

oleic) Oil Scientific Comm

on name

Caproic C6:0 ND-0.7 ND ND ND Caprylic C8:0 4.6-10.0 ND ND ND Capric C10:0 5.0-8.0 ND ND ND Lauric C12:0 45.1-53.2 ND ND-0.1 ND Myristic C14:0 16.8-21.0 ND-0.2 ND-0.2 ND-1 Palmitic C16:0 7.5-10.2 1.5-6.0 8.0-13.5 2.0-6.0 Palmitoleic C16:1 ND ND-3.0 ND-0.2 ND-0.05 Margaric C17:0 ND ND-0.1 ND-0.1 ND-0.05 Heptadecenoic (cis-10) C17:1 ND 0.5-3.1 ND-0.1 ND-0.06 Stearic C18:0 2.0-4.0 8.0-60.0 2.0-5.4 2.9-6.2 Oleic C18:1 5.0-10.0 11.0-23.0 17-30 75 -90 Linoleic C18:2 1.0-2.5 11.0-23.0 48.0-59.0 2-17 Linolenic C18:3 ND-0.2 5.0-13.0 4.5-11.0 ND-0.3 Arachidic C20:0 ND-0.2 ND-3.0 0.1-0.6 0.2-0.4

ND-non detectable, defined as ≤0.05%

According to Table 1, unlike in most of the other natural esters, it is clear that coconut oil contains a high percentage of saturated fats (C12:0, C14:0). This makes the coconut oil more stable when contacting with air [8, 13]. Therefore the oxidative rancidity is less significant in coconut oil than other vegetable oils [22]. However, high pour point is the main disadvantage caused by the high percentage of saturated fat [8].

2.2 PHYSICAL PROPERTIES

Table 2 shows some important physical properties of coconut oil, commercially available vegetable oil for transformers [4] and recommended levels for uninhibited mineral oil of class I [23]. Most of the physical properties such as dielectric strength, flash point, moisture content, etc. are within the recommended limits for mineral oil stipulated by



IEC 60296 [23]. Even though the viscosity of coconut oil is higher than the recommended value, it is less than some of the vegetable oils currently used as alternative oil [8]. The pour point of coconut oil has the highest diversity from the recommended level.

Table 2. Comparison of physical properties of coconut oil and typical vegetable oil.

Property Coconut oil [13]

Vegetable oil [4]

Uninhibited class I mineral oil [23]

Dielectric strength [kV] 60 74 50 Viscosity [cSt at 40C] 29 33-45 13 Pour point [°C] 23 -15 to -25 -40 Flash point [°C] 170-225 310-325 154 Specific gravity at 20C 0.917 0.91-0.95 0.895 Moisture content [mg/kg] 1 50-10 1.5

As proven by some other laboratory experiments partial solidification of coconut oil at temperatures below the melting point does not significantly affect its breakdown voltage [13]. It has been proved that the pour point could be reduced to 12 °C by adding 15% of styrenated phenol to the coconut oil (by weight) [24]. Further it could be reduced to -3 °C and even up -42 °C, by chemical modification (acid catalyzed condensation reaction with coconut oil/castor oil mixture) and by adding lauric and oleic acids as the synthesis respectively [24].

3 EFFECT OF MANUFACTURING PROCESS

Three main types are categorized according to the manufacturing process of the virgin coconut oil, the copra oil and the RBD type oil. The virgin coconut oil, usually produced in small scale, is extracted from fresh coconut meat, milk or residue. This process is considered as a wet method. Chemicals and high heating are not used for in the refining process. The copra oil, known as dry method, is produced by heating and drying the meat extracted from the coconut shell. The oil extracted from copra is then refined, bleached and deodorized to make RBD coconut oil. In general, the manufacturing process of the coconut oil has limited effect on the chemical composition but may affect free fatty acid content significantly [22]. When considering the free fatty acid percentage as lauric acid, copra oil shows the highest acid percentage i.e. 1.41% whereas the RBD coconut oil shows the lowest i.e. 0.02%. Virgin coconut oil has an acid percentage of 0.13% [22].

3.1 SAMPLES

Preliminary investigations were conducted on different types of coconut oil samples to observe the effect of manufacturing process on their properties. Three different types of commercially available coconut oil, namely Baraka, Marina, and N-joy were selected for the study. Baraka oil is a virgin coconut oil which is made from the white scraped coconut. First it is dried below 60 0C to remove the moisture and then cooled back to room temperature. Then the oil is extracted with a cold press method. Finally, the oil is filtered and heated in a water bath between 55 0C – 60 0C. N-joy brand belongs to copra oil type. In the extraction process coconut meat separated from the shelf is dried in a kiln to reduce the

moisture content to less than 6%. The copra is then crushed, cooked using indirect steam heating and mechanically expelled to produce crude coconut oil. The crude oil is then filtered using a micron-sized filter. Marina is a R.B.D. type coconut oil. It is produced from coconuts, converted into Copra (dried meat or kernel of the coconut), by a cold process. Then it is refined physically by steam and vacuum.

3.2 PROCEDURE

Oil samples of 100ml volume from all three types were taken for the investigation. The FDS measurements were taken at room temperature of about 25 0C for all the samples using an Insulation Dielectric Analyzer IDA 200. A test cell consisting of three electrode systems with a geometric capacitance of 70 pF was used for the measurements. The frequency was varied from 1 kHz to 1 mHz at 50 V ac and the corresponding complex capacitance was monitored with respect of the frequency. Then the results were modeled to find out the quality of the oil samples in terms of the conductivity, the permittivity and the response function. In the modeling, frequency variation of complex susceptibility was modeled with Cole-Cole model with two relaxation processes (Δ1, τ1, 1, Δ2, τ2, 2) and inverse power dependence (A and n). The inverse power dependence was used at low frequencies whereas Cole-Cole models were used for high frequencies. The measured complex permittivity (capacitance) values were fitted by adding permittivity at infinity and conductivity terms on the real and imaginary parts of the susceptibility respectively (see equations (1) and (2)).

ε' Aω‐ Re∆

‐

∆‐

ε (1)

ε" Aω‐ cot 1‐n Im∆

‐

∆‐

ε (2)

3.3 RESULTS

Figures 2 and 3 show the variation of measured and modeled real and imaginary permittivity values with respect of frequency. Table 3 shows the estimated modeled parameters for the three different brands. According to Figure 2 all the oil types had nearly equal real permittivity values at high frequencies and the permittivity values increased when the frequency reduced towards the low frequencies. The Baraka oil brand (type B) had the highest permittivity values within the measured frequency range. When considering the dielectric losses, all the oil types followed a linear behavior approximately about -1 gradient towards the low frequencies confirming the effect of the dc conductivity as the dominant part. According to Figure 3 N-Joy oil brand (type C) showed the highest dielectric losses whereas the Marina oil brand (type A) showed the lowest. The estimated conductivity values (σdc) in Table 3 also matched with the FDS results i.e. the type A brand having the lowest conductivity and the type C brand having the highest.



Figure 2. Variation of measured and modeled real part of permittivity with respect of frequency for different coconut oil brands.

Figure 3. Variation of measured and modeled imaginary part of permittivity with respect of frequency for different coconut oil brands.

Table 3. Estimated response function parameters and conductivities for different coconut oil brands.

Brand name Marina new Oil

Baraka new Oil

N-Joy new Oil

Type A B C A [10-3] 20 9.1 18.1 n 0.5 1.8 1.2 ∆ε1 4.2 20 18.4 τ1 [s] 34.6 5.0 8.5 α1 0.07 0.13 0.09 ∆ε2 0.45 0.1 0.6 τ2 [ms] 133.0 19.5 1.0 α2 0.1 0.2 0.1 σdc (pS/m) 69.0 383.0 844.9

The dielectric response also showed a similar behavior by having relaxations in the low frequencies (highest τ1 and τ2 values) for type A brand compared to others (see Table 3). In general, the copra coconut oil (type C brand) showed the highest conductivity (844.9 pS/m) and it might be due to the presence of high content of free fatty acid levels as no refining process is used after manufacturing. Out of the tested samples, type A brand, having the lowest conductivity values, was selected for further improvement of the conductivity (see section 4). The type C, having the worst conductivity, was selected for performance testing under aging and faults conditions (section 5 and 6).

4 IMPROVEMENT OF CONDUCTIVITY

4.1 SAMPLES AND PROCEDURE

The selected oil type brand A was used to improve the conductivity by two methods, namely dehydrating and neutralization. Five coconut oil samples with a volume of 100 ml each from type A brand was selected. In dehydrating, two samples were heated at 105 0C in an air circulating oven (Yamato DS-63 with inner dimensions: 450 mm (width) x 480 mm (depth) x 450 mm (height)) for 24 hours in unsealed conditions to remove absorbed moisture. Afterwards, the samples were cooled for 24 hours in two ways i.e. naturally or under low RH inside a desiccator. In the neutralization, a pre-calculated amount of NaOH was added to another two samples to reduce the acids content. Afterwards the samples were subjected to dehydration. One new oil sample was selected as the reference. Table 4 shows the selected oil samples together with the purification procedure. More details of the test procedure are available in [16]. After the samples were prepared, the FDS measurements were conducted and the results were modeled as described in the section 3.2. Table 4. Details of the coconut oil samples of brand A and the purification processes to improve the conductivity. Sample Description A New oil A1 New oil, Dehydrated and cooled in open air A2 New oil, Dehydrated and cooled under low RH A3 New oil, Neutralized, Dehydrated and cooled in open air A4 New oil, Neutralized, Dehydrated and cooled under low RH

4.2 RESULTS

Figures 4 and 5 show the variation of measured and modeled real and imaginary permittivity values with respect to frequency. Table 5 shows the estimated modeled parameters for the different treatments used to improve conductivity.

Figure 4. Variation of measured and modeled real part of permittivity with respect of frequency by different purification methods. When comparing dried oil with new oil, it is clearly visible that the samples, A1 and A2, had limited changes in the real permittivity values whereas the imaginary permittivity reduced significantly by about 60% in low frequencies confirming the removal of the majority of the absorbed moisture in the oil (see Figures 4 and 5). When the oil samples were neutralized prior to the drying, the real permittivity values reduced significantly about 50% at low frequencies.

1

10

100

0.0001 0.001 0.01 0.1 1 10 100 1000

Perm

ittivity e'

Frequency [Hz]

Baraka oil‐measured

Marina oil‐measured

N‐Joy oil‐measured

Oil‐modeled

0.001

0.01

0.1

1

10

100

1000

10000

100000

0.0001 0.001 0.01 0.1 1 10 100 1000

Perm

ittivity e"

Frequency [Hz]

Baraka oil‐measured

Marina oil‐measured

N‐Joy oil‐measured

Oil‐modeled

0.1

1

10

100

0.0001 0.01 1 100

Perm

ittivity e'

Frequency [Hz]

A‐measured A1‐measured

A2‐measured A3‐measured

A4‐measured Oil‐modeled



Figure 5.Variation of measured and modeled imaginary part of permittivity with respect of frequency by different purification methods. Table 5. Estimated response function parameters and conductivities under different purification processes.

Sample name A A1 A2 A3 A4 A [10-3] 20 1.1 0.6 0.03 0.5 n 0.5 1.7 1.99 1.99 1.1 ∆ε1 4.2 1 1.1 0.038 0.025 τ1 [s] 34.6 30 30 13 0.5 α1 0.07 0.7 0.6 0.41 0.56 ∆ε2 0.45 0.05 0.04 0.001 0.002 τ2 [ms] 133.0 0.1 0.01 0.14 0.1 α2 0.1 0.1 0.1 0.1 0.1 σdc (pS/m) 69.0 23.7 35 30 17.6

At the same time the imaginary frequency values also reduced by 60% at low frequencies, showing the removal of fatty acid content to some extent. More importantly neutralization also reduced imaginary permittivity about 50% at high frequencies. It is also clearly visible that two cooling methods had different effects on the complex permittivity values especially on the imaginary component. The cooling under low RH inside the dessicator indicated a reduction of the imaginary permittivity values by about 50% more than the cooling in normal atmosphere. This difference should be due to the fact that the possibility of moisture absorption during cooling under former case is limited compared to the latter one. The estimated conductivity values also showed a reduction with the purification processes (see Table 4). The neutralized, dehydrated and cooled under low RH sample (A4) showed the best conductivity values i.e. about 75% improvement. According to the estimated response function parameters, the relaxation processes seems to happen at low frequencies for sample A4 compared to the new sample A.

5 THERMAL AGING

5.1 SAMPLES AND PROCEDURE

The oil brand C (N-Joy brand) which showed the worst conductivity levels was selected for the aging test and compared with mineral oil (naphthenic type) [23]. First the moisture content (MC) of both oil types was measured and the values were 47 ppm and 42 ppm for mineral oil and coconut oil respectively. Despite the MC levels were high compared to recommended values, no drying process was used to remove the moisture as the purpose was to age under the worst conditions as much as possible. Three coconut oil samples and

six mineral oil samples of volume of 1200 ml were selected. For all the samples, different metal parts (i.e. copper 2.5 g/l, iron 2.5 g/l, aluminum 0.5 g/l and zinc 0.5 g/l) were added to represent the actual conditions inside a transformer [9]. In addition, naturally-wet pressboard pieces (100 g/1) were added to the prepared coconut oil samples and three samples of mineral oil to investigate the effect of moisture during aging. The rest of the three mineral oil samples were kept in the dry condition. All the oil samples were aged by drying inside the air circulating oven at120 0C under sealed condition. The sealed condition was used as the worst case compared to aging under unsealed condition [25, 26]. Different aging periods such as 2, 5 and 7 weeks were selected for the three different sets of samples and 7 weeks was taken as the longest duration with sufficient aging [25]. Details of the samples are given in Table 6.

After the samples were aged, their performances were evaluated by visual scrutiny, electrical and non-electrical tests. First, color of the oil was obtained by visual inspection. Two non-electrical tests were performed to determine interfacial tension (IFT) and acidity values. The IFT was evaluated by Easy Dyne measuring device which use the du Nouy ring method. Average of five tests had been taken as the final value of IFT. The acidity level was evaluated by titrating the oil sample with 0.1% w/v NaOH solution. Then the results were converted into equivalent milligrams of KOH per gram [16]. Two electrical tests; breakdown voltage and FDS measurements were done. The breakdown voltage was obtained by MEGGER OTS 80AF/2 using an oil container with sphere gaps of 2.5 mm distance. The FDS measurements were performed by using the oil container and the Insulation Dielectric Analyzer (IDA 200) at 50 V from 1 kHz to 1 mHz (same procedure described in section 3.2). The FDS results were finally modeled using the Cole-Cole and inverse power dependences.

Table 6. Details and aged durations of coconut oil and mineral oil samples. Dry Transformer

mineral oil Wet Coconut

oil Wet Transformer

mineral oil Duration [weeks]

TD CA TW 0-new TD2 CA2 TW2 2 TD5 CA5 TW5 5 TD7 CA7 TW7 7

T – Mineral oil, CA – Coconut oil, D – Dry and W - Wet

5.2 RESULTS

Table 7 shows the color variations of the samples during aging. It is clear that all the samples have gone through color changes during aging mainly attributed to the oxidation process. However, in coconut oil as most of other vegetable oils, the oxidation process is weaker due to the higher percentage of saturated fatty acids [9, 22].

Table 7. Color variation of coconut oil and mineral oil samples with respect of the aging period. Sample No.

Color Sample No.

Color Sample No.

Color

TD Pale yellow CA Pale yellow TW Pale yellow TD2 Pale yellow CA2 Pale yellow TW2 Pale yellow TD5 Pale yellow CA5 Yellow TW5 Yellow TD7 Amber CA7 Brown TW7 Amber

T – Mineral oil, CA – Coconut oil, D – Dry and W - Wet

0.001

0.01

0.1

1

10

100

1000

10000

0.0001 0.001 0.01 0.1 1 10 100 1000

Perm

ittivity e"

Frequency [Hz]

A‐measured A1‐measured

A2‐measured A3‐measured

A4‐measured Oil‐measured



Figure 6 shows the IFT test results obtained for both coconut oil and mineral oil samples. Some results at higher aging periods could not be obtained. From the results, it is clearly visible that at the beginning both mineral and coconut oils had similar and higher IFT values than required by the standard [23] which is marked as RL. However, the IFT values reduced in both oil types during aging and coconut oil showed a drastic change in that respect. Usually sludge and wax formation could be a cause for the reduction of IFT. When coconut oil heats for a longer duration, weaker substances with low density might have formed and rise to the oil-air interface so that IFT values reduce with the aging. Water solubility is also an important factor. Further studies should be conducted to understand these observations.

Figure 7 shows the variation of acidity levels during aging. The acid levels increased in both mineral and coconut oil during aging, but the increase for the latter case was significant to about 10 times as the mineral oil. However, when comparing acidity levels of commercially available alternative oils such as BIOTEMP [4], the acidity levels are reported to be in a similar range. According to literature, acidity levels of other types of alternative oils too have increased with aging [8].

Recent reports have shown that new treatment methods such as Fullers Earth method can be used to reduce acidity level of natural esters significantly [27]. Based on that information, the high acidity level of coconut oil seems to be less significant and further studies are recommended to reduce the acidity level.

Figure 6.Variation of the IFT values for both coconut oil and mineral oil samples with respect of the aging period.

Figure 7. Variation of the acidity values for both coconut oil and mineral oil samples with respect of the aging period.

Figures 8 and 9 shows the real and imaginary permittivity values obtained from the FDS measurements. Table 8 shows the corresponding modeled parameters. When considering the Figures 8, 9 and Table 8, coconut oil showed aging behavior by increasing its permittivity, dielectric losses and the estimated conductivities with respect of the aging time. The real permittivity showed a significant increase at the lowest frequency which was about 350% after 7 weeks with respect of the new samples. However, at high frequencies, the permittivity values were in the same order. On the other hand, both dielectric losses and estimated conductivity values after 7 weeks aging had similar values as the beginning. In coconut oil, as for other natural esters, hydrolysis (reverse of the esterification) occurs during aging [9]. This process creates fatty acids while consuming water as shown in Figure 1. Therefore, the moisture which helps to increase oil conductivity for new oil decreases with aging. However, the acid level increases with aging so that the conductivity increases. As far as both dry and wet mineral oil samples were concerned, the permittivity, dielectric losses and estimated conductivities initially decreased and then increased with the aging time. It is reasonable to state that in mineral oil, aging process is followed by drying process.

In general, the estimated conductivity values and dielectric losses for coconut oil were comparatively higher than mineral oil. This was true for both new and aged samples. Increased conductivity is mainly due to presence of moisture and/or acid substances. When the oil samples were new, the absorbed moisture in the new wet coconut oil sample increased the conductivity. When the sample was aged, the formation of free fatty acids caused the increase of conductivity. After considering Figures 6 to 9, the FDS results were in good agreement with the acidity and IFT results.

Figure 10 shows the variation of the breakdown voltages during aging for both coconut oil and mineral oil samples. It is clearly visible that, same as most of the other vegetable oils coconut oil has the highest breakdown voltage than mineral oil with aging [8, 9].

Figure 8.Variation of measured and modeled real part of permittivity with respect of frequency for mineral oil and coconut oil at different aging periods.

42.1

29.9

174.7

11.7

11.3

11.8

42.1

16.1 23.8

20.8

1

10

100

1000

0 2 5 7

IFT (m

N/m

)

Time (weeks)

TW CA TD RL

0.02 0.05

0.05

0.050.08

2.19

2.28

2.36

0.02 0.04

0.04

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10.00

0 2 5 7

Acidity (KOH m

g/oil g

Time (weeks)

TW CA TD RL

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10

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1000

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Perm

ittivity e'

Frequency [Hz]

C‐measuredCA2‐measuredCA5‐measuredCA7‐measuredC‐modeledT‐measuredTW2‐measuredTW5‐measuredTW7‐measuredTW‐modeledTD2‐measuredTD5‐measuredTD7‐measuredTD‐measured



Figure 9. Variation of measured and modeled imaginary part of permittivity with respect of frequency for mineral oil and coconut oil at different aging periods

Table 8. Estimated response function parameters and conductivities for different aging periods.

Sample No.

A [10-

3]

n ∆ε1 τ1 [s] α1 [10-1]

∆ε2 τ2

[ms]

α2 [10-1]

σdc

(pS/m)

C 18.1 1.2 18.4 8.5 0.9 0.6 1 1 845 CA2 1.7 1.8 9 311 1.5 0.01 0.1 5 368 CA5 4.4 1.8 8 144.6 2 0.052 6.1 1 512 CA7 6 1.9 6 51.53 2 0.01 0.1 1 877

T 64.2 1.1 3 1.33 2 2.08 2 2.1 54 TW2 3E-

04 4 45 5.1 1 1.56 5.1 1.4 77.6

TW5 0.9 1.4 0.11 1.3 5 0.01 0.01

3.3 2.2

TW7 0.2 1.8 0.2 20 1 0.001 0.1 0.8 4.1 TD2 60 1.1 9 2 5 0.01 1 5 64.9 TD5 7 1.1 0.1 5.1 1 0.008 0.0

1 1 10

TD7 0.1 1.1 0.2 5 1 0.01 0.01

0.1 7

Interestingly, the breakdown voltages of coconut oil increased with the aging duration and were well above the recommended levels [23]. In general, the breakdown voltages showed a positive impact of coconut oil based on the selected worse coconut oil (i.e. copra type) despite other negative effects such as acidity, IFT and conductivities.

Figure 10. Variation of the breakdown voltage values for both coconut oil and mineral oil samples with respect of the aging period.

6 THERMAL AND ELECTRICAL FAULTS Usually transformers are subjected to thermal and electrical faults during their operations. The thermal faults are categorized T1, T2 and T3 according to temperature band as T1 (<300 0C), T2 (>300 0C) and T3 (>700 0C). The electrical faults are categorized as corona type (PD), low energy discharges (D1) and high energy discharge (D2) [7, 28, 29]. In this study thermal faults of T1 type with different intensities and electrical faults of PD and D1 were simulated [7, 29].

6.1 SAMPLES AND PROCEDURE Copra type coconut oil (brand C) and mineral oil (naphthenic type according to [23]) in volumes of 500 ml were used for the investigation. All samples were prepared inside stainless steel containers of volume 1500 ml under sealed conditions. Metal substances (Copper, Aluminum, Iron and Zinc) and pressboard pieces were included with all samples in the same amounts as for the aging (see section 5.1). The containers were closed by a separate lid and a weight (about 1 kg) was kept on the top of the container. For electrical faults, adjustable pointer was used with the lid. Schematic diagrams for thermal and electrical faults are shown in Figures 11 and 12.

Under thermal faults two sets of samples were aged under sealed condition at 160 0C for 12 h and 200 0C for 1 h to represent two different intensities of T1 [29]. In addition, the samples aged at 120 0C for 7 weeks were also used as another case of T1. The details of the samples are given in Table 9.

Figure 11.Test setup for thermal faults.

Figure 12.Test setup for electrical faults

0.0001

0.001

0.01

0.1

1

10

100

1000

10000

100000

0.0001 0.001 0.01 0.1 1 10 100 1000

perm

ittivity e"

Frequency [Hz]

CCA2‐measuredCA5‐measuredCA7‐measuredC‐modeledTTW2‐measuredTW5‐measuredTW7‐measuredTW‐modeledTD2‐measuredTD5‐measuredTD7‐measured

18.6

13.5

39

31.7

34

69.3

60.7

77.9

18.6

14.5 18.3

31.3

0

10

20

30

40

50

60

70

80

90

0 2 5 7

Break

down voltage (kV

)

Time (weeks)

TW CA TD RL



Table 9. Details of the coconut oil and mineral oil samples prepared for thermal and electrical faults.

Coconut oil Mineral oil Description C T New samples without aging

CTF120 TTF120 Aged at 120 0C for 7 weeks CTF160 TTF160 Aged at 160 0C for 12 hours CTF200 TTF200 Aged at 200 0C for 1hour

CPD TPD Partial discharge for 4 hours CBD TBD 20 electrical breakdowns

TF- Thermal fault, PD – Partial discharge, BD - Breakdown

Under electrical faults two sets of samples were subjected to partial discharges for four hours and 20 electrical breakdowns to represent the PD and D1 respectively [29] (see Table 9). The ac voltage was applied to the oil samples through a rod-plain gap with gap distance of 15 mm by 60 kVA, 60 kV 50 Hz, transformer. First corona inception voltage and breakdown voltage was obtained by increasing the voltage gradually. Then the samples (CPD, TPD) were exposed to PD at corona inception voltage. Afterwards, the other samples (CBD, TBD) were subjected to low density discharges by increasing to breakdown voltage. One minute was kept between two breakdowns.

After the faults were applied the samples were tested by dissolved gas analysis (DGA) and FDS measurements. The DGA was done using Myrkos Transformer Fault Gas Analyzer which uses the Gas chromatography method. The FDS was measured by IDA 200 using the same procedure described in section 3.2. The conductivities were obtained by fitting the FDS curves with models.

6.2 RESULTS Figures 13 and 14 show the comparison of DGA results of

coconut and Mineral oil samples for thermal and electrical faults respectively. The common dissolved gasses are CH4, C2H2, C2H4, C2H6, H2, CO and CO2 [18, 30]. Natural esters which have double bonds between carbon atoms generate dissolved hydrocarbon gases during aging and faults [30]. However, DGA analysis shows that the content of the hydrocarbons in coconut oil were insignificant during aging and faults. On the other hand, the tested mineral oil samples contained considerable amount of gases such as C2H6 and CH4 showing the formation of gasses. Considering coconut oil, when subjected to thermal or electrical faults, formation of hydrocarbons by scission of C-C and C-H bonds seems to be less effective as the C-O bonds of the triglycerides are weaker. Thus formations of hydrocarbon gasses are negligible. At the same time less solubility of hydrocarbon gases in coconut oil may also have become a reason to observe less hydrocarbon gases in coconut oil.

When considering other common gases such as H2, CO and CO2, in electrical faults, coconut oil had shown 10 to 20 times more CO than mineral oil. However, the gasses such as CO2 and H2 contents were in the similar range and for the latter case, H2 appears in the breakdowns only.

On the other hand for thermal faults the contents of those gases were in a similar range for coconut and mineral oils. The above observations could be explained as follows: Usually low energy breakdown releases more energy than partial discharges [7] so that H2 was released in low energy breakdowns. According to [28] decomposition of solid cellulose insulation, moisture and oxygen are the major reason for formation of CO and CO2 gases.

Figure 13. Variation of dissolve gases in coconut oil and mineral oil under simulated thermal faults.

Figure 14. Variation of dissolve gases in coconut oil and mineral oil under simulated electrical faults.

The inclusion of pressboard pieces during faults could be the cause for formation of such gases recorded in both coconut oil and mineral oil samples. In opposition to hydrocarbon gases, CO and CO2 might be more highly soluble in coconut oil than in mineral oil which may be the reason for the higher amount of dissolved CO and CO2 gases in coconut oil for faults over that of mineral oil.

Figures 15 to 18 show the FDS results obtained for coconut oil and mineral oil with thermal and electrical faults respectively. Table 10 shows the estimated conductivity values.

Figure 15. Variation of measured and modeled real part of permittivity with respect of frequency for mineral oil and coconut oil under different thermal faults.

30

631

3003

26

79

573

2784

204

27

223

795

1

10

100

1000

10000

H2 CH4 CO CO2 C2H4 C2H6 C2H2

Gas amount/ (ppm)

Type of gas

CTF120

TTF120

CTF200

18

385

887

20

16

760

9

285

839

28

739

3

1

10

100

1000

H2 CH4 CO CO2 C2H4 C2H6 C2H2

Gas amount/ (ppm)

Type of gas

CBD TBD

CPD TPD

0.5

5

50

0.001 0.01 0.1 1 10 100 1000

Perm

ittivity e'

Frequency [Hz]

New transformer oil measuredTTF120‐measuredTTF160‐measuredTTF200‐measuredNew coconut oil measuredCTF120‐measuredCTF160‐measuredCTF200‐measured



Figure 16.Variation of measured and modeled imaginary part of permittivity with respect of frequency for mineral oil and coconut oil under different thermal faults.

Figure 17. Variation of measured and modeled real part of permittivity with respect of frequency for mineral oil and coconut oil under different electrical faults.

Figure 18. Variation of measured and modeled imaginary part of permittivity with respect of frequency for mineral oil and coconut oil under different electrical faults.

In general, coconut oil had higher values of dielectric losses as well as permittivity’s for both thermal and electrical faults. This increase was significant for electrical faults. The estimated conductivities also showed similar behavior by having higher conductivity values for electrical faults. Once the coconut oil was subjected to faults, with the hydrolysis, acids were formed. Thus the FDS results of coconut oil showed the increased losses and conductivity values.

Table 10. Estimated conductivity values for coconut oil and mineral oil samples under different faults.

Sample No. σdc (pS/m) Sample No. σdc (pS/m) C 845 T 54

CTF120 877 TTF120 4.1 CTF160 59.5 TTF160 8.9 CTF200 161 TTF200 15.1

CPD 1498 TPD 15.1 CBD 1063 TBD 32

TF- Thermal fault, PD – Partial discharge, BD - Breakdown

7 FIELD AGING A coconut oil (purified RBD) filled sealed type distribution transformer had been installed in the Wathara area in Kesbewa, Sri Lanka in 2001 January. Its name plate parameters were three phase, 160 kV, 33 kV/400 V, 50 Hz, etc. This transformer has been supplying a 35 kVA bulk consumer (rubber factory) through a 400 V feeder and domestic consumers through another two 400 V feeders for the last 11 years. During its service, the transformer has been feeding an average load of about 40% and exposed to outdoor tropical weather conditions. The transformer worked well without having any reported failures. Figure 19 shows a photo of this transformer.

On 9th October 2012, an oil sample of volume 300 ml was taken from this transformer. The transformer was first interrupted and then the oil sample was taken from the top of the transformer tank by opening the pressure valve. The oil sample was collected in a glass bottle and kept sealed and brought to the University of Peradeniya for chemical and electrical analysis. The analysis included color, acidity viscosity and FDS measurement as described in section 5.1. The viscosity was measured with a Redwood viscosity meter which measures the time consumed to flow 50 ml of oil out from a specially designed container in constant temperature. Then the results were converted from redwood seconds to centistokes. From the FDS measurements the conductivity was estimated.

Figures 20 and 21 show the obtained permittivity and the loss tangent results from FDS measurements on RBD type field aged coconut oil sample. For comparison, the laboratory aged copra type oil samples (section 5) and neutralized and deodorized RBD type oil sample (section 4) are also included.

Figure 19. Coconut oil filled 160 kVA, 33/0.4 kV distribution transformer installed at Wathara, Kesbewa Sri Lanka in 2001.

0.0001

0.001

0.01

0.1

1

10

100

1000

10000

100000

0.001 0.01 0.1 1 10 100 1000

Perm

ittivity e"

Frequency [Hz]

New transformer oil measuredTTF120‐measuredTTF160‐measuredTTF200‐measuredNew coconut oil measuredCTF120‐measuredCTF160‐measured

0.5

5

50

0.001 0.01 0.1 1 10 100 1000

Perm

ittivity e'

Frequency [Hz]

New transformer oil measured

TPD‐measured

TBD‐measured

New coconut oil measured

CPD‐measured

CBD‐measured

0.01

0.1

1

10

100

1000

10000

100000

0.001 0.01 0.1 1 10 100 1000

Perm

ittivity e"

Frequency [Hz]

New transformer oil measured

TPD‐measured

TBD‐measured

New coocnut oil measured

CPD‐measured

CBD‐measured



Table 11 shows the summary of the test results for the field aged and laboratory tested samples. The color, acidity, conductivity, loss tangent at 50 Hz and permittivity at infinite are included.

According to the FDS measurements, the field-aged sample follows a similar pattern as the laboratory aged samples for both in permittivity and loss tangent plots. It is interesting to note that 11 years of field aging did not show any significant aging effect compared to laboratory aged samples even with the shortest duration of 2 weeks. However, compared to a new sample, the field aged sample showed higher permittivity and losses. The other tested parameters also showed a similar behavior. The acidity value for the field aged samples was higher than the recommended value (0.5 mg KOH/g oil) for mineral oil in working transformers [31]. However, the value was well below the laboratory aged samples (see Table 11). The permittivity, loss tangent value and the estimated conductivity value for the field aged sample was below than the laboratory aged samples. The loss tangent and permittivity values are also below the laboratory aged samples. Transformers in the field may not have undergone any thermal aging of the oil due to the light load. (The temperature might have been below the threshold by a good margin)

Figure 20. Variation of measured permittivity with respect of frequency for field aged coconut oil and thermally aged coconut and mineral oil.

Figure 21. Variation of measured loss tangent with respect of frequency for field aged coconut oil and thermally aged coconut and mineral oil.

The viscosity of the CFA sample was 29.3 cSt at 40 0C which was same as the viscosity of the new coconut oil sample used in [13]. The color of the CFA sample was similar to the

laboratory aged sample CA5. However, greenish color could be seen in the sample. Based on all the results of the tested and estimated parameters, the oil condition of the field aged transformer is still in good condition.

Table 11. Summary of test results.

Sample No.

Color Acidity (mg KOH/g oil)

σdc

(pS/m) Tan delta @ 50Hz

Permittivity at infinite

CA2 Pale

yellow (1)

2.2 368 0.055 2.80

CA5 Yellow

(2) 2.3 512 0.081 2.79

CA7 Brown

(3) 2.4 877 0.121 2.88

CFA Green yellow

0.5 218 0.031 2.70

8 CONCLUSIONS The work summarizes the following results:

1. As far as dielectric properties are concerned, refined, bleached and deodorized (RBD) coconut oil shows the best insulation properties whereas copra coconut oil shows the worst.

2. The conductivity of RBD coconut oil can be improved further by dehydration and neutralization. The sample which was neutralized, heated and cooled under low humid conditions showed the maximum reduction of its conductivity at 75% compared to non-treated oil samples.

3. When copra type coconut oil is thermally aged; its interfacial tension reduces, acidity level increases, dielectric loss increases, and conductivity level increases. The formation of fatty acids by hydrolysis might have caused those parameter changes. On the contrary, the breakdown voltage remains above the recommended level showing the positive aspects of insulation properties of the coconut oil.

4. When copra type coconut oil is subjected to thermal and electrical faults, its dielectric losses and conductivity levels increase due to the formation of fatty acids. However, the obtained dissolved hydrocarbon gasses are negligible compared to mineral oil.

5. The coconut oil filled sealed type distribution transformer works well during 11 years of service without having any insulation failures. The condition of the field-aged coconut oil after 11 years was relatively good compared to laboratory-aged oil samples.

6. The frequency dielectric spectroscopy (FDS) measurement provides useful information about the condition of the tested oil samples. The FDS results were in good agreement with the chemical tests.

7. The FDS can be modeled by Cole-Cole and inverse power dependence models. The estimated conductivity values obtained from those models can be used to interpret the condition of the insulation.

0.1

1

10

100

0.0001 0.001 0.01 0.1 1 10 100 1000

Perm

ittivity e'

Frequency [Hz]

New neutrlized and demoisturized

Laboratory aged (2 weeks)



Field aged (140 months)

0.001

0.01

0.1

1

10

100

1000

0.001 0.01 0.1 1 10 100 1000

Tan delta

Frequency [Hz]

New neutrlized and demoisturized




Field aged (140 months)



Based on the above observations gathered from the laboratory investigations and field performance, it can be concluded that, coconut oil can be a good choice as an alternative to the conventional mineral oil for power transformers. As far as the long-term performances of power transformers are concerned, further studies are suggested to analyze the aging aspects of treated coconut oil together with transformer solid insulation.

ACKNOWLEDGMENT The authors would like to express their deep gratitude to Ceylon Electricity Board Assets management branch for conducting the tests, Lanka Transformers PLC for providing material samples, Department of Chemical and Process Engineering of University of Peradeniya for aging the samples and Department of Electrical and Electronic Engineering and Department of Mechanical Engineering of University of Peradeniya for conducting the tests. A special thank goes to Mr. W.M.D. Wijesundara, Ceylon Electricity Board for assisting the field investigations.

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[6] A. Guushaa, E. W. Westenbrink, C. Rogers, A. S.Vaughan,and S. G. Swingler, “Aging of Biodegradable Oils and Assessment of Their Suitability for High Voltage Applications”, IEEE Trans. Dielectr. Electr. Insul., Vol. 18, No. 3, pp. 728-738, 2011.

[7] Imad-U-Khan, Z.D. Wang, I. Cotton and S. Northcote, “Dissolved Gas Analysis of Alternative Fluids for Power Transformers”, IEEE Electr. Insul. Mag., Vol. 23, No. 5, pp. 5-14, 2007.

[8] J.-I. Jeong, Jung-SikAn and C.-S. Huh, “Accelerated Aging Effects of Mineral and Vegetable Transformer Oils on Medium Voltage Power Transformers”, IEEE Trans. Dielectr. Electr. Insul., Vol. 19, No. 1, pp. 156-161, 2012.

[9] S. Tenbohlen and M. Koch, “Aging Performance and Moisture Solubility of Vegetable Oil for Power Transformers”, IEEE Trans. Power Delivery, Vol. 25, No. 2, pp. 825-830, 2010.

[10] D. Martin, N. Lelekakis, W. Guo, and Y. Odarenko, “Further Studies of a Vegetable-Oil-Filled Power Transformer”, IEEE Electr. Insul. Mag., Vol 27, No. 5, pp. 6-13, 2011.

[11] G .K. Frimpong, T.V. Oommen and R. Asano, “A Survey of Aging Characteristics of Cellulose Insulation in Natural Ester and Mineral Oil”, IEEE Electr. Insul. Mag., Vol. 27, No. 5, pp. 36-48, 2011.

[12] J. Li, Z. Zhang, P. Zou, S. Grzybowski, and M. Zahn, “Preparation of a Vegetable Oil-based Nanofluid and Investigation of Its Breakdown and Dielectric Properties”, IEEE Electr. Insul. Mag., Vol. 28, No. 5, pp. 43-50, 2012.

[13] J.R. Lucas, D.C. Abeysundara, C. Weerakoon, K.B.M.I. Perera, K.C. Obadage and K.A.I. Gunatunga, “Coconut Oil Insulated Distribution Transformer”, Annual Conf. of IEE Sri Lanka, Colombo, Sri Lanka, pp. 1-5, 2001.

[14] S. Ranawana, C.M.B. Ekanayaka, N.A.S.A. Kurera, M.A.R.M. Fernando and K.A.R. Perera, “Analysis of Insulation Characteristics of Coconut Oil as an Alternative to the Liquid Insulation of Power Transformers”, IEEE 3rd Int’l. Conf. Industrial and Information Systems (ICIIS), pp 1-5, 2008.

[15] CODEX STAN 210, “Codex standard for named vegetable oil”, 1999.

[16] D. Hinduja, Gayathri, C.S. Kalpage and M.A.R.M. Fernando, “Laboratory Investigation of Treated Coconut Oil as Transformer Liquid Insulation”, IEEE 6th Int’l. Conf. Industrial and Information Systems (ICIIS2011), Kandy, Sri Lanka, pp. 108-113, 2011.

[17] I. L. Hosier, A. Guushaa, E. W. Westenbrink, C. Rogers, A. S. Vaughan, and S. G. Swingler, “Aging of Biodegradable Oils and Assessment of Their Suitability for High Voltage Applications”, IEEE Trans. Dielectr. Electr. Insul., Vol. 18, No. 3, pp. 728-738, 2011.

[18] N. Lelekakis, D. Martin, W. Guo and J. Wijaya, “Comparison of Dissolved Gas-in-Oil Analysis Methods Using a Dissolved Gas-in-Oil Standard”, IEEE Electr. Insul. Mag., Vol. 27, No. 5, pp. 29-35, 2011.

[19] U. Gafvert, L. Adeen, M. Tapper, P. Ghasemi and B. Jonsson, “Dielectric Spectroscopy in Time and Frequency Domain Applied to Diagnostics of Power Transformers”, Int’l. 6th Conf. Properties and Application of Dielectric Materials (ICPADM), Xi’an, China, Vol. 2, pp. 825-830, 2000.

[20] T.K. Saha, “Review of Modern Diagnostic Techniques for Assessing Insulation Condition in Aged Transformers”, IEEE Trans. Dielectr. Electr. Insul., Vol. 10, No. 5, pp. 903-917, 2003.

[21] C. Ekanayake, S.M. Gubanski, K.D. Mularachchi and M.A.R.M. Fernando, “Diagnostic of Power Transformers in Sri Lanka: Application of Dielectric Spectroscopy in Frequency Domain”, IEEE Conf. Electr. Insul., Cincinnati, USA, pp. 593-596, 2001.

[22] F.M. Dayrit, O.E.M. Buenafe, E.T. Chainani, I.M.S. de Vera, I.K.D. Dimzon, E.G. Gonzales and J.E.R. Santos, “Standards for Essential Composition and Quality Factors of Commercial Virgin Coconut Oil and its Differentiation from RBD Coconut Oil and Copra Oil”, Philippine J. Sci., Vol. 136, No. 2, pp. 119-129, 2007.

[23] IEC 60296, Ed. 2,“Specification for unused mineral insulating oils for transformers and switchgear”,1982.

[24] G. Ajithkumar, Analysis, Modification and Evaluation of the Cold Flow Properties of Vegetable Oils as Base Oils for Industrial Lubricants, Ph.D. Thesis, Cochin University of Science and Technology, India, 2009.

[25] C.F. Ten, M.A.R.M. Fernando and Z.D. Wang, “Dielectric Properties Measurements of Transformer Oil, Paper and Pressboard with the Effect of Moisture and Ageing”, IEEE Conf. Electr. Insul. Dielectr. Phenomena (CEIDP), Vancouver, Canada, pp. 727-730, 2007.

[26] B.S.H.M.S.Y. Matharage, M.A.A.P. Bandara, M.A.R.M. Fernando, G.A. Jayantha, and C.S. Kalpage, “Aging Effect of Coconut Oil as Transformer Liquid Insulation- Comparison with Mineral Oil”, IEEE 7th Int’l. Conf. Industrial and Information Systems (ICIIS), Chennai, India, pp. 1-6, 2012.

[27] M. H. Abderrazzaq, and F. Hijazi, “Impact of Multi-filtration Process on the Properties of Olive Oil as a Liquid Dielectric”, IEEE Trans. Dielectr. Electr. Insul., Vol. 19, No. 5, pp. 1637-1680, 2012.

[28] IEC 60599, Ed. 2, “Mineral Oil-impregnated Electrical Equipment in Service-Guide to the Interpretation of Dissolved and Free Gases Analysis”, 1993.

[29] B.S.H.M.S.Y. Matharage, M.A.R.M. Fernando, E. Tuncer, M.A.A.P. Bandara, and C.S. Kalpage, “Coconut Oil as Transformer Liquid Insulation- Ageing and Simulated Thermal and Electrical Faults”, IEEE Conf. Electr. Insul. Dielectr. Phenomena (CEIDP), Montreal Canada, pp. 839-842, 2012.

[30] C. Perrier, M. Marugan, and A. Beroual, “DGA Comparison between Ester and Mineral Oils”, IEEE Trans. Dielectr. Electr. Insul, Vol. 19, No.5, pp. 1609-1614, 2012.

[31] IEC 60422, Ed. 2, “Supervision and Maintenance Guide for Mineral Insulating Oils in Electrical Equipment”, 1989.



M. A. R. M. Fernando (M’07-SM’10) was born in Colombo, Sri Lanka in 1966. He received the B.Sc. Eng. degree from the University of Peradeniya, Sri Lanka in 1993, the Tech. Lic., degree from the Royal Institute of Technology, Stockholm Sweden in 1997 and the Ph.D. degree from the Chalmers University, Gothenburg, Sweden in 1999. He started his career as an instructor in the University of Peradeniya in 1993 and at present, he is

a Professor. He was a guest researcher at HTWS, Zitau Germany in 1997, a guest researcher in Chalmers University of Technology SWEDEN in 2000 and an academic fellow in the Manchester University UK in 2006/2007. He is a chartered Engineer and an International Professional Engineer. He was the founder chair of IEEE Sri Lanka Power and Energy Society Chapter in 2010, the general chair of IEEE fourth International Conference on Industrial and Information Systems in 2009 and the chair of IEEE Sri Lanka central region subsection in 2009/2010. His research interests include condition monitoring, alternative insulation, problems related to outdoor insulation.

B. S. H. M. S. Y. Matharage was born in Kandy, Sri Lanka in 1987. He received the B.Sc. Eng. degree from the University of Peradeniya, Sri Lanka in 2011. He is currently working as a Ph.D. student in the University of Manchester, UK. His main research interests are Alternative insulation materials, aging effects and dielectric modeling.

M. A. A. P. Bandara was born in Ratnapura, Sri Lanka in 1986. He received the B.Sc. Eng. degree from University of Peradeniya, Sri Lanka in 2010. He started his career as an instructor in the University of Peradeniya from 2010 to 2011. He is currently working as an operation engineer in the Kotmale Power Station, Sri Lanka. His main research interests are condition monitoring and identification of faults of power and current transformers using different methods.

G. A. Jayantha was born in Matara, Sri Lanka in 1959. He received the B.Sc. Eng. degree from University of Peradeniya, Sri Lanka in 1983. He started his career as the engineer in-charge of Udawalawe power station Sri Lanka and later worked as the senior electrical engineer of Samanalawewa Hydro Power project (120 MW), Kukuleganga Hydro power project (70 MW). From

1993-2004 he worked as the senior electrical engineer of Mahaweli Hydro Power Complex in Sri Lanka. From 2004 -2011 he worked as the Chief engineer in-charge of condition monitoring of generators, transformers and HV circuit breakers of all the hydro power stations in Sri Lanka. From 1993 he was involved in fault diagnosing, major repairs and overhaul work of hydro power stations in Sri Lanka. He has served in many technical committees of hydro power projects in Sri Lanka. Since 2011 he has been working as the Deputy General Manager of Generation projects where he is the in-charge of refurbishment and modernization of hydro power stations in Sri Lanka. He has more than 28 years of service as a hydro power engineer. He is a charted engineer. Condition monitoring and fault diagnosing of Generators and Transformers are his main research interest.

C. S. Kalpage was born in Kandy, Sri Lanka in 1968. He received the B.Sc. Eng. degree from the University of Moratuwa, Sri Lanka in 1994, and the Ph.D. degree from the University of Birmingham, UK in 2005. He served in industry at capacities of Production Engineer and Chemical Engineer for nearly four years before joining the University in 1998. At present, he is the Head of the Department of Chemical and Process

Engineering. His research interests include bio-diesel, bio-ethanol and environmental engineering related research particularly of wastewater treatment by low cost materials.


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