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Synchronous fluorescence and excitation emission characteristics of transformer oil ageing
Talanta 70 (2006) 811–817
Synchronous fluorescence and excitation emission characteristics of transformer oil ageing Subbiah Deepa a , R. Sarathi b , Ashok K. Mishra a,∗ a
b
Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India Department of Electrical Engineering, Indian Institute of Technology Madras, Chennai 600 036, India Received 29 November 2005; received in revised form 30 January 2006; accepted 30 January 2006 Available online 9 March 2006
Abstract This paper describes the evaluation of synchronous fluorescence spectroscopy (SFS) and excitation emission matrix fluorescence (EEMF) spectroscopy as means of monitoring transformer oil degradation. When accelerated thermal ageing method is used, the onset of degradation of transformer oil on 17th day and transformer oil with polypropylene and cellulosic paper on 23rd and 27th days is sensitively reflected in the SFS and EEMF fluorescence spectral characteristics. © 2006 Elsevier B.V. All rights reserved. Keywords: Transformer oil; Synchronous scan fluorescence; Excitation emission matrix fluorescence; Ageing
1. Introduction Transformer oil is a derivative of petroleum crude. Mineral oils used for insulation are complex mixtures of linear saturated hydrocarbons (paraffins), cyclic saturated hydrocarbons (naphthenes), aromatic hydrocarbons and a small fraction of non-hydrocarbons, with hundreds of individual compounds. It contains high proportions of polychlorinated biphenyls (PCBs) or polycyclic aromatic hydrocarbons (PAHs) which are fluorescent molecules. Current techniques employed specifically for the analysis of the PAHs in transformer oil are limited to a standard test method for measuring the total mass of polycyclic aromatics in unused oil [1] and a procedure involving analysis of PAHs fingerprint of transformer oil using GC–MS. Both the techniques have lengthy extraction procedures, and although quantitative data can be obtained, the time consumed is long and expense is high. Transformer oils are susceptible to oxidation, which leads to the formation of compounds such as acids, aldehydes, esters, ketones, peroxides and alcohols. The deterioration of oil characteristics under working conditions may be hazardous to the electric equipment and installation. The parameters that accurately indicate the degradation in the transformer
∗
Corresponding author. Tel.: +91 44 2257 4207; fax: +91 44 2257 4202. E-mail address: [email protected] (A.K. Mishra).
0039-9140/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2006.01.045
oils are the dielectric breakdown voltage (electrical parameter), water content, neutralization number and oxidation stability of the oils. The oxidation of oil results in the formation of polar (oxygenated) compounds, which are responsible for the increase in acidity of the oil, and consequently reduces the dielectric breakdown voltage and increases the interfacial tension. High voltage power transformers make use of paper-insulated windings immersed in transformer oil. Replacement of traditional insulating materials like paper with commercially available thermosetting polymers has been practised recently. Several thermoplastics like polyetherimide, polyethylenes, polystyrene, Noryl and imidazole cured epoxy polymers were exposed to variation in humidity, ambient air, temperature and their stability was checked by Frost et al. [2]. Polypropylene films are used in capacitor coils and as cable wrap for layer and phase separation in rotating electrical equipment and transformers. During normal operation of the transformer, the temperature of the windings will increase and over a period of time this causes the cellulose chains in the paper to cleave. As a result, the mechanical strength of the paper measured directly or through its degree of polymerization, degrades with time and this will have an adverse effect on the performance of the insulation. A number of diagnostic test methods are available to monitor the insulation of power transformers. These include techniques such as gas chromatography for dissolved gas analysis (DGA) [3] to determine the hydrocarbon and carbon dioxide concentrations, moisture and
812
S. Deepa et al. / Talanta 70 (2006) 811–817
acidity measurements of the winding insulation, degree of polymerization (DP) measurements on paper samples [4] and high performance liquid chromatography (HPLC) [5] for the analysis of furan, product formed from paper degradation. Thermal ageing experiments has been performed by many authors over the last few decades [6–8] and the results of these experiments were used to make predictions about the transformer oil lifetime. Effects of oxidation on the ageing of oil–paper insulation have been studied by accelerated ageing experiments on paper wrapped insulated conductors in the presence of air and nitrogen environments over a temperature range of 115–145 ◦ C by Saha et al. [7]. Recently, Palmer et al. [6] have characterized the absorption response of transformer oil to in-service and laboratory aged transformer oils. This study indicated a significant increase in the absorbance over a range of wavelengths for both transformer oil degraded in the laboratory and for transformer oil drawn from transformers that have been in service for extended period of time. Recently, Neimanis et al. [9] and Gupta [10] have reported the determination of moisture content in oil impregnated paper using near infrared (NIR) spectroscopy. Fourier transform infrared (FTIR) and near infrared (NIR) spectroscopy has been used to characterize the ageing of cellulosic paper [11]. Spectrofluorimetry has been widely used in the characterization of hydrocarbons in environmental analysis [12–16] and in process monitoring applications. Fluorimetry is a powerful detection method used in separation techniques like liquid chromatography and electrophoresis. One of the most important advantages of the fluorescence method is its selectivity; however in its application to complex natural systems, the selectivity of conventional fluorescence techniques appears to be insufficient. Instead, multidimensional fluorescence techniques like synchronous fluorescence (SFS) and EEMF are used in such applications, providing additional information about the samples. For multifluorophoric systems, the importance of SFS and EEMF has been wellestablished [17–19]. An EEMF spectrum is obtained as a plot of emission intensity as a function of both excitation and emission wavelengths and contains in it all possible information of conventional steady-state fluorimetry. An SF spectrum is obtained by simultaneously scanning both the excitation and emission monochromators in a spectrofluorimeter. The resulting excitation–emission data matrix provides a total intensity profile of the sample over the range of excitation and emission. SFS in particular has been found to be useful for the analysis [20] and identification [21] of oils, but the work has not been focused specifically on oils used primarily for insulation purposes. A recent work on the possible use of fluorescence in monitoring oil quality by SFS technique has been reported [22], which mainly focuses on the chemometric techniques applied to the characterization of transformer oil. The most critical components of a transformer are immersed in its oil. By monitoring and identifying the condition of oil, the state of the transformer can be diagnosed. This investigation is aimed at a detailed analysis of fluorescence characteristics of transformer oil and a comparison of the ageing process of transformer oil in presence and absence of insulating materials (polypropylene film and the kraft paper)
towards obtaining a simple and convenient analytical method of monitoring transformer oil degradation. 2. Experimental 2.1. Materials and methods Oil samples were supplied by Raj Lubricants, Chennai under the trade name of Electrol, with a breakdown voltage of 65 kV. The general life of the transformer oil is much longer than the research duration; these investigations were based on studying the ageing effect with increased speed by thermally accelerated ageing of oils. Three kinds of degradation were designed and performed to investigate the response of transformer oil to various studies. Oil samples were taken in three 500 ml beakers and kept in an air-circulated oven maintained at 100 ◦ C for 31 days. In one 500 ml beaker with oil, kraft paper was immersed and aged thermally. Kraft papers are extensively used in a variety of electrical equipments. The thickness of the paper used for the study was 60 m. In case of insulation, paper absorbs moisture from the surrounding more rapidly than the pressboard material. Hence, before dipping into transformer oil, papers were placed in an oven maintained at 100 ◦ C for 24 h and then impregnated in oil for ageing studies. In the second beaker, biaxially oriented polypropylene film of 20 m thickness was used. The percentage of crystallinity was 71, dielectric constant of the material 2.2 and melting point of the material was 178 ◦ C. The breakdown voltage of the film is 380 V m−1 . The third beaker was in the absence of these insulating materials. Studies were carried out on these oils for every 2 days till the 30th day of ageing processes. 2.2. Apparatus A Perkin Elmer lambda 25 UV–vis spectrophotometer was used for the absorbance measurements. Fluorescence measurements were done by using a Hitachi F 4500 spectrofluorimeter. For SFS measurements, the scan speed was 240 nm s−1 and PMT voltage was at 700 V. Excitation and emission slits were 5 nm each. Excitation source was 100 W xenon lamp. SFS was measured in the excitation wavelength range 200–500 nm. The EEMF spectra were measured in the excitation wavelength range of 250–590 nm with an interval of 10 nm and in the emission wavelength range 270–600 nm within an interval of 5 nm, respectively, maintained at a slit width of 5/5 and scan speed 1200 nm s−1 . FTIR measurements were done in Perkin Elmer IR spectrophotometer by scanning from 550 to 4000 cm−1 . NIR measurements were done to characterize the ageing of cellulosic paper using UV–vis–NIR Cary spectrophotometer. 3. Results and discussion 3.1. UV–vis absorption studies Transformer oil is a highly absorptive material and the absorbance with the neat sample of oil exceeds the range of the spectrophotometer in the wavelength range 200–400 nm. Trans-
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813
Fig. 1. SFS spectra of raw oil at different λ.
former oils are very complex mixtures and may consist of as many as 2900 paraffinic, naphthenic and aromatic hydrocarbon molecule types, 25% of these being aromatic [23]. Oxidation changes the color of the oil, which can be monitored, by the change in the absorbance of the oil by UV–vis spectroscopy [7]. With ageing of oil, a red shift in absorbance was observed. Thus, the raw oil without paper and polypropylene also developed color on ageing, indicating that, the insulating material is not responsible for the formation of the absorbing species and it arises due to oxidation of oil. The degradation of oil was shifted from 17th day in the case of raw oil to 23rd and 27th days in presence of polypropylene and paper insulation. 3.2. Fluorescence studies In the course of investigation into fluorimetric analysis of multifluorophoric systems at higher concentration [24], it was observed that the use of right-angle geometry is capable of providing important analytical usefulness despite the presence of factors like light attenuation (LA), self-absorption (SA), energy transfer (ET) and collisional quenching (CQ). Kao et al. [25] also observed that for the multifluorophoric samples, right-angle geometry exhibits the widest linear dynamic range and lowest detectable fluorophore concentration. Hence, right-angle geometry was used for SFS and EEMF studies in characterizing the transformer oils.
Fig. 3. Variation of SFS intensity with degradation of (a) raw oil, (b) oil with polypropylene and (c) oil with paper at λsfs max 300 nm.
Transformer oil is a multifluorophoric system and as expected, the emission spectra keep changing in their shape and intensity, when the excitation wavelength is varied. This lack of uniqueness makes conventional fluorescence useless of monitoring purposes. Hence, SFS and EEMF spectroscopy were used for the analysis of transformer oil monitoring its ageing process. The crucial parameter in SF spectroscopy is λ, the difference in wavelength of the excitation and the emission monochromators when they are scanned simultaneously. When the λ is very low (<2 nm), scattering interferences are expected. Similarly when λ is very large, those fluorophores, which have a very small stokes shift are not expected to be reflected in the SFS spectrum. Thus, for a particular type of a complex multifluorophoric system, the choice of optimum λ varies. For example, in case of motor fuel characterization, an optimum λ 40 nm is known to be more useful [19]. In order to arrive at an optimized λ for transformer oil, the value was progressively varied in the range 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100 nm and the corresponding SF spectra recorded as shown in Fig. 1. It was found that at λ 5 nm, SF spectra showed better resolved features in the transformer oil spectrum (Fig. 1). For a variety of transformer oil originating from different petroleum crude, the choice of λ for particular transformer oil is not necessar-
Fig. 2. SFS spectra of raw oil (a) and raw oil with polypropylene (b) and with paper (c) with increasing days: 0 day (Virgin), days 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29, 31. The spectra overlapped extensively till day 17th in raw oil and day 23rd in oil with poplypropylene and 25th day in oil with paper. λ = 5 nm. There was a substantial loss of SF intensity at 330 nm on day 19 in raw oil, 25th day in oil with polypropylene and 27th day in oil with paper.
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ily best at λ 5 nm. However as Fig. 1 shows it is possible to identify an SFS band, which does not change much when λ is varied. In the present case, it is at 330 nm (Fig. 1). The variation in SF spectra at λ 5 nm for raw transformer oil and in presence of insulating polypropylene and paper is shown in Fig. 2a–c, respectively. As is evident from the figures, a prominent SFS band at 330 nm was found to decrease substantially with ageing of transformer oil and changes of this band with regard to the presence of the insulating material could be rationalized in terms of progressive ageing of the oil. A drastic decrease in the SFS intensity of 330 nm band with ageing after 17th day in raw transformer oil and 23rd and 27th days in transformer oil with polypropylene and paper was observed. This is accompanied by the appearance of a new band at 450 nm, (shown in the inset of Fig. 2a–c) which appear to be due to the formation of a
new species on degradation. The SFS band intensity at 330 nm is highly invariant to the degradation of the transformer oil in all the three cases as shown in Fig. 3. As is evident from Fig. 3, the degradation of transformer oil with polypropylene and paper starts from 23rd and 27th days whereas it started at 17th day in raw oil. The SFS intensity at 330 nm is fairly constant till the onset of degradation, i.e. 17th, 23rd and 25th days in raw oil and oil with polypropylene and paper, respectively, which then drastically decreases. It is well known that, by the addition of insulating material to oil, the stability of the oil is increased [10]. In EEMF, the fluorescence intensity is plotted as a function of both the excitation and the emission wavelengths. Fig. 4A–C shows the variation of the contours with time of raw oil samples and in presence of polypropylene and paper, respectively. The EEMF contour maximum for raw transformer oil without degra-
Fig. 4. (A) EEMF contour spectra for raw transformer oil with ageing: (a) raw oil, (b) day 19, (c) day 31. (B) EEMF contour spectra for transformer oil with polypropylene film upon ageing: (a) raw oil, (b) day 25, (c) day 31. (C) EEMF contour spectra for transformer oil with paper upon ageing: (a) raw oil, (b) day 27, (c) day 31.
S. Deepa et al. / Talanta 70 (2006) 811–817
dation was (370, 415) nm, which progressively shifted to (420, 485) nm at the end of 31 days, accompanied with a decrease in fluorescence intensity (Fig. 4). Fluorescence intensity drastically decreased at the 17th day and then marginally increased upon ageing till the 31st day in the case of the raw transformer oil whereas, in transformer oil with polypropylene and kraft paper, the ageing process started from 23rd and 27th days with a red shifting of contour maximum to (390, 460) nm with a drastic decrease in intensity. A slight increase in fluorescence intensity was observed after 23rd and 27th days as shown in Fig. 2b and c. Thus, the drastic decrease in fluorescence intensity with ageing of transformer oil has striking similarity with the results reported by Saha et al. [7]. They investigated the conditions of aged insulation samples in the presence of air and nitrogen environments over a temperature range of 115–145 ◦ C. With increase in ageing time, the peak maximum return voltage was found to initially increase and then found to decrease drastically due to lowering of the insulation resistance. Both SFS and EEMF technique were able to report this phenomenon sensitively and faithfully. Fig. 5A–C shows the variation of fluorescence excitation, emission maxima and intensity of raw transformer oil and oil in presence of polypropylene and paper. As can be seen, a red shift in the contour maximum is observed with ageing. Thus, the EEMF fluorescence of the transformer oil agrees well with the SFS results. When transformer oil is degraded, acidity increases, resulting in increase in C O bond and C C double bonds. During thermal decomposition, paraffinic compounds dehydrogenate and form naphthenic compounds. The naphthenes further dehydrogenate and form conjugated C C double bonds and aromatics. Upon thermal oxidation, the unsaturated hydrocarbons in the oil form hydroperoxides, which on subsequent oxidation results in aldehydes and ketones by a free radical mechanism [6]. A kinetic experimental study of the oxidation reactions of naphthenic mineral oil (with and without additives) as a function of temperature has been carried out by Neto et al. [26]. Their results showed that the presence of hindered phenolic antioxidants significantly reduces the formation of oxidation products. Presence of antioxidants in the oil sample inhibits degradation until the antioxidants are consumed [26,27]. This could explain the sudden onset of fluorescence loss on day 17 for raw oil and on days 23 and 27 for oil with polypropylene and paper. There is a distinct possibility that under thermally accelerated ageing conditions in an air circulated oven, there could be volume loss and evaporation of small molecular mass species, which could include fluorophores emitting at shorter wavelengths. Loss of these could result in overall fluorescence loss and a red shift of the fluorescence contour. However, if evaporation of lowmolecular mass fluorophores were affecting the SF or EEM spectra, the spectral profiles would have started changing right from the first sample taken after 2 days, would have been a gradual process, and probably with more significant changes during the initial days. Since transformer oil is a derivative of petroleum crude, the spectral fingerprint and SFS characteristics of transformer oils obtained from different sources may show variations. Thus, no unique fluorescence characteristic can be ascribed to transformer
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Fig. 5. (A) Variation of: (a) excitation maxima, (b) emission maxima and (c) fluorescence intensity with ageing of raw oil. (B) Variation of: (a) excitation maxima, (b) emission maxima and (c) fluorescence intensity with ageing of oil in presence of polypropylene. (C) Variation of: (a) excitation maxima, (b) emission maxima and (c) fluorescence intensity with ageing of raw oil in presence of paper.
oil in general and monitoring of transformer oil characteristics needs to be done starting with the measurement of SFS and/or EEMF spectra of the virgin oil. 3.3. IR studies on the ageing of transformer oil In order to confirm the formation of a new carbonyl species on ageing process, IR spectra of raw and aged oil under different conditions were recorded. The vibration frequency correspond-
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Fig. 7. NIR spectra of (a) raw paper and (b) in oil.
3.4. NIR studies on cellulosic paper It has been reported by Saha et al. [7] that during ageing, the surface of the paper, which is in direct contact with the transformer oil, undergoes a color change and becomes darker since the oxidation products from the oil were attracted to the surface. A significant concentration of doubly bonded carbons and oxygen present in the surface of the samples had been identified using (X-ray photoelectron spectroscopy) XPS [7]. Ali et al. [11] have reported FTIR and NIR spectroscopy to characterize the ageing of cellulosic paper. Fig. 7 shows the NIR spectra of Kraft paper in air and paper impregnated in transformer oil, which agrees well with the spectra, reported in literature [8]. Water/OH features dominate the NIR spectra but the development of carbonyl/carboxyl overtones was also observed in the region of 1700–1900 nm. The oil impregnated paper showed a well-defined water combination band at about 1950 nm and the O-H stretch first overtone at about 1450 nm. 4. Conclusion
Fig. 6. (a) FTIR spectra of raw oil. (b) FTIR spectra of oil in presence of polypropylene. (c) FTIR spectra of oil in presence of paper.
ing to the carbonyl stretching frequency at 1710 cm−1 appeared during the process of ageing, which could be due to the presence of oxidation products of the oil like aldehydes and ketones. Thus, Fig. 6a–c shows the FTIR spectra of raw transformer oil and the polypropylene and paper impregnated oil upon ageing which supports the fluorescence and UV–vis absorption studies.
Synchronous excitation spectra and EEMF of oils can be used as a fingerprint and can be applied, together with fluorescence intensities, as a rapid screening tool to investigate similarities of oils. Thus, when accelerated thermal ageing method is used, the onset of degradation of transformer oil on 17th day and transformer oil with polypropylene and cellulosic paper on 23rd and 27th days is sensitively reflected in the SFS and EEMF spectral characteristics. This was supported by the FTIR and NIR studies on the ageing of transformer oil and the cellulosic paper. Fluorescence is a totally unperturbative technique. Delivery of excitation light and collection of fluorescence light can be easily done using optical fibers. Thus, it should be possible to develop in situ monitoring methods for transformer oil degradation. Acknowledgements S.D. thanks Council of Scientific and Industrial Research (CSIR), New Delhi for fellowship. We are grateful to Sophisti-
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cated Analytical Instrumentation Facility, IIT Madras for FTIR and NIR spectral studies. We thank Raj Lubricants, Chennai for the supply of sample and valuable help. References [1] IP346/92 (BS2000 Pt. 346), Determination of PCA content in unused lubricating base oils and asphaltene-free petroleum fractions—DMSO extraction/refractive index method, The British Standards Institute, 389 Chiswick High Road, London, W4 4AL United Kingdom. [2] N.E. Frost, P.B. Mcgrath, C.W. Burns, IEEE Trans. Power Deliv. 11 (1996) 331–334. [3] J.J. Kelly, IEEE Trans. Ind. Appl. 16 (1980) 777. [4] J. Unsworth, F. Mitchell, IEEE Trans. Electr. Insul. 25 (1990) 737. [5] A.M. Emsley, X. Xiao, R.J. Heywood, M. Ali, IEEE Proc. Sci. Measur. Tech. 147 (2000) 116. [6] J.A. Palmer, X. Wang, A. Mander, D. Torgerson, C. Rich, Conference Record of the 2000 IEEE International Symposium on Electrical Insulation, Anaheim, CA, USA, 2000, p. 460. [7] T.K. Saha, M. Darveniza, Z.T. Yao, D.J.T. Hill, G. Yeung, IEEE Trans. Power Deliv. 14 (1999) 1359. [8] D.J.T. Hill, T.T. Le, M. Darvenzia, T.K. Saha, Polym. Degrad. Stability 49 (1995) 429. [9] R. Neimanis, H. Lennholm, R. Kriksson, 1999 Annual Report Conference on Electrical Insulation and Dielectric Phenomena, 1999, p. 162.
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[10] B.K. Gupta, IEEE Intern. Sympos. Electr. Insulat., Arlington, Virginia, USA, 1998, p. 583. [11] M. Ali, A.M. Emsley, H. Herman, R.J. Heywood, Polymer 42 (2001) 2893. [12] J.M. Shaver, L.B. McGown, Appl. Spectrosc. 49 (1995) 813. [13] M. Picer, N. Picer, Chemosphere 24 (1992) 1825. [14] M. Picer, V. Hocenski, Water Res. 28 (1994) 619. [15] L. De domencio, E. Crisafi, G. Magazzu, A. Puglisi, A. Larosa, Mar. Pollut. Bull. 28 (1994) 587. [16] E.S. Brodskii, S.A. Savchuk, J. Anal. Chem. 53 (1998) 1070. [17] D. Patra, A.K. Mishra, Trends Anal.Chem. 21 (2002) 787. [18] D. Patra, A.K. Mishra, Appl. Spectrosc. 55 (2001) 338. [19] D. Patra, A.K. Mishra, Talanta 53 (2001) 783. [20] J.B.F. Lloyd, Nat. Phys. Sci. (Lond.) 231 (1971) 64. [21] P. John, I. Soutar, Anal. Chem. 48 (1976) 520. [22] C. Bessant, L. Ritchie, S. Saini, B. Pahlavanpour, G. Wilson, Appl. Spectrosc. 55 (2001) 840. [23] A. Sierota, J. Rungis, IEEE Electr. Insul. Mag. 11 (1995) 1. [24] D. Patra, A.K. Mishra, Analyst 125 (2000) 1383. [25] S. Kao, A.N. Asanov, P.B. Oldham, Instrum. Sci. Technol. 26 (1998) 375. [26] R.C.R. Neto, D.O. Lima, T.D.S. Pinhelro, R.F. Almeida, T.N.C. Dantas, M.S.G. Dantas, M.A.S. Araujo, C.L. Cavaleante, D.C.S. Azevedo, Ind. Eng. Chem. Res. 43 (2004) 7428. [27] P.R. Krishnamoorthy, S. Vijayakumari, S. Sankaralingam, IEEE Trans. Electr. Insul. 27 (1992) 271.
Synchronous fluorescence and excitation emission characteristics of transformer oil ageing Subbiah Deepa a , R. Sarathi b , Ashok K. Mishra a,∗ a
b
Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India Department of Electrical Engineering, Indian Institute of Technology Madras, Chennai 600 036, India Received 29 November 2005; received in revised form 30 January 2006; accepted 30 January 2006 Available online 9 March 2006
Abstract This paper describes the evaluation of synchronous fluorescence spectroscopy (SFS) and excitation emission matrix fluorescence (EEMF) spectroscopy as means of monitoring transformer oil degradation. When accelerated thermal ageing method is used, the onset of degradation of transformer oil on 17th day and transformer oil with polypropylene and cellulosic paper on 23rd and 27th days is sensitively reflected in the SFS and EEMF fluorescence spectral characteristics. © 2006 Elsevier B.V. All rights reserved. Keywords: Transformer oil; Synchronous scan fluorescence; Excitation emission matrix fluorescence; Ageing
1. Introduction Transformer oil is a derivative of petroleum crude. Mineral oils used for insulation are complex mixtures of linear saturated hydrocarbons (paraffins), cyclic saturated hydrocarbons (naphthenes), aromatic hydrocarbons and a small fraction of non-hydrocarbons, with hundreds of individual compounds. It contains high proportions of polychlorinated biphenyls (PCBs) or polycyclic aromatic hydrocarbons (PAHs) which are fluorescent molecules. Current techniques employed specifically for the analysis of the PAHs in transformer oil are limited to a standard test method for measuring the total mass of polycyclic aromatics in unused oil [1] and a procedure involving analysis of PAHs fingerprint of transformer oil using GC–MS. Both the techniques have lengthy extraction procedures, and although quantitative data can be obtained, the time consumed is long and expense is high. Transformer oils are susceptible to oxidation, which leads to the formation of compounds such as acids, aldehydes, esters, ketones, peroxides and alcohols. The deterioration of oil characteristics under working conditions may be hazardous to the electric equipment and installation. The parameters that accurately indicate the degradation in the transformer
∗
Corresponding author. Tel.: +91 44 2257 4207; fax: +91 44 2257 4202. E-mail address: [email protected] (A.K. Mishra).
0039-9140/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2006.01.045
oils are the dielectric breakdown voltage (electrical parameter), water content, neutralization number and oxidation stability of the oils. The oxidation of oil results in the formation of polar (oxygenated) compounds, which are responsible for the increase in acidity of the oil, and consequently reduces the dielectric breakdown voltage and increases the interfacial tension. High voltage power transformers make use of paper-insulated windings immersed in transformer oil. Replacement of traditional insulating materials like paper with commercially available thermosetting polymers has been practised recently. Several thermoplastics like polyetherimide, polyethylenes, polystyrene, Noryl and imidazole cured epoxy polymers were exposed to variation in humidity, ambient air, temperature and their stability was checked by Frost et al. [2]. Polypropylene films are used in capacitor coils and as cable wrap for layer and phase separation in rotating electrical equipment and transformers. During normal operation of the transformer, the temperature of the windings will increase and over a period of time this causes the cellulose chains in the paper to cleave. As a result, the mechanical strength of the paper measured directly or through its degree of polymerization, degrades with time and this will have an adverse effect on the performance of the insulation. A number of diagnostic test methods are available to monitor the insulation of power transformers. These include techniques such as gas chromatography for dissolved gas analysis (DGA) [3] to determine the hydrocarbon and carbon dioxide concentrations, moisture and
812
S. Deepa et al. / Talanta 70 (2006) 811–817
acidity measurements of the winding insulation, degree of polymerization (DP) measurements on paper samples [4] and high performance liquid chromatography (HPLC) [5] for the analysis of furan, product formed from paper degradation. Thermal ageing experiments has been performed by many authors over the last few decades [6–8] and the results of these experiments were used to make predictions about the transformer oil lifetime. Effects of oxidation on the ageing of oil–paper insulation have been studied by accelerated ageing experiments on paper wrapped insulated conductors in the presence of air and nitrogen environments over a temperature range of 115–145 ◦ C by Saha et al. [7]. Recently, Palmer et al. [6] have characterized the absorption response of transformer oil to in-service and laboratory aged transformer oils. This study indicated a significant increase in the absorbance over a range of wavelengths for both transformer oil degraded in the laboratory and for transformer oil drawn from transformers that have been in service for extended period of time. Recently, Neimanis et al. [9] and Gupta [10] have reported the determination of moisture content in oil impregnated paper using near infrared (NIR) spectroscopy. Fourier transform infrared (FTIR) and near infrared (NIR) spectroscopy has been used to characterize the ageing of cellulosic paper [11]. Spectrofluorimetry has been widely used in the characterization of hydrocarbons in environmental analysis [12–16] and in process monitoring applications. Fluorimetry is a powerful detection method used in separation techniques like liquid chromatography and electrophoresis. One of the most important advantages of the fluorescence method is its selectivity; however in its application to complex natural systems, the selectivity of conventional fluorescence techniques appears to be insufficient. Instead, multidimensional fluorescence techniques like synchronous fluorescence (SFS) and EEMF are used in such applications, providing additional information about the samples. For multifluorophoric systems, the importance of SFS and EEMF has been wellestablished [17–19]. An EEMF spectrum is obtained as a plot of emission intensity as a function of both excitation and emission wavelengths and contains in it all possible information of conventional steady-state fluorimetry. An SF spectrum is obtained by simultaneously scanning both the excitation and emission monochromators in a spectrofluorimeter. The resulting excitation–emission data matrix provides a total intensity profile of the sample over the range of excitation and emission. SFS in particular has been found to be useful for the analysis [20] and identification [21] of oils, but the work has not been focused specifically on oils used primarily for insulation purposes. A recent work on the possible use of fluorescence in monitoring oil quality by SFS technique has been reported [22], which mainly focuses on the chemometric techniques applied to the characterization of transformer oil. The most critical components of a transformer are immersed in its oil. By monitoring and identifying the condition of oil, the state of the transformer can be diagnosed. This investigation is aimed at a detailed analysis of fluorescence characteristics of transformer oil and a comparison of the ageing process of transformer oil in presence and absence of insulating materials (polypropylene film and the kraft paper)
towards obtaining a simple and convenient analytical method of monitoring transformer oil degradation. 2. Experimental 2.1. Materials and methods Oil samples were supplied by Raj Lubricants, Chennai under the trade name of Electrol, with a breakdown voltage of 65 kV. The general life of the transformer oil is much longer than the research duration; these investigations were based on studying the ageing effect with increased speed by thermally accelerated ageing of oils. Three kinds of degradation were designed and performed to investigate the response of transformer oil to various studies. Oil samples were taken in three 500 ml beakers and kept in an air-circulated oven maintained at 100 ◦ C for 31 days. In one 500 ml beaker with oil, kraft paper was immersed and aged thermally. Kraft papers are extensively used in a variety of electrical equipments. The thickness of the paper used for the study was 60 m. In case of insulation, paper absorbs moisture from the surrounding more rapidly than the pressboard material. Hence, before dipping into transformer oil, papers were placed in an oven maintained at 100 ◦ C for 24 h and then impregnated in oil for ageing studies. In the second beaker, biaxially oriented polypropylene film of 20 m thickness was used. The percentage of crystallinity was 71, dielectric constant of the material 2.2 and melting point of the material was 178 ◦ C. The breakdown voltage of the film is 380 V m−1 . The third beaker was in the absence of these insulating materials. Studies were carried out on these oils for every 2 days till the 30th day of ageing processes. 2.2. Apparatus A Perkin Elmer lambda 25 UV–vis spectrophotometer was used for the absorbance measurements. Fluorescence measurements were done by using a Hitachi F 4500 spectrofluorimeter. For SFS measurements, the scan speed was 240 nm s−1 and PMT voltage was at 700 V. Excitation and emission slits were 5 nm each. Excitation source was 100 W xenon lamp. SFS was measured in the excitation wavelength range 200–500 nm. The EEMF spectra were measured in the excitation wavelength range of 250–590 nm with an interval of 10 nm and in the emission wavelength range 270–600 nm within an interval of 5 nm, respectively, maintained at a slit width of 5/5 and scan speed 1200 nm s−1 . FTIR measurements were done in Perkin Elmer IR spectrophotometer by scanning from 550 to 4000 cm−1 . NIR measurements were done to characterize the ageing of cellulosic paper using UV–vis–NIR Cary spectrophotometer. 3. Results and discussion 3.1. UV–vis absorption studies Transformer oil is a highly absorptive material and the absorbance with the neat sample of oil exceeds the range of the spectrophotometer in the wavelength range 200–400 nm. Trans-
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Fig. 1. SFS spectra of raw oil at different λ.
former oils are very complex mixtures and may consist of as many as 2900 paraffinic, naphthenic and aromatic hydrocarbon molecule types, 25% of these being aromatic [23]. Oxidation changes the color of the oil, which can be monitored, by the change in the absorbance of the oil by UV–vis spectroscopy [7]. With ageing of oil, a red shift in absorbance was observed. Thus, the raw oil without paper and polypropylene also developed color on ageing, indicating that, the insulating material is not responsible for the formation of the absorbing species and it arises due to oxidation of oil. The degradation of oil was shifted from 17th day in the case of raw oil to 23rd and 27th days in presence of polypropylene and paper insulation. 3.2. Fluorescence studies In the course of investigation into fluorimetric analysis of multifluorophoric systems at higher concentration [24], it was observed that the use of right-angle geometry is capable of providing important analytical usefulness despite the presence of factors like light attenuation (LA), self-absorption (SA), energy transfer (ET) and collisional quenching (CQ). Kao et al. [25] also observed that for the multifluorophoric samples, right-angle geometry exhibits the widest linear dynamic range and lowest detectable fluorophore concentration. Hence, right-angle geometry was used for SFS and EEMF studies in characterizing the transformer oils.
Fig. 3. Variation of SFS intensity with degradation of (a) raw oil, (b) oil with polypropylene and (c) oil with paper at λsfs max 300 nm.
Transformer oil is a multifluorophoric system and as expected, the emission spectra keep changing in their shape and intensity, when the excitation wavelength is varied. This lack of uniqueness makes conventional fluorescence useless of monitoring purposes. Hence, SFS and EEMF spectroscopy were used for the analysis of transformer oil monitoring its ageing process. The crucial parameter in SF spectroscopy is λ, the difference in wavelength of the excitation and the emission monochromators when they are scanned simultaneously. When the λ is very low (<2 nm), scattering interferences are expected. Similarly when λ is very large, those fluorophores, which have a very small stokes shift are not expected to be reflected in the SFS spectrum. Thus, for a particular type of a complex multifluorophoric system, the choice of optimum λ varies. For example, in case of motor fuel characterization, an optimum λ 40 nm is known to be more useful [19]. In order to arrive at an optimized λ for transformer oil, the value was progressively varied in the range 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100 nm and the corresponding SF spectra recorded as shown in Fig. 1. It was found that at λ 5 nm, SF spectra showed better resolved features in the transformer oil spectrum (Fig. 1). For a variety of transformer oil originating from different petroleum crude, the choice of λ for particular transformer oil is not necessar-
Fig. 2. SFS spectra of raw oil (a) and raw oil with polypropylene (b) and with paper (c) with increasing days: 0 day (Virgin), days 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29, 31. The spectra overlapped extensively till day 17th in raw oil and day 23rd in oil with poplypropylene and 25th day in oil with paper. λ = 5 nm. There was a substantial loss of SF intensity at 330 nm on day 19 in raw oil, 25th day in oil with polypropylene and 27th day in oil with paper.
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ily best at λ 5 nm. However as Fig. 1 shows it is possible to identify an SFS band, which does not change much when λ is varied. In the present case, it is at 330 nm (Fig. 1). The variation in SF spectra at λ 5 nm for raw transformer oil and in presence of insulating polypropylene and paper is shown in Fig. 2a–c, respectively. As is evident from the figures, a prominent SFS band at 330 nm was found to decrease substantially with ageing of transformer oil and changes of this band with regard to the presence of the insulating material could be rationalized in terms of progressive ageing of the oil. A drastic decrease in the SFS intensity of 330 nm band with ageing after 17th day in raw transformer oil and 23rd and 27th days in transformer oil with polypropylene and paper was observed. This is accompanied by the appearance of a new band at 450 nm, (shown in the inset of Fig. 2a–c) which appear to be due to the formation of a
new species on degradation. The SFS band intensity at 330 nm is highly invariant to the degradation of the transformer oil in all the three cases as shown in Fig. 3. As is evident from Fig. 3, the degradation of transformer oil with polypropylene and paper starts from 23rd and 27th days whereas it started at 17th day in raw oil. The SFS intensity at 330 nm is fairly constant till the onset of degradation, i.e. 17th, 23rd and 25th days in raw oil and oil with polypropylene and paper, respectively, which then drastically decreases. It is well known that, by the addition of insulating material to oil, the stability of the oil is increased [10]. In EEMF, the fluorescence intensity is plotted as a function of both the excitation and the emission wavelengths. Fig. 4A–C shows the variation of the contours with time of raw oil samples and in presence of polypropylene and paper, respectively. The EEMF contour maximum for raw transformer oil without degra-
Fig. 4. (A) EEMF contour spectra for raw transformer oil with ageing: (a) raw oil, (b) day 19, (c) day 31. (B) EEMF contour spectra for transformer oil with polypropylene film upon ageing: (a) raw oil, (b) day 25, (c) day 31. (C) EEMF contour spectra for transformer oil with paper upon ageing: (a) raw oil, (b) day 27, (c) day 31.
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dation was (370, 415) nm, which progressively shifted to (420, 485) nm at the end of 31 days, accompanied with a decrease in fluorescence intensity (Fig. 4). Fluorescence intensity drastically decreased at the 17th day and then marginally increased upon ageing till the 31st day in the case of the raw transformer oil whereas, in transformer oil with polypropylene and kraft paper, the ageing process started from 23rd and 27th days with a red shifting of contour maximum to (390, 460) nm with a drastic decrease in intensity. A slight increase in fluorescence intensity was observed after 23rd and 27th days as shown in Fig. 2b and c. Thus, the drastic decrease in fluorescence intensity with ageing of transformer oil has striking similarity with the results reported by Saha et al. [7]. They investigated the conditions of aged insulation samples in the presence of air and nitrogen environments over a temperature range of 115–145 ◦ C. With increase in ageing time, the peak maximum return voltage was found to initially increase and then found to decrease drastically due to lowering of the insulation resistance. Both SFS and EEMF technique were able to report this phenomenon sensitively and faithfully. Fig. 5A–C shows the variation of fluorescence excitation, emission maxima and intensity of raw transformer oil and oil in presence of polypropylene and paper. As can be seen, a red shift in the contour maximum is observed with ageing. Thus, the EEMF fluorescence of the transformer oil agrees well with the SFS results. When transformer oil is degraded, acidity increases, resulting in increase in C O bond and C C double bonds. During thermal decomposition, paraffinic compounds dehydrogenate and form naphthenic compounds. The naphthenes further dehydrogenate and form conjugated C C double bonds and aromatics. Upon thermal oxidation, the unsaturated hydrocarbons in the oil form hydroperoxides, which on subsequent oxidation results in aldehydes and ketones by a free radical mechanism [6]. A kinetic experimental study of the oxidation reactions of naphthenic mineral oil (with and without additives) as a function of temperature has been carried out by Neto et al. [26]. Their results showed that the presence of hindered phenolic antioxidants significantly reduces the formation of oxidation products. Presence of antioxidants in the oil sample inhibits degradation until the antioxidants are consumed [26,27]. This could explain the sudden onset of fluorescence loss on day 17 for raw oil and on days 23 and 27 for oil with polypropylene and paper. There is a distinct possibility that under thermally accelerated ageing conditions in an air circulated oven, there could be volume loss and evaporation of small molecular mass species, which could include fluorophores emitting at shorter wavelengths. Loss of these could result in overall fluorescence loss and a red shift of the fluorescence contour. However, if evaporation of lowmolecular mass fluorophores were affecting the SF or EEM spectra, the spectral profiles would have started changing right from the first sample taken after 2 days, would have been a gradual process, and probably with more significant changes during the initial days. Since transformer oil is a derivative of petroleum crude, the spectral fingerprint and SFS characteristics of transformer oils obtained from different sources may show variations. Thus, no unique fluorescence characteristic can be ascribed to transformer
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Fig. 5. (A) Variation of: (a) excitation maxima, (b) emission maxima and (c) fluorescence intensity with ageing of raw oil. (B) Variation of: (a) excitation maxima, (b) emission maxima and (c) fluorescence intensity with ageing of oil in presence of polypropylene. (C) Variation of: (a) excitation maxima, (b) emission maxima and (c) fluorescence intensity with ageing of raw oil in presence of paper.
oil in general and monitoring of transformer oil characteristics needs to be done starting with the measurement of SFS and/or EEMF spectra of the virgin oil. 3.3. IR studies on the ageing of transformer oil In order to confirm the formation of a new carbonyl species on ageing process, IR spectra of raw and aged oil under different conditions were recorded. The vibration frequency correspond-
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Fig. 7. NIR spectra of (a) raw paper and (b) in oil.
3.4. NIR studies on cellulosic paper It has been reported by Saha et al. [7] that during ageing, the surface of the paper, which is in direct contact with the transformer oil, undergoes a color change and becomes darker since the oxidation products from the oil were attracted to the surface. A significant concentration of doubly bonded carbons and oxygen present in the surface of the samples had been identified using (X-ray photoelectron spectroscopy) XPS [7]. Ali et al. [11] have reported FTIR and NIR spectroscopy to characterize the ageing of cellulosic paper. Fig. 7 shows the NIR spectra of Kraft paper in air and paper impregnated in transformer oil, which agrees well with the spectra, reported in literature [8]. Water/OH features dominate the NIR spectra but the development of carbonyl/carboxyl overtones was also observed in the region of 1700–1900 nm. The oil impregnated paper showed a well-defined water combination band at about 1950 nm and the O-H stretch first overtone at about 1450 nm. 4. Conclusion
Fig. 6. (a) FTIR spectra of raw oil. (b) FTIR spectra of oil in presence of polypropylene. (c) FTIR spectra of oil in presence of paper.
ing to the carbonyl stretching frequency at 1710 cm−1 appeared during the process of ageing, which could be due to the presence of oxidation products of the oil like aldehydes and ketones. Thus, Fig. 6a–c shows the FTIR spectra of raw transformer oil and the polypropylene and paper impregnated oil upon ageing which supports the fluorescence and UV–vis absorption studies.
Synchronous excitation spectra and EEMF of oils can be used as a fingerprint and can be applied, together with fluorescence intensities, as a rapid screening tool to investigate similarities of oils. Thus, when accelerated thermal ageing method is used, the onset of degradation of transformer oil on 17th day and transformer oil with polypropylene and cellulosic paper on 23rd and 27th days is sensitively reflected in the SFS and EEMF spectral characteristics. This was supported by the FTIR and NIR studies on the ageing of transformer oil and the cellulosic paper. Fluorescence is a totally unperturbative technique. Delivery of excitation light and collection of fluorescence light can be easily done using optical fibers. Thus, it should be possible to develop in situ monitoring methods for transformer oil degradation. Acknowledgements S.D. thanks Council of Scientific and Industrial Research (CSIR), New Delhi for fellowship. We are grateful to Sophisti-
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cated Analytical Instrumentation Facility, IIT Madras for FTIR and NIR spectral studies. We thank Raj Lubricants, Chennai for the supply of sample and valuable help. References [1] IP346/92 (BS2000 Pt. 346), Determination of PCA content in unused lubricating base oils and asphaltene-free petroleum fractions—DMSO extraction/refractive index method, The British Standards Institute, 389 Chiswick High Road, London, W4 4AL United Kingdom. [2] N.E. Frost, P.B. Mcgrath, C.W. Burns, IEEE Trans. Power Deliv. 11 (1996) 331–334. [3] J.J. Kelly, IEEE Trans. Ind. Appl. 16 (1980) 777. [4] J. Unsworth, F. Mitchell, IEEE Trans. Electr. Insul. 25 (1990) 737. [5] A.M. Emsley, X. Xiao, R.J. Heywood, M. Ali, IEEE Proc. Sci. Measur. Tech. 147 (2000) 116. [6] J.A. Palmer, X. Wang, A. Mander, D. Torgerson, C. Rich, Conference Record of the 2000 IEEE International Symposium on Electrical Insulation, Anaheim, CA, USA, 2000, p. 460. [7] T.K. Saha, M. Darveniza, Z.T. Yao, D.J.T. Hill, G. Yeung, IEEE Trans. Power Deliv. 14 (1999) 1359. [8] D.J.T. Hill, T.T. Le, M. Darvenzia, T.K. Saha, Polym. Degrad. Stability 49 (1995) 429. [9] R. Neimanis, H. Lennholm, R. Kriksson, 1999 Annual Report Conference on Electrical Insulation and Dielectric Phenomena, 1999, p. 162.
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