Fourier Transform Infrared (FTIR) Spectroscopy Analysis of Transformer Paper in Mineral Oil-Paper Composite Insulation Under Accelerated Thermal Aging

energies

Article Fourier Transform Infrared (FTIR) Spectroscopy Analysis of Transformer Paper in Mineral Oil-Paper Composite Insulation under Accelerated Thermal Aging

Abi Munajad * ID , Cahyo Subroto ID and Suwarno ID

School of Electrical Engineering and Informatics, Institut Teknologi Bandung, Bandung 40132, Indonesia; [email protected] (C.S.); [email protected] (S.) * Correspondence: [email protected]; Tel.: +62-857-465-76059

Received: 26 December 2017; Accepted: 29 January 2018; Published: 4 February 2018

Abstract: Mineral oil is the most popular insulating liquid for high voltage transformers due to its function as a cooling liquid and an electrical insulator. Kraft paper has been widely used as transformer solid insulation for a long time already. The degradation process of transformer paper due to thermal aging in mineral oil can change the physical and chemical structure of the cellulose paper. Fourier transform infrared (FTIR) spectroscopy analysis was used to identify changes in the chemical structure of transformer paper aged in mineral oil. FTIR results show that the intensity of the peak absorbance of the O–H functional group decreased with aging but the intensity of the peak absorbance of the C–H and C=O functional groups increased with aging. Changes in the chemical structure of the cellulose paper during thermal aging in mineral oil can be analyzed by an oxidation process of the cellulose paper and the reaction process between the carboxylic acids in the mineral oil and the hydroxyl groups on the cellulose. The correlation between the functional groups and the average number of chain scissions of transformer paper gives initial information that the transformer paper performance can be identiﬁed by using a spectroscopic technique as a non-destructive diagnostic technique.

Keywords: transformer paper; mineral oil; thermal aging; Fourier transform infrared (FTIR) spectroscopy

1. Introduction Operational conditions of the transformer such as increased loading and short-circuit power might affect the ability of the transformer to endure the mechanical, electrical, and thermal stresses that occur. Those situations may accelerate the thermal aging of the transformer because electrical and mechanical properties of the transformer become worse due to those stresses. Cellulose paper has been used in power transformers as solid insulation for 100 years [1]. Cellulose is a homopolymer of D-anhydroglucose units bonded together with C1–C4 glycosidic oxygen linkages [2]. Cellulose paper plays an important role in considering the lifetime of power transformers [3–5]. Cellulose has been characterized by the degree of polymerization (DP), which is the average number of glucose rings of its polymeric chain [6,7]. The DP that is related to the tensile strength of the paper decreases with aging. At low DP levels, the ability to withstand high mechanical stresses is signiﬁcantly reduced, increasing the risk of electrical failures such as generating the inrush current through windings [6,7]. By considering the effects of the damage of solid insulation of transformers, it can be concluded that the life of a transformer is directly related to its solid insulation condition. During operation, the degradation process of the cellulose paper takes place due to thermal aging, which is the dominant

Energies 2018, 11, 364; doi:10.3390/en11020364 www.mdpi.com/journal/energies Energies 2018, 11, 364 2 of 12 aging in power transformers. The lifetime of the solid insulation is affected by temperature, water, and oxygen [8]. The aging mechanism of solid insulation can be explained by the degradation process of cellulose, which is the combined effect of pyrolysis, hydrolysis, and oxidation [7]. Cellulose paper and insulating liquid are the main parts of power transformers’ insulation. Nowadays, mineral oil is the most popular insulating liquid for high voltage transformers. Mineral oil has been used not only for liquid insulation but also as a cooling medium of power transformers. Mineral oil is well known as an insulating liquid for power transformers due to several advantages: good compatibility with cellulose paper as solid insulation, good physical and electrical properties, suitable properties such as good electrical arc quenching, availability, low cost, and long history [9–11]. Mineral oil is a complex mixture of several hydrocarbon molecules and its chemical formula consists of alkenes (paraffin), cyclic alkenes (naphthene), and aromatics [9,11]. Oxidation is the major degradation mechanism of hydrocarbon oil and the rate of oxidation depends on temperature [12]. The degradation process due to thermal aging can change the physical and chemical structure of transformer paper in mineral oil. Fourier transform infrared (FTIR) spectroscopy analysis can identify chemical bonds by using an infrared spectrum that is absorbed by the material. Spectroscopy is a powerful non-destructive technique that uses an electromagnetic radiation interaction effect to determine the atomic or molecular structure and the energy level of the substance [13]. So far, many researchers have developed the non-destructive technique to measure the condition of both the oil and the paper in a power transformer using spectroscopy techniques. For insulating liquid performance measurements, the combination of spectroscopy techniques and an artificial neural network is used to obtain the correlation between the spectral response and the chemical and physical properties of the oil. This combined method has been used to investigate a 2-FAL compound in transformer oil, the interfacial tension number of transformer oil, and various dissolved gases within transformer oil [13–15]. For insulating paper performance measurements, the spectroscopy technique and statistical analyses have been developed to obtain a correlation between the spectral response and the chemical properties of the paper. Several works have reported that this combined method has shown the possibility of using the spectroscopy technique for determining the degree of polymerization (DP) and water content of the transformer paper [6,16–18]. Currently, destructive methods such as viscometry have been widely used for the direct measurement of DP [6,19]. This paper reports the chemical structures of transformer paper aged in mineral oil using Fourier transform infrared (FTIR) spectroscopy. Fourier transform infrared (FTIR) spectroscopy shows the intensity of the peak absorbance of the functional groups of transformer paper, which can identify the chemical bond in a molecule of cellulose paper. This study was conducted to provide a better understanding of transformer paper characteristics in mineral oil during thermal aging using Fourier transform infrared (FTIR) spectroscopy. This study also gives initial information that the condition of the cellulose paper as transformer solid insulation, which has a direct correlation with the life of the transformer, can be identiﬁed by using a spectroscopy technique.

2. Experimental Specimens and FTIR Measurement

2.1. Preparation of Experimental Specimens The sample needed for this experiment was a copper conductor, kraft paper, and mineral oil. Mineral oil used in this experiment is an uninhibited transformer oil that is typically used for high voltage transformers and conforms to IEC 60296. The transformer paper (thickness of 0.061 mm and a base weight of 0.0064 g/cm2) used for these studies was kraft paper, which is available commercially for power transformer windings. The initial water content of all kraft paper samples was about 5%. The copper conductor was needed in this experiment in order to obtain the real conditions of a transformer where the transformer coils are insulated by kraft paper and immersed in insulating liquid. IEEE Standard C57.100–2011 also recommends using a copper conductor in experiments relating to transformer solid insulation [20]. A total amount of 10 g kraft paper, 8.75 g copper conductor, and 200 g mineral oil were placed in a hermetical glass bottle [17,21]. A pre-treatment process was carried Energies 2018, 11, 364 3 of 12 out on the mineral oil samples by putting the mineral oil in the oven for 24 h at 100 ◦C[9,11,17]. The purposeEnergies 2018 of, 11 this, x FOR step PEER was REVIEW to reduce the amount of water content inside the mineral oil. 3 of 12 Kraft paper was cut into strips and wrapped around the copper conductor before the paper samples on the mineral oil samples by putting the mineral oil in the oven for 24 h at 100 °C [9,11,17]. The purpose wereof inserted this step intowas heat-resistantto reduce the amount glass of bottles. water content After that,inside eachthe mineral aging oil. test bottle was sealed. In this experiment,Kraft thermal paper was aging cut usinginto strips a sealed and wrapped system wasaround chosen the copper based conductor on the recommendation before the paper samples of the IEEE Standardwere C57.91–1995 inserted into [heat22].-resistant An accelerated glass bottles. thermal After aging that, test each was aging conducted test bottle to was obtain sealed. the agedIn this paper samplesexperiment, in a short thermal time. aging All of using the sealeda sealed glass system bottles was chosen were placedbased on in the two recommendation different ovens of forthe agingIEEE and heatedStandard to 120 C57.91◦C and–1995 150 [22].◦C, An respectively, acceleratedfor thermal a certain aging period test was of conducted time. Samples to obtain were the takenaged paper out of the ovenssamples at regular in a intervals:short time.336, All of 672, the and sealed 1008 glass h. Agingbottles were at a temperatureplaced in two of different 150 ◦C ovens was selectedfor aging according and to a referenceheated to 12 published0 °C and 15 in0 °C IEEE, respectively by Mc Shane, for a certain et al. [ 23period], while of time. aging Samples at a temperature were taken out of of 120 the oven◦C iss a hot at regular intervals: 336, 672, and 1008 h. Aging at a temperature of 150 °C was selected according to a spot temperature according to the IEEE [24]. A list of samples is shown in Table1. reference published in IEEE by Mc Shane et al. [23], while aging at a temperature of 120 °C is a hot spot temperature according to the IEEE [24].Table A li 1.stSample of samples and is treatment. shown in Table 1.

TableSample 1. Sample and treatment Aging . MO.T0 Initial state Sample Aging MO.T1.120 120 ◦C for 336 h MO.T0MO.T2.120 Initial 120 ◦C state for 672 h MO.T1.120MO.T3.120 120 120 °C◦ Cfor for 336 1008 h h ◦ MO.T2.120MO.T1.150 120 150 °C forC for 672 336 h h ◦ MO.T3.120MO.T2.150 120 150 °C forCfor 1008 672 h h MO.T3.150 150 ◦C for 1008 h MO.T1.150 150 °C for 336 h MO.T2.150 150 °C for 672 h 2.2. Fourier Transform Infrared (FTIR)MO.T3.150 Spectroscopy 150 °C for 1008 h

Electromagnetic2.2. Fourier Transform radiation Infrared that(FTIR) interacts Spectroscopy with a substance can be absorbed, transmitted, reflected, scattered, or have photoluminescence (PL), which provides significant information on the molecular Electromagnetic radiation that interacts with a substance can be absorbed, transmitted, reflected, structure and the energy level transition of that substance [13,25]. Paper samples placed in the path of scattered, or have photoluminescence (PL), which provides significant information on the molecular an infrared beam will absorb and transmit light and then the light signal will penetrate the sample structure and the energy level transition of that substance [13,25]. Paper samples placed in the path to theof detector. an infrared The beam detector will absorb measures and transmit the intensity light and of then the the radiation light signal moving will penetrate into a sample the sample and the intensityto the of detector. the radiation The detector transmitting measures through the intensity a sample. of the Figure radiation1a showsmoving the into schematic a sample and diagram the of an FTIRintensity spectrometer. of the radiation Its output transmitting as a function through ofa sample. time is Figure converted 1a shows into the a plot schematic of absorption diagram againstof wavenumberan FTIR spectrometer. by a computer Its usingoutput a as Fourier a function transform of time methodis converted [25]. into In this a plot experiment, of absorption the agains applicationt of anwavenumber ALPHA FTIR by spectrometera computer using was useda Fourier to measure transform infrared method spectra [25]. Inof the this functional experiment, groups the of transformerapplication paper. of an An ALPHA ALPHA FTIR FTIR spectrometer spectrometer was used is shown to measure in Figure infrared1b. spectra FTIRusing of the anfunctional attenuated totalgroups reflection of transformer (ATR) technique paper. An was ALPHA used inFTIR this spectrometer experiment is toshown investigate in Figure the 1b. structuralFTIR using changes an attenuated total reflection (ATR) technique was used in this experiment to investigate the structural of cellulose paper by obtaining its infrared spectra. The penetration depth of a light beam using changes of cellulose paper by obtaining its infrared spectra. The penetration depth of a light beam µ an ATRusing technique an ATR technique into a sample into a is sample about is 0.5–3 aboutm 0.5 [–73]. μm This [7]. measurement This measurement only neededonly needed small small samples of transformersamples of transformer paper and paper each and sample each sample was conducted was conducted twice twice to ensure to ensure the the infrared infrared spectra of of the investigatedthe investigated paper samples. paper samples. The observed The observed spectra spectra are the are absorbance the absorbance of the of different the different paper pa samplesper − versussamples the wavenumber versus the wavenumber range 4000–400 range cm4000–1400. cm−1.

(a) (b)

Figure 1. (a) Schematic diagram of Fourier transform infrared (FTIR) spectrometer. (b) ALPHA FTIR Figure 1. (a) Schematic diagram of Fourier transform infrared (FTIR) spectrometer. (b) ALPHA spectrometer. FTIR spectrometer.

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3. Results and Discussion EnergiesEnergies 2018Energies ,2018 11Energies, x ,2018 11FOR, x2018, 11 FORPEER, x,, 11 FORPEER REVIEW,, xx FOR FORPEER REVIEW PEERPEER REVIEW REVIEWREVIEW 4 of 124 of 124 of 124 of 12

3.3.1. Results3. Fourier Results3. Resultsand3. TransformResults and Discussion andDiscussion and Discussion Infrared Discussion (FTIR) Spectroscopy Analysis of Transformer Paper During operation, the oil–paper composite insulation within a transformer experiences an aging 3.1. 3.1.Fourier 3.1.Fourier Transform3.1.Fourier TransformFourier Transform Infrared Transform Infrared Infrared(FTIR) Infrared(FTIR) Spectroscopy (FTIR) Spectroscopy (FTIR) Spectroscopy Spectroscopy Analysis Analysis Analysis of TransformerAnalysis of Transformer of Transformer of Transformer Paper Paper Paper Paper process that is caused by a combination of thermal, electrical, and mechanical stresses. Thermal stress in oil–paperDuringDuring Duringoperation, composite Duringoperation, operation, the operation, insulation oilthe– paperoilthe– papertheoil composite– is oilpaper the composite–paper most composite insulation composite signiﬁcant insulation insulation within insulation factorwithin a within transformer in awithin transformer insulation a transformer a transformer exper aging experiences experiences [26 exper an].iences Dueaging aniences aging an to aging thermal an aging processstress,process processthat the processthat is degradation caused that is caused that is bycaused is a bycaused combination process a by combination a by combination ofa combination insulation of thermal, of thermal, of thermal, materialselectricalof thermal, electrical electrical, and withinelectrical, and mechanical, andmechanical transformers,, and mechanical mechanicalstresses. stresses. stresses. leadsThermal stresses. Thermal to Thermal changesstress Thermal stress stress in stress the in oil–paper composite insulation is the most significant factor in insulation aging [26]. Due to thermal inchemical oilin– paperoilin– paperoil structurein composite– paperoil composite–paper composite of insulation composite the insulation insulation insulation is insulation the is mostthe material. is mostthe significant is mostthe significant Amost significant chemical factor significant factor in factor reaction insulation in factor insulation in insulation takesin aging insulation aging place [26]. aging [26]. duringDue aging [26]. Due to [26].t Duehermal theto t Duehermal degradationto t hermal to t hermal stressstress, thestress, thedegradationstress, thedegradation,, thedegradation degradation process process processof insulation processof insulation of insulation of materialsinsulation materials materials within materials within transformerwithin transformerwithin transformer transformers leadss leads sto leads change sto leads change to schange toin schangethe in sthe ins the in the stressprocess., the Tabledegradation2 shows process the visual of insulation appearances materials of within paper transformer surfaces ageds leads in mineralto change oil.s in Thethe color of chemicalchemicalchemical structurechemical structure structure of structure the of insulationthe of insulationthe of theinsulation material insulation material material. A materialchemical. A chemical. A chemical.. Areaction chemical reaction reaction takes reaction takes place takes place takesduring place during place theduring degradationtheduring degradationthe thedegradation degradation the paper surface becomes darker with aging. Solid dielectrics become darker as the aging time and process.process.process. Tableprocess. Table 2 Tableshows 2 Tableshows 2 the shows 2 visualthe shows visualthe appearances thevisual appearances visual appearances appearances of paper of paper of surfac paper of surfac papere ssurfac agede ssurfac aged eins agedmineral eins agedmineral in mineral oil.in mineral Theoil. Theoil.color oil.Thecolor of The colorthe of colorthe of the of the papertemperaturepaper surfacepaper surfacepaper surfacebecomes increase. surfacebecomes becomes darker becomes It darker can withdarker be withdarker statedaging. with aging. with thatSolidaging. Solidaging. andielectrics Solid old dielectrics Solid transformerdielectrics becomedielectrics become becomedarker might become darker asdarker have theasdarker the aging darkeras theagingas time theaging solid time agingand time dielectricsand time and and temperaturecomparedtemperaturetemperaturetemperature to increase a newincrease increase. one.It increasecan. It When can.be It stated.can.be ItIt higher statedcancanbe that statedbebe temperatures that statedstatedan that oldan thatthat oldtransformeran oldtransformeranan were oldoldtransformer transformer applied,transformer might might have might the have mightmight insulationdarker have darker havehave soliddarker papersoliddarkerdarker dielectrics solid dielectrics became solidsolid dielectrics dielectricsdielectrics darker comparefaster.comparecompare Thed tocompared visuala tonew da tonew one appearanced a tonew .one Wa newhen .one W hen higherone. W of henhigher.. W the temperaturehen higher paper temperature higher temperature sample temperatures weres were changed applied,s were applied,s were applied, withthe applied, insulationthe aging, insulationthe insulationthe which paper insulation paperindicates became paper became paper becamedarker changes becamedarker darker indarker the faster.microstructurefaster. Thefaster. Thefaster.visual Thevisual and appearanceThe visual chemicalappearance visual appearance appearance of structure the of thepaper of paperthe of of sample thepaper the sample paper cellulose samplechanged samplechanged paper.changed with changed with aging The with aging chemical ,with which aging, whichaging ,indicates which structure ,, indicates whichwhich indicates changes indicates of changes the changes transformerin changes in in in thepaper themicrostructure agedthemicrostructure themicrostructure in microstructure mineral and and chemical oil andchemical can and chemical bestructure chemical identiﬁed structure structure of structure the byof theusingcelluloseof thecelluloseof Fourier thecellulose paper. cellulose paper. transform Thepaper. Thepaper.chemical Thechemical infrared The chemical stru chemical structure (FTIR) structure of structure spectroscopy. theofcture theof theof the transformerFTIRtransformer wastransformertransformerused paper paper to aged paper identify aged paper in agedmineral in the agedmineral in functional mineral oilin mineral canoil canbeoil groups identified canbeoil identified canbe identified ofbe by theidentified usingby investigated usingby Fourier usingby Fourier using Fouriertransform material, Fouriertransform transform infrared transform which infrared infrared is(FTIR) showninfrared (FTIR) (FTIR) by (FTIR) the spectroscopy.intensityspectroscopy.spectroscopy. ofspectroscopy. peakFTIR FTIR absorbance.was FTIR was used FTIR was used to was usedidentify to usedidentify to identify theto identify functionalthe functionalthe thefunctional groupfunctional groups ofgroup sthe groupof sinvestigatedthe of sinvestigatedthe of theinvestigated investigated material material material, which material, which ,is which ,,is which is is shownshown byshown the byshown intensitythe by intensity theby theintensity of intensitypeak of peak ofabsorbance. peak ofabsorbance. peak absorbance. absorbance. Table 2. Visual appearances of the paper sample. TableTable 2. TableVisual 2. TableVisual 2 appearances. Visual 2 appearances.. Visual appearances appearancesof the of paperthe of paper the ofsample paperthe sample paper. sample. sample. .. Aging Time (h) AgingAging TimeAging Time Aging(h) Time (h) Time (h) (h) TemperatureTemperatureTemperatureTemperature Temperature 0 0 0 00 336 336 336 336 672 672 672 672 672 10081008 1008 1008

120 °120C °120C °120C120 °C◦ C

◦ 150 °150C °150C °150C150 °C C

FigureFigureFigure 2Figure 2shows shows2Figure shows 2 theshows the2 theFTIRshows FTIR FTIRthe spectra theFTIR spectraspectra FTIR spectraof transformer spectra of transformer of transformer of transformer paper paper paper aged paper aged paper in aged agedmineral in aged mineral in in mineral mineral oilin mineral foroil 1008 for oil 1008 foroil h. 1008forTable 1008h. 1008Table hs h.. 3Table h Tabless.. 3TableTable s 3 3s 3 and 4 show andthe peak4 show absorbance the peak absorbancevalues of the values paper of samples. the paper There samples. is a significant There is a difference significant between difference between andandand 44 show show and4 show the4 theshow peakthe peak peakthe absorbance absorbancepeak absorbance absorbance values valuesvalues of values the of of paperthe the of paperthe paper samples. paper samples. samples. samples. There There is There There a significantis a is significantis aa significantsignificant difference difference difference difference between between between between FTIRFTIR spectra,FTIR spectra,FTIR spectra, especially spectra, especially especially theespecially thepeak thepeak absorbance thepeak absorbance peak absorbance absorbancevalue value of value the of value theinfrared of theinfraredof theinfrared spectra infrared spectra spectraof the spectraof thepaper of thepaperof sample thepaper sample paper sampleat sampleat at at FTIR spectra, especially the peak absorbance value of the infrared spectra of the paper sample at agingaging temperatureaging temperatureaging temperature temperatures of s12 of0 s12°C of0 sand12°C of0 and 1215°C00 and °C15°C0 and of15°C the0 of 15°C 0samethe of°C samethe of a gingthesame a gingsame times. aging times. a gingThis times. This times.confirms This confirms This confirms that confirms that thermal that thermal that thermal stress thermal stress stress stress aging temperatures of 120 ◦C and 150 ◦C of the same aging times. This confirms that thermal stress influenceinfluenceinfluences theinfluences chemicalthes chemicalthes thechemical structure chemical structure structure of structure paper of paper of samples. paper of samples. paper samples. samples. influences the chemical structure of paper samples.

0.2 0.2 0.2 0.2 O-HO -HO -HCO -H-HC -HC -HC -H C=OC=O C=O C=O C -OC -OC -OC -O 0.150.15 0.150.15

0.1 0.1 0.1 0.1 Absorbance Absorbance Absorbance Absorbance Absorbance Absorbance Absorbance Absorbance 0.050.05 0.050.05

0 0 0 0 954 832 711 589 954 832 711 589 954 832 711 589 954 832 711 589 954 832 711 589 954 832 711 589 954 832 711 589 954 832 711 589 3998 3876 3754 3632 3511 3389 3267 3145 3024 2902 2780 2658 2537 2415 2293 2171 2050 1928 1806 1685 1563 1441 1319 1198 1076 3998 3876 3754 3632 3511 3998 3389 3876 3267 3754 3145 3632 3024 3511 2902 3389 2780 3267 2658 3145 2537 3024 2415 2902 2293 2780 2171 2658 2050 2537 1928 2415 1806 2293 1685 2171 1563 2050 1441 1928 1319 1806 1198 1685 1076 1563 1441 1319 1198 1076 3998 3876 3754 3632 3511 3389 3267 3145 3024 2902 2780 2658 2537 2415 2293 2171 2050 1928 1806 1685 1563 1441 1319 1198 1076 3998 3876 3754 3632 3511 3389 3267 3145 3024 2902 2780 2658 2537 2415 2293 2171 2050 1928 1806 1685 1563 1441 1319 1198 1076 3998 3876 3754 3632 3511 3389 3267 3145 3024 2902 2780 2658 2537 2415 2293 2171 2050 1928 1806 1685 1563 1441 1319 1198 1076 3998 3876 3754 3632 3511 3389 3267 3145 3024 2902 2780 2658 2537 2415 2293 2171 2050 1928 1806 1685 1563 1441 1319 1198 1076 3998 3876 3754 3632 3511 3389 3267 3145 3024 2902 2780 2658 2537 2415 2293 2171 2050 1928 1806 1685 1563 1441 1319 1198 1076 WavenumberWavenumberWavenumberWavenumber (cm¯¹) (cm¯¹) (cm¯¹) (cm¯¹)

MO.T0MO.T0MO.T0MO.T0MO.T3.120MO.T3.120MO.T3.120MO.T3.120MO.T3.150MO.T3.150MO.T3.150MO.T3.150

FigureFigureFigure 2.Figure 2.FTIR 2FTIRFigure. FTIR spectra 2. spectraFTIR spectra2.. FTIR of spectra transformer of ofspectra transformertransformer of transformer of papertransformer paper paper aged paper aged in agedpaper mineralaged in in mineralaged in mineral oilmineral in at oilmineral aging oilat oil aging at attemperature oil agingaging attemperature aging temperaturestemperature stemperature of s120 of °C120s of ofand °Cs120 120of and °C120◦ Cand °C and and 150 °C for150 1008 °C150 h.for °C 1008 for 1008 h. h. 150150 °C150◦ Cfor °C150 for 1008 for °C 1008 1008 forh. h.1008 h. h.

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◦ TableTable 3. 3Peak. Peak absorbance absorbance values of of IR IR spectra spectra of ofpaper paper samples samples aged aged in mineral in mineral oil at oil120 at °C 120. C.

−1 WavenumberWavenumber (cm ) (cm−1) MO.T0MO.T0 MO.T1.120 MO.T1.120 MO.T2.120 MO.T2.120 MO.T3.120MO.T3.120 2922 0.03842 0.0672 0.08303 0.10985 2922 0.03842 0.0672 0.08303 0.10985 2854 0.03715 0.0557 0.06799 0.08303 2854 0.03715 0.0557 0.06799 0.08303 1745 17450.0188 0.0188 0.0283 0.0283 0.03432 0.03432 0.058270.05827 1454 14540.03391 0.03391 0.04389 0.04389 0.03745 0.03745 0.057590.05759 1369 13690.04387 0.04387 0.04976 0.04976 0.04438 0.04438 0.055490.05549 1314 13140.05322 0.05322 0.05592 0.05592 0.05142 0.05142 0.059790.05979 1159 11590.06123 0.06123 0.06773 0.06773 0.06436 0.06436 0.08340.0834 1104 11040.09064 0.09064 0.09366 0.09366 0.08533 0.08533 0.104570.10457 1050 10500.14004 0.14004 0.14092 0.14092 0.1278 0.1278 0.149070.14907 1026 10260.15272 0.15272 0.15453 0.15453 0.14076 0.14076 0.162230.16223 815 0.05484 0.06236 0.05699 0.0658 815 0.05484 0.06236 0.05699 0.0658

◦ TableTable 4. 4Peak. Peak absorbance absorbance values of of IR IR spectra spectra of ofpaper paper samples samples aged aged in mineral in mineral oil at oil150 at °C 150. C.

Wavenumber (cm−1) MO.T0 MO.T1.150 MO.T2.150 MO.T3.150 Wavenumber (cm−1) MO.T0 MO.T1.150 MO.T2.150 MO.T3.150 2922 0.03842 0.06619 0.07869 0.10029 29222854 0.038420.03715 0.05521 0.06619 0.06416 0.078690.07608 0.10029 28541745 0.037150.0188 0.03462 0.05521 0.04943 0.064160.0555 0.07608 1745 0.0188 0.03462 0.04943 0.0555 1454 0.03391 0.04588 0.05008 0.05566 1454 0.03391 0.04588 0.05008 0.05566 1369 0.04387 0.05201 0.05488 0.05503 1369 0.04387 0.05201 0.05488 0.05503 13141314 0.053220.05322 0.06098 0.06098 0.06271 0.062710.06098 0.06098 11591159 0.061230.06123 0.07295 0.07295 0.07874 0.078740.07974 0.07974 11041104 0.090640.09064 0.09967 0.09967 0.10123 0.101230.10112 0.10112 10501050 0.140040.14004 0.14617 0.14617 0.14438 0.144380.14046 0.14046 10261026 0.152720.15272 0.15797 0.15797 0.15712 0.157120.15071 0.15071 815815 0.054840.05484 0.06179 0.06179 0.06755 0.067550.06297 0.06297

FromFrom Figure Figure2, the 2, FTIR the FTIR spectrum spectrum of new of cellulose new cellulose paper shows paper absorbance shows absorbance peaks at peaks wavenumbers at wavenumbers 3700–3000 cm−1 (which is an O–H functional group), at wavenumbers 3000–2700 cm−1 3700–3000 cm−1 (which is an O–H functional group), at wavenumbers 3000–2700 cm−1 (which is a C–H (which is a C–H functional group), and at wavenumbers 1500–900 cm−1 (which is a C–O functional functional group), and at wavenumbers 1500–900 cm−1 (which is a C–O functional group). These group). These functional groups identified are consistent with the basic structure of cellulose as functionalshown in groups Figure 3. identiﬁed are consistent with the basic structure of cellulose as shown in Figure3.

Figure 3 3.. MolecularMolecular structure structure of cellulose. of cellulose.

Figure 4a,b presents the FTIR spectra of transformer paper aged in mineral oil at aging temperaturFigure4a,bes of presents 120 °C and the 15 FTIR0 °C. These spectra figures of transformerrelate to the hydroxyl paper aged groups in of mineral the spectra. oil atThe aging ◦ ◦ temperaturespeak absorbance of 120 at 3329C and cm 150−1 representsC. These the ﬁgures O–H functional relate to thegroup. hydroxyl The peak groups absorbance of the located spectra. close The peak absorbanceto 3340 cm at−1 3329is a typical cm−1 characteristicrepresents the of O–Hcellulose functional [27]. As group.shown in The these peak figures, absorbance it is clear located that the close to 3340peak cm absorbance−1 is a typical value characteristic of the O–H functional of cellulose group [27]. located As shown close in to these3329 cm ﬁgures,−1 decrease it is clears with that aging the peak absorbancedue to the value oxidation of the process. O–H functionalThe products group of degradation located close of c toellulose 3329 cm paper−1 decreases are CO, CO with2, H2 agingO, H2, due to CH4, and furans, which are dissolved in the mineral oil [28]. During thermal aging, the cleavage of the oxidation process. The products of degradation of cellulose paper are CO, CO2,H2O, H2, CH4, andcellulose furans, chains which involves are dissolved an O– inH thefunctional mineral group. oil [28 An]. During O–H group thermal reacts aging, with the a hydrogen cleavage ofatom cellulose generating a water molecule. An alcohol is transformed into ketone by an oxidation process. In this chains involves an O–H functional group. An O–H group reacts with a hydrogen atom generating a water molecule. An alcohol is transformed into ketone by an oxidation process. In this process, the hydrogen of the O–H group is displaced toward the carbon group and the CH2OH groups react Energies 2018, 11, x FOR PEER REVIEW 6 of 12 EnergiesEnergies2018 ,201811,, 364 11, x FOR PEER REVIEW 6 of 126 of 12 process, the hydrogen of the O–H group is displaced toward the carbon group and the CH2OH groupsprocess, react the with hydrogen oxygen of linkingthe O–H the group glucose is displaced molecules, toward thereby the carbon cleaving group the and chain the as CH shown2OH in withFiguregroups oxygen 5 [29,30 react linking]. withThis the oxintensityygen glucose linking ofmolecules, peak the absorbance glucose thereby molecules, located cleaving closethereby the to chain3329cleaving cm as − shown1the can chain be in associated as Figure shown5[ within29 ,30 a]. −1 ThisreductionFigure intensity 5in [29,30 molecular of peak]. This absorbance intensityweight or of thepeak located DP absorbance value close [7]. to located 3329 cm close−1 canto 3329 be cm associated can be associated with a reduction with a in reduction in molecular weight or the DP value [7]. molecular weight or the DP value [7].

0.07 0.07 0.07 0.07 0.06 0.06 0.06 0.06 0.050.05 0.050.05 0.040.04 0.040.04 0.030.03 0.030.03

0.020.02 0.020.02

Absorbance Absorbance Absorbance Absorbance 0.010.01 0.010.01 0 0 00

3700 3645 3590 3535 3479 3424 3369 3314 3259 3203 3148 3093 3038 3700 3645 3590 3535 3479 3424 3369 3314 3259 3203 3148 3093 3038

3700 3645 3590 3535 3479 3424 3369 3314 3259 3203 3148 3093 3038 3700 3645 3590 3535 3479 3424 3369 3314 3259 3203 3148 3093 3038 WavenumberWavenumber (cm¯¹) (cm¯¹) WavenumberWavenumber (cm¯¹) (cm¯¹)

MO.T0MO.T0 MO.T1.120MO.T1.120 MO.T0MO.T0 MO.T1.150MO.T1.150 MO.T2.120 MO.T3.120 MO.T2.150 MO.T3.150 MO.T2.120 MO.T3.120 MO.T2.150 MO.T3.150 (a) (b) (a) (b) −1 Figure 4. FTIR spectra at 3700–3000 cm−1 of transformer paper aged in mineral oil at aging FigureFigure 4.4. FTIRFTIR spectra spectra at at 3700–3000 3700–3000 cm cm−1 ofof transformer transformer paper paper aged aged in in mineral mineral oil oil at at aging aging temperatures of (a) 120◦ °C and (b) 150◦ °C. temperaturestemperatures of of ( a(a)) 120 120 °CCand and ((bb)) 150150 °CC..

Figure 5. Cleavage of cellulose chains.

−1 Figure 6a,b illustrates the FigurevariationsFigure 5.5. CleavageCleavage of the FTIR ofof cellulose spectra chains.chainsaround. 2900 cm , which represent a C–H functional group. The intensity of these absorbance peaks at 2922 cm−1 and 2854 cm−1 increase withFigure aging 6a due,b illustrate to the adsorptions the variations process ofof the the mineral FTIR spectraoil to the around paper surface 2900 cm during−1, which aging represent [7]. The a −1 C–HmajorFigure functional degradation6a,b illustrates group. of Thethe the aging intensity variations mechanism of of these the in FTIRabsorbance mineral spectra oil ispeaks around oxidation at 2900 2922 [7,29 cm cm]. Oxidation−1 ,and which 2854 representdecomposes cm−1 increase a C–H −1 −1 functionalwiththe aging hydrocarbon group. due to The the molecules intensityadsorption into of process theseother absorbancesubstances of the mineral: hydroperoxide, peaks oil at to 2922 the paper cmalcohol, surfaceand aldehyde, 2854 during cm ketones agingincrease, [7].and withThe agingmajoresters. due degradation toThe the oxidation adsorption of the reaction aging process ofmechanism mineral of the mineraloil in also mineral generates oil to oil the iscarboxylic paper oxidation surface acids [7,29 during (R].– OxidationCOOH) aging, which [ 7decomposes]. The bind major degradationthe tohydrocarbon alcohol of thereby the molecules aging generating mechanism into esters. other The in substances mineral degradation oil: hydroperoxide, is process oxidation of transformer [7 ,alcohol,29]. Oxidation paper aldehyde, aged decomposes inketones mineral, and the oil is due to the combined effects of pyrolysis, hydrolysis, and oxidation [7]. Temperature, moisture, hydrocarbonesters. The oxidation molecules reaction into other of mineral substances: oil also hydroperoxide, generates carboxylic alcohol, acids aldehyde, (R–COOH) ketones,, which and esters.bind and oxygen are the key factors that determine the aging rate of cellulose paper [7]. CO2, water, furanic Theto alcohol oxidation thereby reaction generating of mineral est oilers. also The generates degradation carboxylic process acids of transformer (R–COOH), paper which aged bind in to mineral alcohol compounds, and carboxyl acids (R–COOH) form through the oxidative degradation of cellulose therebyoil is due generating to the combined esters. The effect degradations of pyrolysis, process hydrolysis of transformer, and oxidation paper aged[7]. Temperature, in mineral oil moisture is due to, paper [7]. During thermal aging, the carboxylic acids generated from the oxidation process of mineral the combined effects of pyrolysis, hydrolysis, and oxidation [7]. Temperature, moisture, and oxygen andoil oxygen can react are the with key hydroxyl factors that groups determine on the the cellulose aging rate forming of cellulose a cellulose paper graft [7]. COpolymer2, water, via furanic a are the key factors that determine the aging rate of cellulose paper [7]. CO , water, furanic compounds, compoundscondensation, and reaction carboxyl [31]: acids (R–COOH) form through the oxidative2 degradation of cellulose andpaper carboxyl [7]. During acids thermal (R–COOH) aging, form the throughcarboxylic the acids oxidative generated degradation from the of oxidation cellulose process paper [of7]. mineral During thermaloil can aging, react the with carboxylic hydroxylRCOOH acids + groupsHO generated-[Cellulose] on the from ↔ cel RCOOlulose the oxidation-[Cellulose] forming process + a H cellulose2O of mineral graft oil polymer can react via with a hydroxylcondensation groups reaction on the [31]: cellulose forming a cellulose graft polymer via a condensation reaction [31]:

↔ 2 RCOOHRCOOH + HO + HO-[Cellulose]-[Cellulose] RCOO↔ RCOO-[Cellulose]-[Cellulose] + H +O H 2O

Energies 2018, 11, 364 7 of 12

Energies 2018, 11, x FOR PEER REVIEW 7 of 12 Carboxylic acids are an organic compound that contain a carbon–oxygen double bond and an oxygen–hydrogenCarboxylic acids single are an bond. organic R is compound a hydrocarbon that contain group thata carbon has a–oxygen lot of C–H double functional bond and groups. an Theoxygen reactionEnergies–hydrogen 2018 between, 11, x FOR single the PEER carboxylic bond. REVIEW R is acidsa hydrocarbon in the mineral group oilthat and has the a lot hydroxyl of C–H functional groups on groups. the7 of cellulose 12 The paper is the cause of the increasing C–H functional group on the cellulose paper after a hydrocarbon reaction Carboxylicbetween the acids carboxylic are an organic acids in compound the mineral that oil contain and the a hydroxyl carbon–oxygen groups double on the bond cellulose and paperan group (-R) in the mineral oil is moved to the cellulose paper. The radicals generated on the cellulose is theoxygen cause– hydrogenof the increasing single bond. C–H R functional is a hydrocarbon group groupon the that cellulose has a lotpaper of C after–H functional a hydrocarbon groups. group The (- surfaceR) inreaction facilitatethe mineral between the oil the physical is carboxylicmoved attachment to acids the cellulose in the of mineral lowpaper. molecularoil Theand theradicals hydroxyl weight generated groups oil molecules on thethe cellulose cellulose and leadpaper surface to the formationfacilitateis the of causethe a layerphysical of the of increasing hydrocarbonattachment C– Hof laminationfunctional low molecular group on the weighton the cellulose cellulose oil molecules surface paper after and lead ana hydrocarbon increase to the formation in group the intensity ( -of a of thelayer correspondingR) ofin thehydrocarbon mineral tooil C–H laminationis moved vibrations to onthe thecellulose [7]. cellulose Carboxyl paper. surface acidsThe radicals in and the an mineralgenerated increase oil on andin the the thecellulose intensity hydroxyl surface of groups the of cellulosecorrespondingfacilitate paper the tophysical are C– notH vibrationattachment only thes main[7].of low Carboxyl factors molecular increasingacids weight in theoil the moleculesmineral C–H functionaloil and and lead the to group thehydroxyl formation but groups are of also a of the maincellulose factorslayer paperof in hydrocarbon the are emergence not only lamination the of main the on C=O factor the functional scellulose increasing surface group the C and– onH functionalthean celluloseincrease group in paper the but intensity surface.are also ofthe the main factorFigurecorrespondings in7 thea,b emergence presents to C–H the of vibration the FTIR C=O spectras [7].functional Carboxyl around group acids 1700 on in the cmthe cellulose−mineral1, which oil paper representsand surface.the hydroxyl the C=O groups functional of cellulose paper are not only the main factors increasing the C–H−1 functional group but are also the main group. TheFigure intensity 7a,b presents of this absorbancethe FTIR spect peakra ataround 1745 cm1700−1 cmincreases, which with represents aging. Thisthe C=O peak functional emerges on group.factor Thes in intensity the emergence of this of absorbance the C=O functional peak at group 1745 cmon −1the increases cellulose withpaper aging. surface. This peak emerges on the cellulose paper after thermal aging in mineral oil. There−1 is no absorbance peak at this functional the celluloseFigure paper 7a,b presentsafter thermal the FTIR aging spect in ramineral around oil. 1700 There cm is, which no absorbance represents peak the C=O at this functional functional groupgroup. region The for intensity new transformer of this absorbance paper. peak The at 1745 emergence cm−1 increases of the with C=O aging. functional This peak groupemerges is on due to group region for new transformer paper. The emergence of the C=O functional group is due to interactionsthe cellulose between paper the after low thermal molecular aging weight in mineral acids oil. dissolved There is no in absorbance the mineral peak oil andat this the functional cellulose [31]. interactions between the low molecular weight acids dissolved in the mineral oil and the cellulose Carboxylicgroup acids region containing for new transformer a carbon–oxygen paper. The double emergence functional of the group C=O reactfunctional with thegroup hydroxyl is due to groups [31].interactions Carboxylic betw acidseen containing the low molecular a carbon weight–oxygen acids double dissolved functional in the groupmineral react oil and with the the cellulose hydroxyl on the cellulose, forming a cellulose graft polymer and H2O. A carboxylate of the carboxylic acid groups on the cellulose, forming a cellulose graft polymer and H2O. A carboxylate of the carboxylic (RCOO-)[31]. in Carboxylic mineral oilacids binds containing to the cellulose;a carbon–oxygen this creates double the functional emergence group of react the C=Owith the functional hydroxyl group acidgroups (RCOO on-) the in mineralcellulose, oil forming binds ato cellulose the cellulose; graft polymer this creates and Hthe2O. emergence A carboxylate of theof the C=O carboxylic functional on the cellulose paper surface, as shown in Figure8. groupacid on (RCOO the cellulose-) in mineral paper oil surface, binds to as the shown cellulose; in Figure this creates 8. the emergence of the C=O functional group on the cellulose paper surface, as shown in Figure 8. 0.15 0.15 0.15 0.15 0.1 0.1 0.1 0.1 0.05 0.05

Absorbance 0.05 Absorbance 0.05

0 Absorbance 0 Absorbance 0 0 3000 2966 2932 2898 2864 2830 2796 2762 2728 3000 2968 2937 2906 2875 2844 2813 2782 2750 2719 3000 2966 2932 2898 2864 2830 2796 2762 2728 3000 Wavenumber2968 2937 2906 2875 2844 (cm2813 ¯¹2782 ) 2750 2719 Wavenumber (cm¯¹) Wavenumber (cm¯¹) Wavenumber (cm¯¹)

MO.T0 MO.T1.120 MO.T0 MO.T1.150 MO.T0 MO.T1.120 MO.T0 MO.T1.150 MO.T2.120MO.T2.120 MO.T3.120MO.T3.120 MO.T2.150MO.T2.150 MO.T3.150MO.T3.150

(a()a ) (b()b )

FTIR spectra at 3000–2700 cm−−11 of transformer paper aged in mineral oil at aging FigureFigureFigure 6. 6.FTIR 6. FTIR spectra spectra at at 3000–2700 3000–2700cm cm−1 of transformer transformer paper paper aged aged in mineral in mineral oil at oil aging at aging temperaturetemperatures ofs of(a )( a120) 120 °C °C and and (b (b) )150 150 °C °C. . temperatures of (a) 120 ◦C and (b) 150 ◦C.

0.080.08 0.060.06 0.05 0.06 0.05 0.06 0.040.04 0.04 0.03 0.04 0.03 0.02 0.02 0.02 Absorbance 0.02Absorbance 0.01 Absorbance Absorbance 0 0.010 0 0 1801 1791 1781 1771 1761 1751 1741 1731 1721 1711 1702 1801 1791 1781 1771 1761 1751 1741 1731 1721 1711 1702 1801 1791 1781 1771 1761 1751 1741 1731 1721 1711 1702 Wavenumber (cm¯¹) 1801 1791 Wavenumber1781 1771 1761 1751 (cm1741 ¯¹1731 ) 1721 1711 1702 Wavenumber (cm¯¹) Wavenumber (cm¯¹) MO.T0 MO.T1.120 MO.T0 MO.T1.150 MO.T0 MO.T1.120 MO.T2.120 MO.T3.120 MO.T2.150MO.T0 MO.T3.150MO.T1.150 MO.T2.120 MO.T3.120 MO.T2.150 MO.T3.150 (a) (b) Figure 7. FTIR spectra(a) at 1800–1700 cm−1 of transformer paper aged in(b ) mineral oil at aging Figure 7. FTIR spectra at 1800–1700 cm−1 of transformer paper aged in mineral oil at aging Figuretemperature 7. FTIRs ofspectra (a) 120 °Cat and1800 (b–1700) 150 °Ccm. −1 of transformer paper aged in mineral oil at aging temperatures of (a) 120 ◦C and (b) 150 ◦C. temperatures of (a) 120 °C and (b) 150 °C.

Energies 2018, 11, 364 8 of 12 Energies 2018, 11, x FOR PEER REVIEW 8 of 12

FigureFigure 8. 8A. A carboxylate carboxylate of a a carboxlic carboxlic acid acid..

3.2. Correlation between Average Number of Chain Scissions and the Structural Changes of Transformer 3.2. CorrelationPaper Aged in between Mineral Average Oil Number of Chain Scissions and the Structural Changes of Transformer Paper Aged in Mineral Oil The intensity of the peak absorbance of the functional groups within transformer paper changes duringThe intensity thermal of aging the peakdue to absorbance the chemical of process. the functional The higher groups the withintemperature transformer and the paperlonger changesthe duringduration thermal of thermal aging dueaging to of the transformer chemical paper process. in mineral The higheroil, the higher the temperature is the thermal and energy the longervalue. the durationThe chemical of thermal reaction aging isof faster transformer in systems paper with higher in mineral thermal oil, energy the higher. Svante is theArrhenius thermal showed energy the value. The chemicalrelationship reaction between is temperature faster in systems and the with rate higherconstant thermal by the reaction energy. in Svante Equation Arrhenius (1): showed the 퐸 relationship between temperature and the rate constant− 푎 by the reaction in Equation (1): 푘 = 퐴 ∙ 푒 푅∙푇 (1) − Ea where k is the rate constant at temperaturek T= (K),A ·Ae isR a·T constant called ‘frequency factor’, Ea is the (1) activation energy for the reaction (J/mol), and R is the universal gas constant (8.314 J/mole/K). The whereactivationk is the energy rate constant represents at the temperature energy that theT (K), moleculeA is in a constantthe initial state called of ‘frequencythe process must factor’, haveEa is the activationbefore it can energy take part for in the the reaction reaction. (J/mol), The reaction and Rrateis at the any universal time is assumed gas constant to be proportiona (8.314 J/mole/K).l to The activationthe number energy of unbroken represents polymer the chain energy bonds that in the themolecule aging of paper in the [8,32 initial]. The state Arrhenius of theprocess equation must about the thermal aging of transformer paper in mineral oil is shown in Equation (2) [33]: have before it can take part in the reaction. The reaction rate at any time is assumed to be proportional 퐸 to the number of unbroken polymer chain bonds in the aging of paper [−8 ,32푎 ]. The Arrhenius equation 퐴푣푒푟푎푔푒 푛푢푚푏푒푟 표푓 푐ℎ푎𝑖푛 푠푐𝑖푠푠𝑖표푛 = 퐴 ∙ 푒 푅∙푇 ∙ 푡 (2) about the thermal aging of transformer paper in mineral oil is shown in Equation (2) [33]: In this calculation of the average number of chain scissions, the value of A depends on the initial moisture content within the paper sample and the type of insulating− paper.Ea The value of A used for Average number of chain scission = A·e R·T ·t (2) this calculation was 21 × 108 for non-upgraded kraft paper with a high moisture content [6,34]. For the expected lifetime of the transformer, the energy activation of 111 kJ/mol was used in this In this calculation of the average number of chain scissions, the value of A depends on the initial calculation [6,34,35]. moisture content within the paper sample and the type of insulating paper. The value of A used Table 5 shows the average number of chain scissions of each paper sample. The average number × 8 for thisof chain calculation scission wass of the 21 paper10 samplefor non-upgraded increases with kraft an increasing paper with aging a high temp moistureerature at content the same [6 ,34]. For theaging expected time. The lifetime probability of the of molecule transformer,s colliding the energywith each activation other increase of 111s with kJ/mol aging wastemperature. used in this calculationAs a result, [6,34 the,35 ].kinetic energy of the molecules increases, making the effect to the activation energy ofTable a reaction.5 shows The the thermal average energ numbery increase of chaind due scissions to the kinetic of each energy paper of sample. the molecule The averages making number the of chaindegradation scissions process of the at paper a temperature sample of increases 150 °C higher with than an increasing at a temperature aging of temperature 120 °C. at the same aging time. The probability of molecules colliding with each other increases with aging temperature. As a result, the kinetic energyTable 5. Average of the molecules number of chain increases, scission makings of the paper the samples. effect to the activation energy of a reaction. The thermal energySample increased dueAvg. to the Chain kinetic Scission energy (×1000) of the molecules making the degradation process at a temperatureMO of 150 ◦C higher than at0 a temperature of 120 ◦C. MO.T1.120 1244 MO.T2.120 2487 Table 5. Average number of chain scissions of the paper samples. MO.T3.120 3731 MO.T1.150 13,839 × MO.T2.150Sample Avg. Chain Scission27,678 ( 1000) MO.T3.150MO 041,517 MO.T1.120 1244 Figure 9 shows the correlationMO.T2.120 between the intensity 2487 of the absorbance peaks of the functional MO.T3.120 3731 −1 −1 −1 −1 groups at 3329 cm , 2922 cm MO.T1.150, 2854 cm , and 1745 cm 13,839and the average chain scissions of the paper samples. The intensity of the peakMO.T2.150 absorbance of the functional 27,678 group within the transformer paper that changes with aging correlateMO.T3.150s with the aging mechanisms 41,517 during thermal aging. The intensity of the O–H functional group correlates with the average chain scissions due to the breakage of the Figure9 shows the correlation between the intensity of the absorbance peaks of the functional groups at 3329 cm−1, 2922 cm−1, 2854 cm−1, and 1745 cm−1 and the average chain scissions of the paper samples. The intensity of the peak absorbance of the functional group within the transformer paper Energies 2018, 11, 364 9 of 12

that changes with aging correlates with the aging mechanisms during thermal aging. The intensity Energies 2018, 11, x FOR PEER REVIEW 9 of 12 of the O–H functional group correlates with the average chain scissions due to the breakage of the hydrogenhydrogen bonds bonds of of cellulose. cellulose. The The reduced reduced intensity intensity of the O–HO–H functionalfunctional groupgroup can can be be attributed attributed to to the reduction of the molecular weight/degree of polymerization [7]. The average number of chain the reduction of the molecular weight/degree of polymerization [7]. The average number of chain scissions correlates with the molecular weight as shown in Equation (3) [7]: scissions correlates with the molecular weight as shown in Equation (3) [7]: 11 11 Average number of chain scission = − (3) Average number of chain scission = DP − DP (3) 퐷푃t푡 퐷푃00

DPDP0 0representsrepresents the the DPDP valuevalue of of new new transformer paper and and DPDPtt representsrepresents the theDP DPvalue value of of transformertransformer paper paper after after thermal thermal aging. aging. From From these these figures, ﬁgures, it it can be seen seen that that there there is is a a linear linear correlationcorrelation between between the the intensity intensity of of the the peak peak absorbance absorbance of the O–HO–H functionalfunctional group group and and the the average average numbernumber of of chain chain scission scissions.s. Those Those figures ﬁgures also also show show that that there there is is a acorrelation correlation between between the the intensity intensity of theof peak the peak absorbance absorbance of the of the C– C–HH and and the the C=O C=O functional functional groups groups and and the average numbernumber of of chain chain scissionscissions.s. It Itindicates indicates that that the the reaction reaction of of the the carboxylic carboxylic acids in thethe mineralmineral oiloil withwith the the hydroxyl hydroxyl groupsgroups on on the the cellulose cellulose occurred occurred at at the the same time asas thethe processprocessof of chain chain scission scission on on the the cellulose. cellulose.

(a)

(b)

FigureFigure 9. 9. IntensityIntensity of of the the absorbance absorbance peak peakss of of the the functional functional groupsgroups asas aa functionfunction of of the the average average chain chain scissionscissionss of of paper paper samples samples at at aging aging temperature temperaturess of ((aa)) 120120 ◦°CC( (bb)) 150150◦ °CC..

Table 6 shows the value of R2 from the correlation between the functional group and the average number of chain scissions of transformer paper. R2 is the coefficient of determination, which has a range of 0–1 and measures the closeness of the data to the regression line. A value of R2 of 1 means that the correlation has a linear relationship. This result gives initial information that there is a correlation between the structural changes of transformer paper during thermal aging with the average number of chain scissions.

Energies 2018, 11, 364 10 of 12

Table6 shows the value of R 2 from the correlation between the functional group and the average number of chain scissions of transformer paper. R2 is the coefﬁcient of determination, which has a range of 0–1 and measures the closeness of the data to the regression line. A value of R2 of 1 means that the correlation has a linear relationship. This result gives initial information that there is a correlation between the structural changes of transformer paper during thermal aging with the average number of chain scissions.

Table 6. The value of R2 from the correlation between the average number of chain scissions and the functional groups of the transformer paper.

Sample Testing Method Parameter R2 120 Intensity of absorbance peak of O–H 0.949 FTIR 150(3329 cm−1) functional group 0.9993 120 Intensity of absorbance peak of C–H 0.9889 FTIR 150(2922 cm−1) functional group 0.9804 120 Intensity of absorbance peak of C–H 0.9937 FTIR 150(2854 cm−1) functional group 0.9793 120 Intensity of absorbance peak of C=O 0.9968 FTIR 150(1745 cm−1) functional group 0.9115

4. Conclusions Accelerated thermal aging experiments on transformer paper in mineral oil was conducted under aging temperatures of 120 ◦C and 150 ◦C with durations of up to 1008 h. Visually the paper samples became darker as time elapsed, which indicates the structural changes of the cellulose paper after thermal aging. The chemical structure of transformer paper in mineral oil was identified using Fourier transform infrared (FTIR) spectroscopy analysis. FTIR spectra of new cellulose paper show absorbance peaks located close to 3329 cm−1 that represents an O–H functional group, located close to 2922 and 2854 cm−1 that represent a C–H functional group, and located close to 1050 cm−1 that represents a C–O functional group. These functional groups are consistent with the basic structure of cellulose. The intensity of the peak absorbance at 3329 cm−1, which is a typical characteristic of cellulose, decreased with aging due to oxidation processes. The intensity of the absorbance peaks located close to 2922 cm−1 and 2854 cm−1, which represent a C–H functional group, increased with aging due to the adsorption process of mineral oil to the paper surface during aging. The reaction between the carboxylic acids in the mineral oil with hydroxyl groups on the cellulose paper was the cause of the increased C–H functional group of the cellulose paper after moving a hydrocarbon group (-R) in mineral oil to the cellulose paper. The intensity of this absorbance peak at 1745 cm−1 increased with aging, which represents a C=O functional group. This peak emerged on the cellulose paper after thermal aging in mineral oil. There was no absorbance peak at this functional group region for new transformer paper. The emergence of the C=O functional group was due to the interaction between the low molecular weight acids dissolved in mineral oil and the cellulose. Carboxylate of the carboxylic acid (RCOO-) in mineral oil binds to cellulose, which creates the emergence of the C=O functional group on the cellulose paper surface. The intensity of the peak absorbances of the functional groups within the transformer paper that change with aging correlates with the aging mechanisms during thermal aging. The value of R2 > 0.9 from the correlation between functional groups and the average number of chain scissions of transformer paper gives direct evidence that transformer paper performance, which has a direct correlation with the life of transformers, can be investigated by using spectroscopy techniques.

Acknowledgments: This research was carried out with financial support from the Ministry of Research, Technology and Higher Education of Indonesia (KEMENRISTEKDIKTI). Author Contributions: The authors contributed collectively to the experimental setup, testing and measurement, data analysis and manuscript preparation. Conflicts of Interest: The authors declare no conflict of interest. Energies 2018, 11, 364 11 of 12

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