A Novel Approach for Analyzing Water Diffusion in Mineral and Vegetable Oil-Paper Insulation

Transcription

1 Sesors & Trasducers, Vol. 168, Issue 4, April 014, pp Sesors & Trasducers 014 by IFSA Publishig, S. L. A Novel Approach for Aalyzig Water Diffusio i ieral ad Vegetable Oil-Paper Isulatio Bi Du, * Jia Li, Zhaotao Zhag State Key Laboratory of Power Equipmet & System Security ad New Techology College of Electrical Egieerig Chogqig Uiversity, Chogqig, , Chia * Tel.: , fax: * lijia@cqu.edu.c Received: 14 February 014 /Accepted: 14 arch 014 /Published: 30 April 014 Abstract: Water diffusio characteristics of mieral ad vegetable oil-paper isulatio systems are importat for isulatio coditio evaluatio of oil-filled trasformers. I this paper, we describe a ovel applicatio method of i situ atteuated total reflectio Fourier trasform ifrared (ATR-FTIR) approach for aalyzig the diffusio process of water molecules i oil-immersed isulatig paper. Two-dimesioal correlatio was used to aalyze the cm -1 to 3000 cm -1 hydroxyl peak. The observed results idicated that water molecules form two types of hydroxyl (OH) with oil-impregated paper i the diffusio process are weak ad strog hydroge bods, respectively. D ifrared correlatio aalysis revealed that three OH stretchig vibratio spectra absorptio peaks was existed i hygroscopic vegetable oil-immersed isulatig paper. Ad there are four OH stretchig vibratio spectra absorptio peaks i mieral oil-immersed isulatio paper. Furthermore, mieral oil-impregated paper ad vegetable oil-impregated paper diffusio coefficiets were obtaied by oliear fittig. Copyright 014 IFSA Publishig, S. L. Keywords: ATR-FTIR, Oil-paper isulatio, Water diffusio, Diffusio coefficiet. 1. Itroductio Isulatio paper, pressboard, ad isulatig oils are major isulatio materials i oil-filled power trasformers. These materials should be maitaied uder virtually dry coditios for their etire lives to esure good isulatio ad safe operatio of the oil-filled power trasformers. A automatic Coulometric Karl Fischer Titratio approach to determie water cotets i isulatig liquids, oil-impregated paper ad pressboard was preseted i the IEC stadard [1]. However, water cotet i oil-impregated paper ad pressboard is difficult to be measured after oil-filled trasformers are put ito operatio []. Dew poit method was also used to measure the water cotets of oil-paper isulatio. The mai disadvatage of the dew poit method is that high requiremets for equipmet. This work was motivated by two problems posed i the aforemetioed publicatios for future applicatios. First, these methods i the above publicatios oly measure the water cotet of the oil-paper isulatio ad caot reflect the water molecular diffusio process i the oil-paper isulatio. Secodly, the water diffusio characteristics i vegetable oil-paper isulatios have seldom bee ivestigated although vegetable isulatig oils are used i medium-voltage trasformers [8]. The moisture saturatio level of vegetable isulatig oils is greater tha that Article umber P_

2 Sesors & Trasducers, Vol. 168, Issue 4, April 014, pp of mieral isulatig oils, because the hydrophilic bods exist i the ester molecules [9, 10]. The water diffusio characteristics of vegetable oil-paper isulatios are thus differet from those of mieral oil-paper isulatios. The paper shows a experimet system to simulate water diffusio i vegetable ad mieral oil-paper ad preset a ovel approach of two dimesioal/atteuated total reflectio (D ATR) ifrared correlatio spectroscopic aalysis of water diffusio i mieral ad vegetable oil-paper isulatio. The correlatio aalysis of the D ATR techology is a effective method of studyig the water diffusio process at molecular level. The D ATR techology does ot oly improve the spectral resolutio, but also provides dyamic iformatio over the sequece of fuctioal groups. Based o Fick s secod law, the curves is deduced to simulate the water diffusio characteristics i mieral ad vegetable oil-paper isulatio. The water diffusio curves betwee vegetable ad mieral oil-paper isulatios are also compared.. Experimet.1. Preparatio of aterials ad Samples A type of covetioal aphtheic-based mieral isulatig oil ad a type of rapeseed isulatig oil were used i the experimet. The paper used for the experimet was a ordiary, 0.3 mm thick isulatig Kraft paper. The isulatig paper specimes were dried i a vacuum vessel at 50 Pa ad 90 C for 48 hours to decrease the water cotet of each paper specime to less tha 0.5 %. The isulatig oils were dried at 85 C i a vacuum vessel for 48 hours to lower the water cotet of oil sufficietly. After the isulatig specimes were prepared, the paper specimes were impregated i the degassed ad dried isulatig oil at 40 C uder a vacuum for 4 hours. Fially, the oil-impregated paper specimes were moved ito the well-sealed glass bottle for use... Diffusio Experimets All ifrared spectra for the water diffusio were performed at 5 C usig a Nicolet Nexus Smart ARK-FTIR spectrometer equipped with a DTGS-KBr detector, solid cell accessories, ad a ZSe IRE crystal. Before each experimet, the crystals were cleaed by washig with acetoe ad were dried uder the ifrared lamp. A 0.3 mm thick oil-impregated paper was sadwiched betwee the ZSe reflectio elemets. The filter paper ad was mouted i the ATR cell, as show i Fig. 1. The spectrum of the dry film was collected as the backgroud spectra. Distilled water was ijected ito the filter paper, while the data acquisitio was performed at 1 mi itervals. Spectra were obtaied betwee 4,000 cm cm -1 at a resolutio of 4 cm -1. All of the origial spectra were baseliecorrected usig Omic 8.1 software. Fig. 1. Schematic descriptio of the ATR-FTIR experimetal arragemet..3. D Correlatio Aalysis For the geeralized D correlatio aalysis, 0 spectra at itervals of 6 mi i a wave\umber rage of 3000 cm -1 to cm -1 were selected. The D software used was a macro program amed D-shige. I the D correlatio maps, ushaded regios idicate positive correlatio itesities, whereas shaded regios idicate egative correlatio itesities. Give that the symmetric or atisymmetric relatioship exists i the sychroous ad asychroous spectra, we oly discussed the upper left corer area. 3. Results ad Discussio D Spectral Aalysis Fig. shows FTIR spectra of pure water ad water i oil- impregated paper i the rage of 4,000 cm -1 to 800 cm -1. I the spectra of pure water, the bads aroud cm -1 to 3000 cm -1 ad 1800 cm -1 to 1500 cm -1 wave umber rages are the stretchig ν(oh) ad deformatio vibratio bads δ(oh) of water, respectively. I the spectra of the mieral ad vegetable oil-impregated paper after moisture absorptio, the stretchig bad ad deformatio vibratio bads were evidet i the cm -1 to 3000 cm -1 ad the 1800 cm -1 to 1500 cm -1 regios. The ν(oh) absorptio peak i pure water ad hygroscopic oil- impregated paper located at 3378 ad 3335 cm -1, respectively. Compared with pure water, the ν(oh) absorptio bad peak i oil-impregated paper move to the lower wave umber of 43 cm -1. Compared with the strog itesity of the ν(oh) absorptio bad, the δ(oh) bad is relatively weak ad does ot vary very sigificatly. The time-evolved spectra for water diffusio i two types of oil-impregated paper withi the rage of cm -1 to 3000 cm -1 are show i Fig. 3. The absorptio peaks gradually icreased with time as water diffused ito oil-impregated paper. The 18

3 Sesors & Trasducers, Vol. 168, Issue 4, April 014, pp absorptio peaks at the two kids of oil- impregated paper were located at 3335 cm -1. Fig.. ATR-FTIR spectra of absorbed water i oil-impregated isulatio paper. cocetratio, or ay other physical variable. This tool provides more detailed iformatio ad isights ito molecular iteractios by correlatig the absorptio bad itesities of differet fuctioal groups. Two kids of correlatio maps were used, amely, sychroous ad asychroous spectra, which are geerated based o a set of dyamic spectra. I the D sychroous spectra, the sychroous spectrum is symmetric about the leadig diagoal positio. I the D correlatio map, two kids of peaks Φ (ν1, ν) are preset, amely, autopeak ad cross-peak. The auto-peak is located i the leadig diagoal positio (ν1=ν), which represets the sesitivity of absorptio bads to a certai perturbatio effect. The cross-peak is located i the o-leadig diagoal positio, which represets the same or differet chages i the two peaks. The sychroous correlatio spectra of both vegetable ad mieral oil-impregated paper i the spectral rage of cm -1 to cm -1 are show i Fig. 4. Oe auto-peak was observed at about 3380 cm -1 i Fig. 4(a), which represets the OH stretchig vibratio. The auto-peak idicates that the OH absorptio peak gradually chaged durig water diffusio. The same iformatio ca also be show i Fig. 4(b). oreover, a larger auto-peak at 3360 cm -1 was observed. (a) (a) -9.50E E E E E E E E-04 (b) Fig. 3. ATR-FTIR spectra of absorbed water i oil paper as fluctuatio of time: (a) Vegetable oil-impregated paper; (b) ieral oil-impregated paper. 3.. Two-dimesioal Correlatio Aalysis The geeralized D correlatio spectroscopy is used to evaluate various kids of fluctuatig perturbatios, icludig temperature, pressure, ad (b) Fig. 4. Sychroous D correlatio spectra: (a) Vegetable oil; (b) mieral oil E E E E E E E-04.00E-04 19

4 Sesors & Trasducers, Vol. 168, Issue 4, April 014, pp ore iformatio ca be obtaied from the correspodig asychroous correlatio spectrum of both vegetable ad mieral oil-impregated paper i Fig. 5. A asychroous cross-peak φ (ν1, ν) idicates that the bads ν1 ad ν varied out of phase with each other accordig to the Noda rule [7]. If φ (ν1, ν) > 0, Φ(ν1, ν) is positive (ushaded area), bad ν1 varies before bad ν. If Φ(ν1, ν) is egative (shaded area), implies the opposite pheomea, which is bad ν1 varies after bad ν does. If φ (ν1, ν) < 0, this rule is reversed. I the upper left triagle of Fig. 5(a), two positive cross-peaks (3413, 3198, 3335, ad 3198 cm -1 ) domiate the asychroous map, which idicates the broad ν(oh) water bad i the spectral regio of cm -1 to cm -1 is split ito three separate bads located at 345, 3357 ad 316 cm -1. I additio, this result shows that durig water diffusio, the bad at 345 ad 3357 cm -1 chaged prior to the bad at 316 cm -1. Similarly, i Fig. 5(b), oe positive cross-peak (3346, 316 cm-1) ad two egative cross-peaks (368, 3346 cm-1, 345, ad 3346 cm -1 ) were observed i the upper left triagle, which idicates that the broad water bad i the spectral rage of cm cm -1 is composed of four separate bads located at 368, 345, 3346 ad 316 cm -1. Durig water absorptio, the bad at 3346 cm -1 varies prior to the bad at 316 cm -1 ad the bad at 368 ad 345 cm -1 chages after 316 cm (a) E E E E E E E E E E E E E-06 The 345, 3346, ad 316 cm -1 bads express the symmetric ad asymmetric stretchig vibratio absorptio peaks of the hydroxyl of strog hydroge bods betwee water molecules. The 368 cm -1 bad expresses the symmetric ad asymmetric stretchig vibratio absorptio peak of the hydroxyl of weak hydroge bods betwee water molecules ad the hydrophilic group of the isulatig paper. oreover, the 3346 cm -1 itesity chaged before 368 cm -1 accordig to the Noda rule. This result idicated that water i the form of molecules diffused ito the mieral oil-immersed isulatig paper, ad formed hydroge bods with hydrophilic groups i the isulatig paper. However, i the vegetable oil-immersed isulatig paper, this pheomeo is ot very obvious Diffusio Characteristics The moisture diffusio i specimes alog the thickess directio ca be explaied by the secod Fick's diffusio law i oe dimesio, as follows: C C = D, (1) t z where C is the moisture cotet i isulatig paper at the locatio alog the specime thickess directio x at time t. D is the effective diffusio coefficiet. The solutio to Equatio (1) with boudary ad iitial coditios ca be give by (, ) C z t C = 0 4 ( 1) ( + 1) π ( + 1) π = 0 1 4L L D t πz = 1 exp cos + Based o Equatio (), the mass-trasported liquid at time t compared with the equilibrium mass is give by Equatio (3): () (b) E E E E E E E E E E E E-06.65E E E E E-06 t = 1 = 0 8 ( + 1) π D( + 1) exp 4L π t, (3) where t is the absorbed mass at time t, is the equilibrium mass, ad L is the oil impregated paper thickess. I a short time, t / is very small, the Equatio (3) ca be simplified ito Equatio (4): t 4 Dt = L π (4) Fig. 5. Asychroous correlatio spectrum of vegetable ad mieral oil-impregated paper. I the diffusio experimet, the absorptio of ifrared radiatio occurred at the iterface betwee the samples ( 1 ) ad ATR crystals ( ). The 130

5 Sesors & Trasducers, Vol. 168, Issue 4, April 014, pp ifrared decayed with icreasig distace from the surface. The evaescet wave electric field decay ca be represeted as follows: ( γz) E = E0 exp, (5) where E 0 is the electrical field stregth at the surface of the ATR crystal, ad γ is the peetratio depth of the evaescet wave. The γ ca be defied usig Equatio (6): ( si ) γ = λ π θ, (6) where λ is the wavelegth of the ifrared beam i the ATR elemet, θ is the icidece agle of radiatio at the paper/elemet iterface, ad 1 is the ratio of the refractive idex of oil-impregated paper to that of the elemet. Cosiderig the covolutio of the evaescet wave electric field (Equatio (5)) with the diffusio profile, Equatio (3) becomes: D( 1 ) π t ( 1) + + π exp ( γ L ) + ( 1) ( γ ) A 8 4L L t γ = 1, A π 1 exp( Lγ ) (7) = 0 ( + 1) π ( + 1) 4γ + L 1 where A t is the bad absorbace of ATR-FTIR spectra at time t, ad A is the bad absorbace at equilibrium. The diffusio coefficiet ca be calculated by Equatio (6). I the 3800 cm -1 to 3000 cm -1 rage, the OH absorptio peak height was used as the moisture absorptio amout. The fuctioal relatio betwee the area ad time was fitted i a o-liear curve. Fittig data calculatio ito the program ad the fittig curve was obtaied, as show i Fig. 6. The results are show i Table 1. mieral oil-immersed isulatig paper ad formed hydroge bods with hydrophilic groups i the isulatig paper. I vegetable oil-immersed isulatig paper, this pheomeo is ot very evidet. ) D ifrared correlatio aalysis revealed three OH stretchig vibratio spectra absorptio peaks i hygroscopic vegetable oil-immersed isulatig paper. The peaks were 345, 3357 ad 316 cm -1, respectively. The four OH stretchig vibratio spectra absorptio peaks i hygroscopic mieral oil-immersed isulatio paper were 345, 3346, 316 ad 368 cm -1. 3) The diffusio coefficiets of water i mieral oil-immersed isulatio paper ad i vegetable oil-immersed isulatig paper were calculated usig a oliear curve fittig, which were mm s -1 ad mm s -1, respectively. Ackowledgemets Fig. 6. Fittig curves of absorbed water i oil-paper isulatio. Table 1. Fittig results from ATR-FTIR spectra. Parameter ieral oil-paper 7. Coclusios L (mm) D (10-6 mm s -1 ) The preset work focused o the characteristics of moisture diffusio i oil-paper isulatio usig D ifrared correlatio spectroscopy. The experimet ad aalysis results are summarized as follows: 1) Based o the D ifrared correlatio aalysis, water i the form of molecules diffused ito the The authors ackowledge the Natioal Sciece Foudatio of Chia (No ad No ). Project 111 of the iistry of Educatio, Chia (No. B08036) is also appreciated. Refereces [1]. L. E. Ludgaard, W. Hase, D. Lihjell, et al., Agig of oil-impregated paper i power trasformers, IEEE Trasactios o Power Delivery, 19, 1, 004, pp []. Y. Du,. Zah, B. C. Lesieutre, et al., oisture equilibrium i trasformer paper-oil systems, IEEE Electrical Isulatio agazie, 15, 1, 1999, pp [3]. T. V. Oomme, oisture equilibrium charts for trasformer isulatio dryig practice, IEEE Trasactios o Power Apparatus ad Systems, 103, 10, 1984, pp [4]. F.. Clark, Factors affectig the mechaical deterioratio of cellulose isulatio, Browse Jourals & agazies, 61, 10, 194, pp [5]. A.. Emsley, X. Xiao, R.J. Heywood,. Ali, 131

6 Sesors & Trasducers, Vol. 168, Issue 4, April 014, pp Degradatio of cellulosic isulatio i power trasformers. Part 3: Effects of oxyge ad water o ageig i oil, IEEE Proceedigs: Sciece, easuremet ad Techology, 147, 3, 000, pp [6]. G. T. Fieldso, T. A. Barbari, The use of FTi.r.-a.t.r. spectroscopy to characterize peetrat diffusio i polymers, Polymer, 34, 6, 1993, pp [7]. I. Noda, Two-dimesioal ifrared spectroscopy, Joural of the America Chemical Society, 111, 6, 1989, pp [8]. I. Noda, Advaces i two-dimesioal correlatio spectroscopy, Vibratio Spectroscopy, 36, 6, 004, pp [9]. P. Wu, H. W. Siesler, Two-dimesioal correlatio aalysis of variable-temperature Fourier-trasform mid- ad ear-ifrared spectra of polyamide 11, Joural of olecular Structure, 51, 1 3, 000, pp [10]. J. Zhag, Hideto Tsuji, I. Noda, Weak itermolecular iteractio durig the melt crystallizatio of poly (L-lactide) ivestigated by two dimesioal ifrared correlatio spectroscopy, The Joural of Physical Chemistry B, 108, 36, 004, pp Copyright, Iteratioal Frequecy Sesor Associatio (IFSA) Publishig, S. L. All rights reserved. ( 13