Comparative Study of Breakdown Voltage of Mineral, Synthetic and ...

energies Article

Comparative Study of Breakdown Voltage of Mineral, Synthetic and Natural Oils and Based Mineral Oil Mixtures under AC and DC Voltages Abderrahmane Beroual 1, *, Usama Khaled 2,3 , Phanuel Seraphine Mbolo Noah 1 and Henry Sitorus 4 1 2 3 4

*

Ecole Centrale de Lyon, University of Lyon, Ampere CNRS UMR 5005, 36 avenue Guy Collongue, 69134 Ecully, France; [email protected] Electrical Engineering Department, College of Engineering, King Saud University, Riyadh 11421, Saudi Arabia; [email protected] Electrical Engineering Department, Faculty of Energy Engineering, Aswan University, Aswan 81528, Egypt Electrical Engineering Department, Engineering Faculty, Universitas Lampung (Unila), Bandar Lampung, Lampung 35141, Indonesia; [email protected] Correspondence: [email protected]; Tel.: +33-47-2186-110

Academic Editor: Pierre Trambouze Received: 27 February 2017; Accepted: 23 March 2017; Published: 10 April 2017

Abstract: This paper deals with a comparative study of AC and DC breakdown voltages of based mineral oil mixtures with natural and synthetic esters mainly used in high voltage power transformers. The goal was to analyze the performances of oil mixtures from the dielectric withstand point of view and to predict the behavior of transformers originally filled with mineral oil and re-filled with synthetic or natural ester oils when emptied for maintenance. The study concerns mixtures based on 20%, 50%, and 80% of natural and synthetic ester oils. AC breakdown voltages were measured using a sphere-sphere electrode system according to IEC 60156 specifications; the same specification was adopted for DC measurements since there is no standard specifications for this voltage waveform. A statistical analysis of the mean values, standard deviations, and histograms of breakdown voltage data was carried out. The Normal and Weibull distribution functions were used to analyze the experimental data and the best function that the data followed was used to estimate the breakdown voltage with risk of 1%, 10%, and 50% probability. It was shown that whatever the applied voltage waveforms, ester oils always have a significantly higher breakdown voltage than mineral oil. The addition of only 20% of natural or synthetic ester oil was sufficient to considerably increase the breakdown voltage of mineral oil. The dielectric strength of such a mixture is much higher than that of mineral oil alone and can reach that of ester oils. From the point of view of dielectric strength, the mixtures constitute an option for improving the performance of mineral oil. Thus, re-filling of transformers containing up to 20% mineral oil residues with ester oils, does not present any problem; it is even advantageous when considering only the breakdown voltage. Under AC, the mixtures with natural ester always follow the behavior of vegetable oil alone. With the exception of the 20% mixture of natural ester in DC, the breakdown voltage values of all the tested mixtures were in accordance with the normal distribution, which made it possible to define the breakdown voltages for the risk levels of 1%, 10%, and 50% of probability. Keywords: insulating oils; vegetable oil; synthetic oil; mineral oil; oil mixtures; AC breakdown voltage; DC breakdown voltage; re-filling of power transformers; statistical analysis; normal distribution; Weibull distribution

Energies 2017, 10, 511; doi:10.3390/en10040511

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1. Introduction Mineral oil (petroleum product) remains today, the most used dielectric liquid in power transformers because of its lower cost and good physicochemical properties. However, its impact on the environment and in particular on aquatic resources and soils (biodegradability of mineral oil below 30%) is a problem and not the least one that must be taken into account in the manufacture of new units (transformers) or the maintenance of old ones and their re-filling. Re-filling is a process of removing insulating liquid in a transformer in operation and replacing it with new oil. In general the liquid replaced is mineral oil. Indeed, it is more economical to replace the insulating liquid than to replace the transformers with new ones. However, governmental and non-governmental organizations presently recommend the replacement of mineral oils with vegetable oils whenever possible. It is therefore necessary to find new oils based on natural products (vegetable oils) therefore biodegradable whose properties would be near or even better than those of mineral oils. The problem of re-filling with other oil than mineral oil is that between 10% and 20% of the old liquid remains in the transformer. So it is necessary to ensure compatibility between the replaced oil and replacement oil and check for non-deterioration of the dielectric and physicochemical properties of the replacement oil; the mixture which will be then in the transformer must have at least the same breakdown voltage. Perrier et al [1,2] investigated mixtures consisting of 80% of mineral oil with 20% of two other kinds of insulating liquids, namely silicon and synthetic ester oils; the aim being the improvement of the performance of mineral oil. They analyzed and compared the main properties required for an insulating liquid for power transformers such as miscibility, viscosity, heat transfer, breakdown voltage (BDV), ageing stability, and electrostatic charging tendency (ECT). These authors demonstrated that a mixture of mineral oil with 20% synthetic ester oil was sufficient to improve the characteristics of mineral oil, particularly its dielectric strength, aging stability, water solubility, and all without changing its viscosity. In fact, the good solubility of water in ester oil reduces the influence of moisture on the breakdown voltage of mineral oil. The purpose of this paper is: (1) to compare the breakdown voltages of two ester oils, one synthetic and the other natural, with the breakdown voltage of mineral oil; and (2) to analyze the breakdown voltages of different mixtures based on mineral oil and these two esters at different concentrations. The goal is to predict the dielectric performances of each oil mixture before their use for re-filling high voltage oil-filled equipment especially power transformers. 2. Experiment 2.1. Oils and Oil Mixtures The tested oils are inhibited naphthenic mineral oil (naphtenic type provided by Nynas AB, Stockholm, Sweden), vegetable oil (EnvirotempTM FR3TM , Cooper Power Systems, Waukesha, WI, USA) and synthetic ester oil (MIDEL 7131, M&I Materials Ltd Hibernia, Manchester, UK). Their main characteristics are given in Table 1. Table 1. Properties of tested oils. The flash point is measured according to Pensky-Martens (PM) closed-cup method according to ASTM D93. Type of Oil

Properties

Density (20 ◦ C) g/cm3 Viscosity (40 ◦ C) mm2 /s Acid index (mg KOH/g) Pour point ◦ C Flash point ◦ C (PM method) Dissipation factor (90 ◦ C) Dielectric constant (20 ◦ C) Breakdown voltage kV Relative water content at 20 ◦ C (%)

Napthenic Mineral Oil Nynas

Synthetic Ester Oil MIDEL 7131

Natural Ester Oil EnvirotempTM FR3TM

0.89 8.9 10 ppm). water content (>10 ppm). Note that the mean value of breakdown voltage, Umean , is computed using the following equation Note that the mean value of breakdown voltage, Umean , is computed using the following equation U n௡ ܷ U =෍ mean ൌ ∑1 n௜i ܷ୫ୣୟ୬ ଵ ݊ whereU Ui iisisthe themeasured measuredbreakdown breakdownvoltage voltageof oftest testi iand andnnthe thenumber numberofofmeasurements measurementsofofthe theseries. series. where

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Energies 2017, 10, 511 AC BREAKDOWN VOLTAGE

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AC BREAKDOWN VOLTAGE

140 120 120 100 120 100 80 100 80 60 80 60 40 60 40 20 40 20 020 0 01

5 of 17

AC BREAKDOWN VOLTAGE

Breakdown voltage (kV)

Breakdown voltagevoltage (kV) Breakdown (kV)

140

6

1 1

6

11 16 21 6 11 11 16 16 21 Series 2126

26 26 31

31 31 36

SO VO MOSO VO MO

SO VO MO

36 36

Series Series

Figure 2. Distribution of AC breakdown voltages of mineral oil (MO), vegetable oil (VO), and Figure 2.Distribution Distributionof of AC AC breakdown voltages of of mineral oil (MO), vegetable oil (VO), synthetic Figure breakdown voltages mineral oil (MO), oil and (VO), and (VO), and synthetic oilDistribution (SO). Figure2. 2. of AC breakdown voltages of mineral oilvegetable (MO), vegetable oil oil (SO). synthetic oil (SO). synthetic oil (SO). DC BREAKDOWN VOLTAGE

80 80 7080 70

DC BREAKDOWN VOLTAGE

DC BREAKDOWN VOLTAGE

70

60 60 Breakdown voltage (kV) Breakdown voltage (kV) Breakdown voltage (kV)

5060 50 4050 40 30 3040 20 2030

SO VO MO

10 1020 0 10 0 1

6

01

11

6

1

16

11 Series 11

Series

6

SO VO SO MO VO MO

16 16

Figure 3. Distribution of DC breakdown voltages ofSeries mineral oil (MO), vegetable oil (VO), and

Figure breakdown voltages of mineral oil (MO),oilvegetable oil (VO), and Figure3.oil 3.Distribution Distribution ofofDCDC breakdown voltages of mineral oil (MO), vegetable (VO), and synthetic synthetic (SO). oil (SO). synthetic oilDistribution (SO). Figure 3. of DC breakdown voltages of mineral oil (MO), vegetable oil (VO), and synthetic oil (SO). SO en AC

8

8

6

6

4

2

6 4 2

4

2

0

0

2 50

60

70

80

90

100

110

050 50

60

70

80

90

100

110 Figure 4. Cont. voltage90(kV) 100 60 Breakdown 70 80 110 Breakdown voltage (kV)

6

4

4

2

40 2 45

0


0

SO en DC

8

Frequency Frequency

Frequency

Frequency Frequency Frequency

6

SO en DC

8

SO en AC

6 8 4

SO en DC

8

SO en AC

50

55

60

65

70

75

80


0 40 45 50 55 60 65 70 75 80 40 45 Breakdown 50 55 voltage 60 65(kV)70 75 80 Breakdown voltage (kV)

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6

2

Frequency

Frequency

4

4

2

2

0

0 50

14 12 10

MO en AC MO en AC

6 4 2 0

8

14 12

6

10

Frequency

8

0 60 70 80 90 100 110 120 50 60 70 80 90 Breakdown voltage (kV) 100 110 120 Breakdown voltage (kV)

Frequency

Frequency

6

8

4

6

2

4

VO en DC VO en DC

8

6 Frequency

8

6

Frequency

8

4

2

0 40

45 50 55 60 65 70 75 80 40 45 Breakdown 50 55 voltage 60 65(kV)70 75 80 Breakdown voltage (kV) MO en DC MO en DC

8

6 Frequency

VO en AC VO en AC

8

4

6 of617of 17 6 of 17

4

2

2 0 30

0 30

40 50 60 40 50 (kV) 60 Breakdown voltage Breakdown voltage (kV)

70

70

0 40

45 50 55 60 65 70 75 80 40 45 50 55 60 65 70 75 80 Breakdown voltage (kV) Breakdown voltage (kV)

Figure 4. Histograms of ACAC and DCDC breakdown voltages of oilsoils alone. Figure Figure 4. 4. Histograms Histograms of of AC and and DC breakdown breakdown voltages voltages of of oils alone. alone.

The values of the average breakdown voltage always follow the same order from the highest to The values values of of the the average average breakdown breakdown voltage voltage always always follow follow the the same same order order from from the the highest highest to to The the smallest for both voltage waveforms: first synthetic oil, then natural ester, and finally mineral oil the smallest for both voltage waveforms: first synthetic oil, then natural ester, and finally mineral oil the both voltage waveforms: first synthetic oil, then natural ester, and finally mineral (Figure 5). It can be noted that the breakdown voltage depends strongly on the waveform of the (Figure 5). 5). It can bebe noted the oil (Figure It can notedthat thatthe thebreakdown breakdownvoltage voltagedepends dependsstrongly strongly on on the the waveform of the applied voltage: it is markedly higher in AC than in DC for ester oils. This difference can be explained applied voltage: it is markedly higher in AC than in DC for ester oils. This difference can be explained applied voltage: it is markedly higher in AC than in DC for ester oils. difference can explained by the application time of the voltage which is longer for DC. On the other hand, as regards mineral by the the application application time time of of the the voltage voltage which which is is longer longer for for DC. DC. On On the the other other hand, hand, as as regards regards mineral mineral by oil, the breakdown voltage varies very little depending on the shape of the applied voltage. oil, the the breakdown breakdown voltage voltage varies variesvery verylittle littledepending dependingon onthe theshape shapeof ofthe theapplied appliedvoltage. voltage. oil, 120 100

Uaver (kV)

Uaver (kV)

80 60 40 20 0

DC 120

AC DC AC

100 80 60 40

91.25 91.25 63.2 63.2

87.25 87.25 59.4 59.4

20 0

SO

SO

VO Oils

VO

52.9 51.6 52.9 51.6

MO

MO

Oils

Figure 5. Average breakdown voltage of oils alone; the bars indicate the standard deviation. Figure 5. Average Average breakdown breakdown voltage voltage of of oils oils alone; alone; the the bars bars indicate indicatethe thestandard standarddeviation. deviation.

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The standard deviation represents the dispersion of the breakdown voltage measured with respect to its mean value. Also, vegetable oil with the highest standard deviation implies that breakdown can occur at a very distant voltage value (greater or smaller) than its mean value. 3.1.2. Hypothesis Test and Statistical Analysis of Breakdown Voltages For the statistical analysis of BDV probability, we first applied a hypothesis test to identify the distribution that the experimental data follow. In this study, we used Normal and Weibull distributions with a significance level of test α of 5% (α = 0.05). Normal and Weibull distribution hypothesis tests were performed by Shapiro Wilk [7] and Anderson-Darling tests, respectively [8]. Other criteria were also used to check the results of the hypothesis test [9]. All computations of the hypothesis test were carried out using software R [10]. The hypotheses null (H0 ) and alternative (H1 ) are:

• •

H0 : the breakdown voltages measured follow the distribution Normal (resp. Weibull); H1 : the breakdown voltages measured do not follow the normal distribution (resp. Weibull).

To identify the distribution law that the experimental data follow one computes the p-value that is the probability of making an error if the null hypothesis is rejected. In case the p-value is greater than α, the null hypothesis is accepted, and the data then follow a statistical law. Tables 3–6 summarize the compliance hypothesis test results. Table 3. Hypothesis test of conformity to normal distribution of oils alone in AC. Oil

W

p-Value

Conformity to Normal Distribution

SO VO MO

0.9549 0.9593 0.9310

0.2026 0.2793 0.0276

Yes Yes No

Table 4. Hypothesis test of conformity to Weibull distribution of oils alone in AC. Oil

A

p-Value

Conformity to Weibull Distribution

SO VO MO

0.2464 1.0485 1.4527

0.6591 0.0078 0.0008

Yes No No

Table 5. Hypothesis test of conformity to normal distribution of oils alone in DC. Oil

W

p-Value


SO VO MO

0.9277 0.9599 0.9369

0.1796 0.5099 0.2738

Yes Yes Yes

Table 6. Hypothesis test of conformity to Weibull distribution of oils alone in DC. Oil

A

p-Value


SO VO MO

0.4073 0.2111 0.8656

0.3215 0.7283 0.0213

Yes Yes No

With the exception of mineral oil, the breakdown voltage of ester oils conforms to the normal distribution irrespective of the shape of the applied voltage. In DC, for the Weibull law, the null hypothesis is accepted for ester oils, but only for synthetic ester in AC. Similar results have been reported by others [11]. Therefore, we chose the Normal law for the statistical study. The probability

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oilbetter betterfollows followsthe thenormal normaldistribution distributionwith withrespect respect tovegetable vegetableoil. oil.Furthermore, Furthermore,atatlow low oil of the breakdown voltage of tested oils is given in Figures 6to and 7. We note that under AC, synthetic probability, the breakdown voltages of natural ester are lower than the adjustment curve. In DC, the probability, the breakdown of natural ester are lower thanoil. theFurthermore, adjustment curve. DC, the oil better follows the normalvoltages distribution with respect to vegetable at low In probability, breakdown voltage at low and high probabilities lies far from the adjustment curve for all types breakdown voltage at low highester probabilities farthe from the adjustment curve types ofof the breakdown voltages ofand natural are lowerlies than adjustment curve. In DC, for theall breakdown oils. oils. voltage at low and high probabilities lies far from the adjustment curve for all types of oils. The coefficientsofofasymmetry asymmetry (skewness)and and flattening(kurtosis) (kurtosis) werecalculated calculated inorder order to The Thecoefficients coefficients of asymmetry(skewness) (skewness) andflattening flattening (kurtosis)were were calculatedin in orderto to observethe thevariations variationsbetween betweenthe thestatistical statisticalvalues valuesand andthe thereal realvalues values(Figure (Figure8).8).The Theasymmetry asymmetry observe observe the variations between the statistical values and the real values (Figure 8). The asymmetry coefficient makesit itpossible possible tocheck check if thedistribution distribution ofBDV BDV is moreororless less asymmetrical.For For a coefficient coefficientmakes makes it possibleto to checkififthe the distributionof of BDVisismore more or less asymmetrical. asymmetrical. Foraa Normal distribution it is equal to 0. Whatever the electrical constraint (DC or AC), the asymmetry Normal Normaldistribution distributionititisisequal equalto to0.0. Whatever Whateverthe theelectrical electricalconstraint constraint(DC (DCor orAC), AC),the theasymmetry asymmetry coefficientofofsynthetic syntheticester esteris isnegative. negative.This Thismeans meansthat thatthe thedistribution distributionpresents presentsananasymmetry asymmetry to coefficient coefficient of synthetic ester is negative. This means that the distribution presents an asymmetryto to the left,i.e., i.e., themajority majority ofvalues values areconcentrated concentrated tothe the rightofofthe the meanvalue. value. Forvegetable vegetable oil, the theleft, left, i.e.,the the majorityof of valuesare are concentratedto to theright right of themean mean value.For For vegetableoil, oil, this coefficient is positive in AC and negative in DC. this thiscoefficient coefficientisispositive positivein inAC ACand andnegative negative in in DC. DC. The coefficientofofflattening flattening ofa anormal normal distributionis is3;3;it itmeasures measures thedegree degree ofcrushing crushing of The Thecoefficient coefficient of flatteningof of a normaldistribution distribution is 3; it measuresthe the degreeof of crushingof of the distribution. In AC, this coefficient is less than 3 for ester oils; these distributions are then called the thedistribution. distribution.In InAC, AC,this thiscoefficient coefficientisisless lessthan than33for forester esteroils; oils;these thesedistributions distributionsare arethen thencalled called platikurtic, i.e., they have wider peaks than that a normal distribution, and therefore the values platikurtic, that ofof a normal distribution, and therefore the values ofof platikurtic,i.e., i.e.,they theyhave havewider widerpeaks peaksthan than that of a normal distribution, and therefore the values BDV are dispersed around the mean value. In addition, the probabilities of extreme values are lower BDV are dispersed around the mean value. In addition, the probabilities of extreme values are lower of BDV are dispersed around the mean value. In addition, the probabilities of extreme values are than those of a normal distribution. With the exception of natural ester which maintains a continuous than those a normal With the exception natural ester which maintains continuous lower thanofthose of adistribution. normal distribution. With the of exception of natural ester whicha maintains a flattening coefficient less than 3, the tetra-esters and the mineral oils have a coefficient greater than flattening coefficient ofof less than 3,of the tetra-esters the mineral a coefficient than continuous flattening coefficient less than 3, theand tetra-esters andoils thehave mineral oils havegreater a coefficient 3.InInthis thiscase, case, themeasured measuredvalues valuesofofBDV BDVare aremainly mainly concentratedaround aroundthe themean mean value;the the 3.greater than 3. the In this case, the measured values of BDV concentrated are mainly concentrated aroundvalue; the mean distribution presents a sharper peak and thicker tails than those of a normal distribution. distribution presents a sharper and thicker tailsthicker than those of a normal value; the distribution presentspeak a sharper peak and tails than those ofdistribution. a normal distribution.

Figure Probability normal distribution AC breakdown voltage oils alone. Figure 6.6.6. Probability ofofof normal distribution ofofof AC breakdown voltage of oils alone. Figure Probability normal distribution AC breakdown voltage ofof oils alone.

Figure 7. Probability of normal distribution of DC breakdown voltage of oils alone. Figure Probability normal distribution DC breakdown voltage oils alone. Figure 7. 7. Probability ofof normal distribution ofof DC breakdown voltage ofof oils alone.

We used the conformance the normal distribution compute BDV probabilities U1%, U10%, We used the conformance toto the normal distribution toto compute BDV probabilities U1%, U10%, andU50% U50%that thatare arethe thebreakdown breakdownvoltages voltagesfor forthe the1%, 1%,10%, 10%,and and50% 50%probability probabilityrisk risklevels levels and (Figure9).9).Calculating Calculatingthe thelow lowprobability probabilityvoltage voltage(1%) (1%)makes makesit itpossible possibletotodetermine determinea asafety safety (Figure

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9ofof1717 conformance to the normal distribution to compute BDV probabilities U1%,9U10%, and U50% that are the breakdown voltages for the 1%, 10%, and 50% probability risk levels (Figure 9). coefficient when designing electrical equipment. standard deviation (std) represents representswhen the coefficient when designing electrical standard deviation (std) the Calculating the low probability voltageequipment. (1%) makesThe itThe possible to determine a safety coefficient dispersion BDVwith with respecttotoits itsmean. mean. Thehigher higherthe the(std) standard deviation, themore moredispersed dispersed dispersion BDV respect The standard deviation, the isis designingofof electrical equipment. The standard deviation represents the dispersion of BDV with BDV. BDV. respect to its mean. The higher the standard deviation, the more dispersed is BDV.

33

55

Kurtosis Kurtosis Skewness Skewness

Kurtosis Kurtosis Skewness Skewness

44

22

33 22

11

11 00

00 SO SO

VO VO

-1-1

-1-1

SO SO

VO VO

MO MO

-2-2

(a) (a)

(b) (b)

Figure8.8.Coefficients Coefficientsofofasymmetry asymmetryand andflattening flatteningofofthe thenormal normaldistribution distributionofofAC AC(a) (a)and andDC DC(b) (b) Figure Figure 8. Coefficients of asymmetry and flattening of the normal distribution of AC (a) and DC (b) breakdownvoltages voltagesofofoils. oils. breakdown breakdown voltages of oils.

Breakdown voltage (kV) Breakdown voltage (kV)

100 100

U1% U1% U5O% U5O% U10% U10%

8080 6060 4040 2020 00

SO SO

VO VO

(a) (a) 7070

U1% U1% U5O% U5O% U10% U10%


6060 5050 4040 3030 2020 1010 00

SO SO

VO VO

MO MO

(b) (b) Figure Probability AC (a) and DC (b) breakdown voltages oils alone. Figure 9.9.9. Probability ofofof AC (a) and DC (b) breakdown voltages ofof oils alone. Figure Probability AC (a) and DC (b) breakdown voltages of oils alone.

Notethat thatfor forthe therisk risklevels levelsofof1%, 1%,10%, 10%,and and50% 50%probability, probability,synthetic syntheticester esterhas hasthe thehighest highest Note breakdownvoltages, voltages,followed followedby byvegetable vegetableoil oiland andfinally finallymineral mineraloil. oil. breakdown Theabove aboveexperimental experimentalresults resultsshow showthat thatwhatever whateverthe theshape shapeofofthe theapplied appliedvoltage voltage(DC (DCororAC), AC), The thedielectric dielectricstrength strengthofofester esteroils oils(natural (naturaland andsynthetic) synthetic)isisalways alwaysbetter betterthan thanthat thatofofmineral mineraloil. oil. the

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Note that for the risk levels of 1%, 10%, and 50% probability, synthetic ester has the highest breakdown voltages, followed by vegetable oil and finally mineral oil. Energies 2017, 10, 511 10 of 17 The above experimental results show that whatever the shape of the applied voltage (DC or AC), the dielectric strength of ester oils (natural and synthetic) is always bettertothan that ofinmineral Indeed, the good solubility of water in esters makes them very insensitive moisture contrastoil. to Indeed, the good solubility of water in esters makes them very insensitive to moisture in contrast to mineral oil. Moreover, the breakdown voltage is a function of the shape and time duration of the mineral oil. Moreover, the breakdown voltage is athan function of the shape and AC. time duration of the stress stress application. In DC, it is considerably less that measured under application. In DC, it is considerably less than that measured under AC. It is important to note that the dielectric strength is not defined by a single value of the It is important noterather that the strength not defined by aofsingle value of theextensive breakdown breakdown voltagetobut by dielectric a range of values,isand this range values is more for voltage butoil rather a range of values, and this of values more obtained extensiveunder for vegetable and vegetable and by more reduced for mineral oil.range Moreover, the is values DC are oil always more for mineral oil. Moreover, the values obtained under DC is arerecorded always more concentrated morereduced concentrated around the mean value, whereas an inverse result in AC, where they around thedispersed mean value, inverse result is recorded in AC, where they are fairly dispersed are fairly (far whereas from the an mean value). (far from the mean value).we came to the conclusion, through statistical analysis, that the breakdown As in other studies, As in other studies, camea to the conclusion, through statistical analysis, that the breakdown voltage of oils generally we follows normal distribution. voltage of oils generally follows a normal distribution. 3.2. Comparison of Breakdown Voltages of Oil Mixtures 3.2. Comparison of Breakdown Voltages of Oil Mixtures

3.2.1. Histograms of Breakdown Voltage 3.2.1. Histograms of Breakdown Voltage a. Mineral oil/natural ester mixtures a. Mineral oil/natural ester mixtures Figures 10 and 11 depict the distribution of BDV of mineral (MO)/vegetable oil (VO) mixtures Figures 10 and 11 depict the distribution of BDV of mineral (MO)/vegetable oil (VO) mixtures under AC and DC, respectively. It is of note that the highest BDV is obtained with the mixture of 20% under AC and DC, respectively. It is of note that the highest BDV is obtained with the mixture of natural ester and 80% mineral oil, which also corresponds to the highest voltage of vegetable oil alone. 20% natural ester and 80% mineral oil, which also corresponds to the highest voltage of vegetable oil Moreover, it can be observed that the smallest breakdown voltage value of the various mixtures alone. Moreover, it can be observed that the smallest breakdown voltage value of the various mixtures always remains higher than the highest value of that of mineral oil alone. always remains higher than the highest value of that of mineral oil alone. 140


120 100 80 60 40 MO 80%MO+20%VO 50%MO+50%VO 20%MO+80%VO VO

20 0 1

6

11

16

21

26

31

36

Series Figure10. 10.Distribution Distribution of breakdown voltages AC of mineral (MO)/vegetable oil (VO) Figure of breakdown voltages underunder AC of mineral (MO)/vegetable oil (VO) mixtures. mixtures.

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Breakdown voltage Breakdown voltage (kV)(kV)

90 90 80 80 70 70 60 60 50 50 40 40 30 30 20 20 10 10 0 01 1

11 of 17 11 of 17 11 of 17

MO 80%MO+20%VO MO 50%MO+50%VO 80%MO+20%VO 20%MO+80%VO 50%MO+50%VO VO 20%MO+80%VO

6 6

11

Series 11 Series

16 16

VO

Figure 11. Distribution of breakdown voltages under DC of mineral (MO)/vegetable oil (VO) Figure11. 11.Distribution Distribution of breakdown voltages DC of mineral (MO)/vegetable oil (VO) mixtures. Figure of breakdown voltages underunder DC of mineral (MO)/vegetable oil (VO) mixtures. mixtures.

The The cumulative cumulative frequency frequency breakdown breakdown probability probability curves curves of of the the various various mixtures mixtures with withvegetable vegetable The cumulative frequency breakdown probabilitythe curves ofcurves, the various mixtures with vegetable oil oil are are given given in in Figures Figures 12 12 and and 13. 13. One One observes observes from from the trend trend curves, that that the the addition addition of of natural natural oil are mineral given in Figures 12 and 13. One observes from the trendincurves, that the addition of natural ester ester in in mineral oil oilconsiderably considerably modifies modifies its its breakdown breakdown voltage voltage in AC AC and and DC DCexcept exceptfor forthe themixture mixture ester in mineral oil considerably modifies its breakdown voltage in AC and DC except for the mixture at at the the 50:50 50:50 ratio ratio under under DC DC stress stress which which exhibits exhibits aa behavior behavior very very similar similar to to that that of of mineral mineral oil oil alone. alone. at the 50:50the ratio under of DC stress which exhibits amoderate behaviorvalues very similar tobetween that of mineral alone. In natural ester gives of BDV, those ofoil the InDC, DC, theaddition addition of20% 20% natural ester gives moderate values of BDV, between those oftwo the In DC, the addition ofalone. 20% natural ester gives moderate values of BDV, between those of the two mineral and vegetable oils The 80% mixture exhibits a dielectric strength very close to that of two mineral and vegetable oils alone. The 80% mixture exhibits a dielectric strength very close to that mineral and vegetable oils alone. The 80% mixture exhibits a dielectric strength very close to that of the vegetable oil oil alone. of the vegetable alone. the vegetable oil alone.

Figure 12. 12. Linear Lineartrend trend of of the the cumulative cumulative frequency frequency of of DC DC breakdown breakdown voltage voltage of of mineral mineral oil oil and and Figure natural ester mixtures. Figure ester 12. Linear trend of the cumulative frequency of DC breakdown voltage of mineral oil and natural mixtures. natural ester mixtures.

Under DC voltage, it should be noted that the behavior of the three tested oil mixtures is similar to that of vegetable oil alone: the characteristic of natural ester oil predominates over that of mineral oil. The above remarks were confirmed by the calculation of the mean values of the breakdown voltage of the various samples as shown in Figure 14.

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Figure 13. Linear trend of the cumulative frequency of AC breakdown voltage of mineral oil and natural ester mixtures.

Under DC voltage, it should be noted that the behavior of the three tested oil mixtures is similar to that of vegetable oil alone: the characteristic of natural ester oil predominates over that of mineral oil. The above remarks were confirmed by the calculation of the mean values of the breakdown Figure Lineartrend trendofofthe thecumulative cumulative frequency frequency of oiloil and Figure 13.13.Linear of AC AC breakdown breakdownvoltage voltageofofmineral mineral and voltage of the various samples as shown in Figure 14. natural ester mixtures. natural ester mixtures.


120voltage, it should be noted that the behavior of the three tested oil mixtures is similar Under DC DC to that of vegetable oilAC alone: the characteristic of natural ester oil predominates over that of mineral 100 oil. The above remarks were confirmed by the calculation of the mean values of the breakdown 80 voltage of the various samples as shown in Figure 14. 60 120 40 100 20 80

DC AC

52,9 51,6

88,6 57

86,25 52,8

87,25

84,3 59,7

59,4

0 60 88,6

40 20

52,9 51,6

57

86,25 52,8

87,25

84,3 59,7

59,4

Figure 14. Average breakdownvoltage voltage of of mineral mineral oil ester oiloil mixtures; the bars indicate Figure 14. Average breakdown oiland andnatural natural ester mixtures; the bars indicate 0 the standard deviation. the standard deviation.

b. Mineral oil/synthetic ester mixtures b. Mineral oil/synthetic ester mixtures Figures 15 and 16 depict the distribution of BDV of mineral (MO)/Synthetic oil (SO) mixtures Figures 15 and 16 depict the distribution of BDV of mineral (MO)/Synthetic oil (SO) mixtures under AC and DC, respectively. The highest breakdown voltage is obtained with a mixture at 50%. 14. DC, Average breakdownThe voltage of mineral oil and natural ester mixtures;with the bars indicateat 50%. under Figure AC and respectively. highest breakdown voltage is oil obtained a mixture In AC, the smallest value of this mixture remains higher than the highest value of mineral oil alone. the standard deviation. In AC, the value of this mixture is remains than theofhighest of whether mineralitoil alone. Thesmallest breakdown voltage of mixtures always higher better than that mineral value oil alone is in DC or in AC. It is also of note that the 50% mixture exhibits the highest breakdown voltage, which is b. Mineral oil/synthetic ester mixtures closest to synthetic oil alone. This voltage is slightly higher than that of synthetic oil under AC stress. The 20% and have similar dielectric strength (Figures 17 and 18). Figures 1580% andmixtures 16 depict theadistribution of BDV of mineral (MO)/Synthetic oil (SO) mixtures

under AC and DC, respectively. The highest breakdown voltage is obtained with a mixture at 50%. In AC, the smallest value of this mixture remains higher than the highest value of mineral oil alone.

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140 120

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

100 140 80 120 60 100 40 MO 80%MO+20%SO 50%MO+50%SO 20%MO+80%SO SO

80 20 60 0 40 1

6

11

16

Series

21

26

31

36

MO 80%MO+20%SO Figure20 15. Distribution of breakdown voltages under AC of mineral (MO)/Synthetic50%MO+50%SO oil (SO) mixtures. 20%MO+80%SO SO

800 70

1

6

11

16

Series

21

26

31

36

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

Figure 15. Distribution Distributionof ofbreakdown breakdownvoltages voltagesunder underAC ACof ofmineral mineral(MO)/Synthetic (MO)/Synthetic oil oil (SO) (SO) mixtures. mixtures. 6015. Figure 50 80 40 70 30 60

MO 80%MO+20%SO 50%MO+50%SO 20%MO+80%SO SO

20 50 10 40 0 30

1

6

Series

11

16

MO

20 80%MO+20%SO Figure 16. Distribution of breakdown voltages under DC of mineral (MO)/Synthetic oil (SO) mixtures. 50%MO+50%SO 10 20%MO+80%SO

The breakdown voltage of mixtures is always better than that of mineralSO oil alone whether it is 0 in DC or in AC. It is also of note that the 50% mixture exhibits the highest breakdown voltage, which 1 6 16 Series 11 is closest to synthetic oil alone. This voltage is slightly higher than that of synthetic oil under AC stress. The 20% and 80% mixtures havevoltages a similar dielectric strength (Figures 17 and 18).mixtures. Figure 16. of under DC (MO)/Synthetic oil Figure 16. Distribution Distribution ofbreakdown breakdown voltages under DCof ofmineral mineral (MO)/Synthetic oil (SO) (SO) mixtures. The breakdown voltage of mixtures is always better than that of mineral oil alone whether it is in DC or in AC. It is also of note that the 50% mixture exhibits the highest breakdown voltage, which is closest to synthetic oil alone. This voltage is slightly higher than that of synthetic oil under AC stress. The 20% and 80% mixtures have a similar dielectric strength (Figures 17 and 18).

Figure 17. Linear trend of the cumulative frequency of DC breakdown voltage of mineral oil and synthetic ester mixtures.

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Figure 17. Linear 17.511 Linear trend trend of of the the cumulative cumulative frequency frequency of of DC DC breakdown breakdown voltage voltage of of mineral mineral oil oil and and EnergiesFigure 2017, 10, 14 of 17 synthetic synthetic ester ester mixtures. mixtures.

Figure Figure 18. the cumulative AC breakdown voltage of mineral oil and Figure 18. 18. Linear Linear trend trend of of the the cumulative cumulative frequency frequency of of AC AC breakdown breakdown voltage voltage of of mineral mineral oil oil and and natural ester mixtures. natural ester mixtures. natural ester mixtures.

The mean values of the breakdown voltage of mixtures are shown in Figure 19. In AC, while the The Themean meanvalues valuesof ofthe thebreakdown breakdownvoltage voltageof ofmixtures mixturesare areshown shownin inFigure Figure19. 19.In InAC, AC,while whilethe the mixture with only 20% synthetic ester is sufficient to considerably improve mineral oil, the mixture mixture toto considerably improve mineral oil,oil, thethe mixture at mixturewith withonly only20% 20%synthetic syntheticester esterisissufficient sufficient considerably improve mineral mixture at 80% deteriorates that of synthetic oil. In DC, the variations in breakdown voltage between oils 80% deteriorates that that of synthetic oil. Inoil. DC, variations in breakdown voltagevoltage betweenbetween oils remain at 80% deteriorates of synthetic Inthe DC, the variations in breakdown oils remain fairly remain fairly moderate. moderate. fairly moderate.

Breakdownvoltage voltage(kV) (kV) Breakdown

120 120 100 100

DC DC AC AC

80 80 60 60

20 20

93,25 93,25

84,5 84,5

40 40 52,9 52,9 51,6 51,6

56,6 56,6

59,6 59,6

91,25 91,25

85,15 85,15 56,2 56,2

63,2 63,2

00

Figure 19. Average Average breakdown voltage of mineral oil and synthetic ester oil mixtures; the bars indicate Figure Figure 19. 19. Average breakdown breakdown voltage voltage of of mineral mineral oil oil and and synthetic synthetic ester ester oil oil mixtures; mixtures; the the bars bars indicate indicate the standard deviation. the the standard standard deviation. deviation.

3.2.2. Statistical Analysis of Mixtures 3.2.2. 3.2.2. Statistical Statistical Analysis Analysis of of Mixtures Mixtures The analysis ofthe the conformity to the normal and Weibull distributions of BDV of the The The analysis analysis of of the conformity conformity to to the the normal normal and and Weibull Weibull distributions distributions of of BDV BDV of of the the different different different mixtures, by taking into account the previously stated points (hypothesis test, value of α), mixtures, mixtures, by by taking taking into into account account the the previously previously stated stated points points (hypothesis (hypothesis test, test, value value of of α), α), is is is summarized in Tables7–10. 7–10. summarized in Tables summarized in Tables 7–10. It can be noted that the mixtures follow above all a anormal distribution whether they areare in DCDC or It It can can be be noted noted that that the the mixtures mixtures follow follow above above all all a normal normal distribution distribution whether whether they they are in in DC AC. The fact of whether oil alone follows a statistical law or not does not influence the conformity of or or AC. AC. The The fact fact of of whether whether oil oil alone alone follows follows aa statistical statistical law law or or not not does does not not influence influence the the conformity conformity the mixture. It is forfor example, under DC stress, the case of mineral and vegetable oils, which alone of of the the mixture. mixture. It It is is for example, example, under under DC DC stress, stress, the the case case of of mineral mineral and and vegetable vegetable oils, oils, which which alone alone follow a normal law, contrary to the case of a mixture with 20% ester oil. follow a normal law, contrary to the case of a mixture with 20% ester oil. follow a normal law, contrary to the case of a mixture with 20% ester oil.

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Table 7. Hypothesis test of conformity to normal distribution of oil mixtures in AC. Oil Mixtures

W

p-Value


20% VO + 80% MO 50% VO + 50% MO 80% VO + 20% MO 20% SO + 80% MO 50% SO + 50% MO 80% SO + 20% MO

0.973 0.951 0.964 0.971 0.984 0.950

0.533 0.142 0.372 0.484 0.870 0.119

Yes Yes Yes Yes Yes Yes

Table 8. Hypothesis test of conformity to Weibull distribution of oil mixtures in AC. Oil Mixtures

A

p-Value


20% VO + 80% MO 50% VO + 50% MO 80%VO + 20% MO 20% SO + 80% MO 50% SO + 50% MO 80% SO + 20% MO

1,022 1,044 1,188 0,484 0,444 0,413

0.009 0.007 0.003 0.217 0.271 0.320

No No No Yes Yes Yes

Table 9. Hypothesis test of conformity to normal distribution of oil mixtures in DC. Oil Mixtures

W

p-Value


20% VO + 80% MO 50% VO + 50% MO 80% VO + 20% MO 20% SO + 80% MO 50% SO + 50% MO 80% SO + 20% MO

0.862 0.935 0.962 0.977 0.965 0.921

0.006 0.257 0.570 0.879 0.633 0.116

No Yes Yes Yes Yes Yes

Table 10. Hypothesis test of conformity to Weibull distribution of oil mixtures in DC. Oil Mixtures

A

20% VO + 80% MO 50% VO + 50% MO 80% VO + 20% MO 20% SO + 80% MO 50% SO + 50% MO SO + 20% MO Energies 2017,80% 10, 511

2.666 0.310 0.494 0.301 0.380 1.525


10−6

1.1 × 0.510 0.198 0.530 0.368 0.0004

U1%

No Yes Yes Yes Yes No

U50%

U10%


100 90 80 70 60 50 40 30 20 10 0

p-Value

Figure 20. Probability of AC breakdown voltage of oil mixtures. Figure 20. Probability of AC breakdown voltage of oil mixtures. 70

ge (kV)

60 50

U1%

U50%

U10%

16 of 17

Breakdown voltage (

70 60 50 40 30 Energies 2017, 10, 511 16 of 17 20 10 Figures 20 and 21 give the0 voltages U1%, U10%, and U50% determined using compliance with the

Normal law. We observe that mineral oil has the least interesting dielectric strength compared to ester oils and mixtures; it has the smallest breakdown voltages with risks of 1%, 10%, and 50% probabilities. The mixtures giving the best BDV in DC are mixtures with 80% natural ester and 50% synthetic ester Figure 20. Probability of AC breakdown voltage of oil mixtures. while in AC the best mixture is that with 50% synthetic ester oil. 70

U1%

U50%

U10%


60 50 40 30 20 10 0

Figure 21. 21. Probability Probability ofofDC Figure DCbreakdown breakdownvoltage voltageofofoil oilmixtures. mixtures.

4. Conclusions Conclusions 4. This work work showed showed that: that: This Whateverthe thetype type of voltage or ester AC),oils ester oils have always have a significantly higher Whatever of voltage (DC (DC or AC), always a significantly higher breakdown breakdown voltage than mineral oil. Thus vegetable oil can be used as a satisfactory alternative to voltage than mineral oil. Thus vegetable oil can be used as a satisfactory alternative to mineral oil in mineral oil in power transformers. power transformers. The mixtures mixtures may may be be an an option option for for improving improving the the performance performance of of mineral mineral oil, oil, from from the the point point of of The view of dielectric strength. The addition of only 20% of natural or synthetic ester oil is sufficient to view of dielectric strength. The addition only 20% of natural or synthetic ester oil is sufficient to considerably increase increasethe thebreakdown breakdownvoltage voltageof ofmineral mineraloil. oil. The The dielectric dielectric strength strength of of such such aa mixture mixture considerably is much much higher higher than thanthat thatof ofmineral mineraloil oilalone aloneand andcan canreach reachthat thatof ofester esteroils. oils. is The re-filling re-filling of of transformers transformers can can be be considered considered with with mixtures mixtures composed composed of of 20% 20% of of mineral mineral oil oil The and 80% 80% of of ester ester oil. oil. In In the case of vegetable oil, the breakdown breakdown voltage voltage of of the the mixture mixture remains remains equal equal and to that thatofofnatural natural ester alone. In contrast, the replacement used replacement oil is synthetic the to ester alone. In contrast, whenwhen the used oil is synthetic ester, the ester, dielectric dielectricofstrength of the is decreased in comparison with that ofoil. synthetic oil. strength the mixture is mixture decreased in comparison with that of synthetic Under AC, AC, the the mixtures mixtures with with natural natural ester ester always always follow follow the the behavior behavior of of vegetable vegetable oil alone, alone, Under unlike mixtures with synthetic ester, which have a breakdown voltage which varies considerably unlike mixtures with synthetic ester, which have a breakdown voltage which varies considerably with with added concentration to exceeding of synthetic oil alone. added concentration up to up exceeding that ofthat synthetic oil alone. Under DC, DC, whatever whatever the the nature nature of of ester ester (natural (natural or or synthetic), synthetic), the the dielectric dielectric strength strength of of mixtures mixtures Under variesas as aa function function of of the the concentration concentrationadded addedto to mineral mineraloil oil and and is is between between that that of of mineral mineral oil oil alone alone varies and ester ester oils oils alone. alone. and The breakdown breakdownvoltage voltagegenerally generallyfollows followsaanormal normaldistribution distributionwhatever whateverthe theoil oilor ormixture. mixture. The WithWith the the exception of the 20% mixture of of natural ester in exception of the 20% mixture natural esterininDC, DC,all allthe the tested tested mixtures mixtures were in accordancewith withthe thenormal normaldistribution, distribution,which which made it possible to define breakdown voltages accordance made it possible to define thethe breakdown voltages for for risk the risk levels of 1%, of probability. the levels of 1%, 10%,10%, and and 50%50% of probability. Acknowledgments: The authors would like to thank the International Scientific Partnership Program (ISPP) at King Saud University for funding this research work through ISPP#0047. The authors extend their appreciation to Twasol Research Excellence Program, “TRE Program”, King Saud University, Saudi Arabia for its support. Author Contributions: All the authors contributed to the realization of experimental arrangements, preparation of oil mixtures and all experimental measurements as well as the writing of the paper. Conflicts of Interest: The authors declare no conflict of interest.

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References and Note 1. 2.

3. 4.

5. 6. 7. 8. 9. 10. 11.

Perrier, C.; Beroual, A.; Bessede, J.-L. Improvement of power transformer by using mixtures of mineral oil with synthetic esters. IEEE Trans. Dielectr. Electr. Insul. 2006, 13, 556–564. [CrossRef] Perrier, C.; Beroual, A.; Bessede, J.-L. Improvement of mineral oil properties by mixing it with synthetic esters. In Proceedings of the 15th IEEE International Conference on Dielectric Liquids (ICDL), Coimbra, Portugal, 26 June–1 July 2005. Norme CEI 60814 (deuxième édition), Isolants liquides—Cartons et papiers imprégnés d'huile Détermination de la teneur en eau par titrage coulométrique de Karl Fischer automatique, August 1997. Mettler Toledo Titrators DL32/DL39, Fundamentals of the Coulometric Karl Fischer Titration with Selected Application, Application Brochure. Available online: http://www.mt.com/fr/fr/home.html (accessed on 31 March 2017). Insulating Liquids—Determination of the Breakdown Voltage at Power Frequency—Test Method; IEC 60156 Ed. 2; International Electrotechnical Commission (IEC): Geneva, Switzerland, 1995. IEEE Guide for Acceptance and Maintenance of Natural Ester Fluids in Transformers; IEEE Std C57.147; IEEE Standard Association: Washington, DC, USA, 2008. Shapiro, S.S.; Wilk, M.B. An analysis of variance test for normality (complete samples). Biometrika 1965, 52, 591–611. [CrossRef] Anderson, T.W.; Darling, D.A. Asymptotic theory of certain “goodness-of-fit” criteria based on stochastic processes. Ann. Math. Stat. 1952, 23, 193–212. [CrossRef] Pearson, K. On lines and planes of closest fit to systems of points in space. Philos. Mag. 1901, 2, 559–572. [CrossRef] The R Project for Statistical Computing. Available online: https://www.r-project.org/ (accessed on 31 March 2017). Dang, V.H.; Beroual, A.; Perrier, C. Comparative Study of Statistical Breakdown in Mineral, Synthetic and Natural Oil under AC Voltage. IEEE Trans. Dielectr. Electr. Insul. 2006, 13, 556–564. © 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).