chromatography mass spectrometry (HS-GC-MS) setup. Result showed that ... ordinary kraft paper inside sealed glass bottles. Methanol and ethanol in oil were ...
Ageing Assessment of Transformer Paper Insulation through Detection of Methanol in Oil S. Y. Matharage1, Q. Liu1, Z. D. Wang1, P. Mavrommatis2, G. Wilson3 and P. Jarman3 1
The University of Manchester, Manchester, M13 9PL, UK TJ/H2b Analytical Services Ltd, Bromborough, CH62 4SU, UK 3 National Grid, Warwick, CV34 6DA, UK
2
Abstract—Ageing assessment of transformer paper insulation is important due to its irreversible ageing nature. Due to the poor accessibility of the paper samples in field transformers, large efforts were made to seek for chemical parameters in oil to indicate paper ageing, e.g. furanic compounds and dissolved carbon oxides. In recent years, alternative parameters including methanol and ethanol in oil were proposed to be related with paper ageing. In this study, a laboratory ageing experiment was conducted at 120 °C with an inhibited mineral oil and an ordinary kraft paper. Methanol and ethanol in oil were measured through an in-house developed method based on a headspace gas chromatography mass spectrometry (HS-GC-MS) setup. Result showed that methanol in oil increases linearly with the chain scission in paper which suggests its suitability to indicate early paper ageing. Keywords—transformer; ageing; methanol; ethanol; degree of polymerisation; paper
I. INTRODUCTION Transformer paper insulation with its irreversible ageing nature is considered as a factor that determines the lifetime of power transformers. However, poor accessibility of paper samples has caused difficulties in measuring the ageing state of paper directly through measurements of tensile strength and degree of polymerization (DP). Therefore, researchers developed methods to assess ageing state of paper indirectly through chemical indicators in oil that are related to paper ageing. For example, furanic compounds and carbon oxides have been used in the field to indicate paper ageing for the past decades [1, 2]. Although it is widely accepted that furans are generated exclusively from paper ageing, the mechanism is yet to be fully understood. High performance liquid chromatography (HPLC) has been used to measure the furanic compounds in oil. Out of the six furanic compounds that are related to paper ageing, 2FAL showed high amount of generation and better stability than the other furanic compounds. Therefore 2-FAL has been widely used to assess the ageing state of paper insulation in field transformers. Researchers proposed several different mechanisms for the generation of furanic compounds from cellulose through both hydrolysis and pyrolysis [3]. Nevertheless some researchers also suggested that 2-FAL could originate from the degradation of pentoses in hemicellulose which affected the assurance of 2-FAL as an
ageing indicator for cellulosic paper [4]. In addition, 2-FAL tends to indicate the late stage of paper degradation [5]. Carbon oxides including carbon monoxide (CO) and carbon dioxides (CO2) are measured through gas chromatography (GC) techniques. In addition to paper, generation of these gases was found from thermal degradation of other components in transformers including oil, paint, varnish and phenolic resins [6]. Moreover, in free-breathing transformers carbon dioxide level could be highly affected by the leakages from the atmosphere [6]. According to IEC 60599, CO2/CO ratio of less than 3 is generally considered to be an indication of a fault involved with paper degradation. Nevertheless it has been mentioned that the correction for the absorption of CO2 from the atmosphere and the correction for the background CO2 and CO are required to obtain reliable results. The availability of existing paper ageing indicators never stop research initiatives on investigating new paper ageing indicators, e.g. low molecular weight acid [7], methanol (CH3OH) and ethanol (C2H5OH) in oil [5]. In one such investigation headspace gas chromatography mass spectrometry (HS-GC-MS) technique was employed to observe the volatile compounds in aged transformer oil that are related to paper ageing. Different ageing experiments including ageing of oil/paper, ageing of components in paper such as cellulose, hemicellulose, lignin and ageing of several intermediate paper ageing products such as levoglucosan and glucose were conducted in the investigations [5]. As a result of these investigations, methanol was related to the ageing of cellulose and ethanol was related to the ageing of levoglucosan [5]. Furthermore, these two indicators showed good stability at transformer working temperatures. In addition, methanol showed a linear increase with paper ageing, whereas 2-FAL showed an exponential increase. Furthermore the amount of methanol was higher than the amount of 2-FAL till the DP reached ~400 [8]. These results imply that methanol can be used to indicate early ageing state of paper [9]. On the other hand, ethanol showed a high yield from ageing of levoglucosan, which is a pyrolysis by-product of cellulose ageing. Therefore, it was suggested to use ethanol to indicate abnormal paper ageing in transformers. The statement was further supported through high amount of ethanol measurements in a service-aged transformer that had abnormal paper ageing as a result of the arcing inside the transformer [10].
This paper investigates the variation of methanol and ethanol with paper ageing through a laboratory ageing experiment conducted using conventional mineral oil and ordinary kraft paper inside sealed glass bottles. Methanol and ethanol in oil were measured through an in-house method developed with a HS-GC-MS system and the ageing state of paper was measured through degree of polymerisation (DP). II. EXPERIMENTAL DESCRIPTION A. Laboratory Ageing Experiment A laboratory based accelerated thermal ageing experiment was conducted at 120 °C with an inhibited mineral oil (Gemini X) and an ordinary kraft paper. The oil was first filtered using a nylon membrane (0.2 µm) to remove particles and then dried at 85 °C in vacuum condition below 500 Pa (5 mbar) for 48 hours in order to remove the moisture. Moisture in paper was removed by drying paper strips of 230 mm × 28 mm at 105 °C for 24 hours in an air circulation oven. Next, samples prepared by mixing 200 g of dried oil and 10 g of dried paper inside glass bottles were left in vacuum condition below 500 Pa (5 mbar) at 85 °C for another 24 hours in order to impregnate the paper with the oil. Finally, the oil and impregnated paper samples were enclosed in the bottles with caps and aged inside an air circulation oven (Binder FD 115) at 120 °C for up to 14 weeks. Samples taken out from the oven were kept at room temperature for 72 hours before measurements in order to allow the chemical indicators to partition between oil and paper. Finally both oil and paper samples were used for measurements including methanol in oil, ethanol in oil and DP of paper. B. Methanol and Ethanol Measurements in Oil An in-house developed method using a HS-GC-MS setup with a CTC combi PAL headspace sampler unit, Varian CP 3800 GC unit and Varian Saturn 2200 ion trap type MS unit was used for the measurements [11]. Fig. 1 shows the block diagram of the setup. In the automated measurement process, a 20 ml headspace vial filled with 10 g of oil was first heated while being agitated in order to extract volatile compounds into the air. Then the extracted volatile compounds were injected into the GC unit using a 1 ml gastight syringe attached to the headspace unit. Compounds in the mixture were separated using a 60 m VF-624 ms mid-polar GC column (0.25 mm internal diameter and 1.4 µm film thickness).
Headspace sampler Computer Injector GC oven with GC column GC unit
Transfer line
Ion trap unit MS unit
Fig. 1. Block diagram of the HS-GC-MS setup
Vacuum pump
Next, these separated compounds were sent into the MS unit through a heated transfer line. Finally the compounds were ionised and scanned from mass-to-charge (m/z) ratios 10 to 100 amu inside the MS unit. Internal standard calibration with ethanol d-6 isotope was used in quantification. Calibration curves obtained from the internal standard calibration method were used to convert the peak heights obtained from the MS unit into concentration values in oil. Calibration samples from 0.05 ppm to 2 ppm of methanol and ethanol in transformer oil were prepared by diluting a stock solution. Furthermore, 5 µl of the internal standard solution (~2000 ppm of ethanol d-6 in transformer oil) was added into all the samples including both calibration and measuring samples. Calibration curves were obtained by plotting peak height ratio between the indicators and the internal standard compound as in Fig. 2. Peak heights of methanol and ethanol were obtained from extracted ion chromatograms (EIC) plotted using the ion counts with m/z ratio of 31 whereas peak heights of ethanol d-6 isotope were obtained from EIC plotted for m/z ratio of 33.
Fig. 2. Calibration curves obtained for methanol and ethanol using internal standard method
III. RESULTS AND DISCUSSION A. Measurements of Methanol and Ethanol in Oil Fig. 3 and Fig. 4 show the variation of methanol and ethanol in oil with ageing. Methanol shows a rapid increase during the early stage of ageing. Then, at the latter stage the increase rate reduces and the amount of methanol only increased up to ~3 ppm at the end of the ageing period. This initial fast increasing rate is possibly due to the degradation of weak glycosidic bonds in the amorphous region at the early ageing stage. In contrast to methanol, ethanol in oil is less than the detection limit during early stage of ageing and then shows a slight increase with ageing. Nevertheless, even at the latter stage, the rate of increase of ethanol was substantially lower than that of methanol. At the end of the ageing period, the amount of ethanol reaches ~0.35 ppm which is only one eighth of the amount of methanol measured at the same time.
B. Measurements of DP in Paper Fig. 5 shows the reduction of average viscometric degree of polymerisation (DP) in paper with ageing. It was measured according to ASTM D4243. DP of paper reduces from ~1100 to 200 in an exponential manner during the ageing period. DP shows a drastic reduction down to 600 within the first two weeks of ageing and then reduces in a slower rate at the latter stage.
Fig. 3. Variation of methanol in oil with ageing at 120 °C
Fig. 5. Variation of the DP of paper with ageing at 120 °C
C. Variation of the Number of Ruptured Bonds in Paper Average number of ruptured bonds in paper is a molecular level property of cellulose which can be used to indicate ageing state of paper [14]. This is obtained from the DP measurements using (1). Average Number of ruptured bonds = (DPv(0) ∕ DPv(t)) − 1
(1)
Where, DPv(0) represents the initial DP value and DPv(t) represents the DP value at ageing time of t. Fig. 4. Variation of ethanol in oil with ageing at 120 °C
Fig. 6 shows the variation of the number of ruptured bonds in paper with ageing.
These measurements conform to the previous finding [10, 12] that the amount of ethanol generation from oil paper systems is lower than the amount of methanol generation at the investigated temperature. Apart from that, it is difficult to state any conclusions such as the origination of ethanol. According to the literature, ethanol could be generated from either oil or paper [5, 10]. Paper ageing at the investigated temperature of 120 °C can generate small amount of levoglucosan which would eventually lead to ethanol generation. Furthermore it is a well-known fact that alcohols are intermediate by product of oil oxidation [13]. Therefore small amount of ethanol could be generated from ageing of oil itself. Nevertheless, contributions from oil and paper in both methanol and ethanol generations are yet to be understood.
Fig. 6. Variation of number of ruptured bonds in paper with ageing at 120 °C
Chain scission of paper shows a linear increase with ageing during the early ageing stage which then reduces to a slower rate at the late ageing stage. The high chain scission rate might be due to the degradation of the weak links in the amorphous region. D. Relationship between Methanol and Paper Ageing Fig. 7 shows the relationship between methanol in oil and paper ageing represented by the number of ruptured bonds. It is evident that Methanol in oil increases linearly with the number of ruptured bonds. Therefore, methanol in oil could be a promising ageing parameter to indicate paper ageing due to the high amount of generation at early ageing stage and a linearly increasing relationship with paper ageing.
ACKNOWLEDGMENT The authors would like to express their gratitude to M&I Materials, National Grid, Scottish Power, Shell Global Solutions, TJ|H2b Analytical Services, UK Power Network and WEIDMANN Electrical Technology for their financial and technical contributions to the transformer research consortium at The University of Manchester. REFERENCES [1] [2]
[3]
[4]
[5]
[6]
[7]
[8] Fig. 7. Variation of methanol with paper ageing at 120 °C [9]
IV. CONCLUSIONS A laboratory ageing experiment was conducted with an ordinary kraft paper and an inhibited mineral oil in order to investigate the promising new paper ageing indicators including methanol and ethanol in oil. An in-house developed method based on a headspace gas chromatography mass spectrometry (HS-GC-MS) unit was used to measure methanol and ethanol in transformer oils. Paper ageing was observed through measurements of average viscometric degree of polymerisation. Methanol shows a linear increase with paper ageing which confirms its suitability of indicating the early paper ageing. The amount of ethanol in oil was much lower than the amount of methanol at the investigated temperature. Nevertheless, further experiments on the contributions of oil ageing to methanol and ethanol generation are proposed to understand the origination of these chemical indicators.
[10]
[11]
[12]
[13] [14]
R. Tamura, H. Aneti, T. Ishii, and T. Kawamura, "Diagnostic of aging deterioration of insulating paper," JIEE Proc. A, 1981, pp. 30 - 36. P. J. Burton, M. Carballeira, M. Duval, C. W. Fuller, J. Graham, A. de Pablo, et al., "Application of liquid chromatography to the analysis of electrical insulating materials," ClGRE, International Conference on Large High Voltage Electric Systems, Paris, France, Paper No. 12-12, 1988. A. M. Emsley, "The kinetics and mechanisms of degradation of cellulosic insulation in power transformers," Polymer Degradation and Stability, vol. 44, pp. 343-349, 1994. J. Scheirs, G. Camino, M. Avidano, and W. Tumiatti, "Origin of furanic compounds in thermal degradation of cellulosic insulating paper," Journal of Applied Polymer Science, vol. 69, pp. 2541-2547, 1998. J. Jalbert, R. Gilbert, P. Tétreault, B. Morin, and D. Lessard-Déziel, "Identification of a chemical indicator of the rupture of 1,4-β-glycosidic bonds of cellulose in an oil-impregnated insulating paper system," Cellulose, vol. 14, pp. 295-309, 2007. L. Lundgaard, D. Allan, I. Hohlein, R. Clavreul, M. Dahlund, H.-P. Gasser, et al., "Ageing of cellulose in mineral-oil insulated transformers," Cigre Brochuer:2007. N. Azis, Q. Liu, and Z. D. Wang, "Ageing assessment of transformer paper insulation through post mortem analysis," IEEE Transactions on Dielectrics and Electrical Insulation, vol. 21, pp. 845-853, 2014. D. Laurichesse, Y. Bertrand, C. Tran-Duy, and V. Murin, "Ageing diagnosis of MV/LV distribution transformers via chemical indicators in oil," 2013 IEEE Electrical Insulation Conference (EIC), pp. 464-468, 2013. J. Jalbert, R. Gilbert, Y. Denos, and P. Gervais, "Methanol: A Novel Approach to Power Transformer Asset Management," IEEE Transactions on Power Delivery, vol. 27, pp. 514-520, 2012. E. Rodriguez-Celis, J. Jalbert, S. Duchesne, B. Noirhomme, M. C. Lessard, and M. Ryadi, "Chemical markers use for the diagnosis and life estimation of power transformers: a preliminary study of their origins," 2012 CIGRE Canada Conference, Montreal, Quebec, 2012. S. Y. Matharage, Q. Liu, E. Davenport, G. Wilson, D. Walker, and Z. D. Wang, "Methanol detection in transformer oils using gas chromatography and ion trap mass spectrometer," 2014 IEEE 18th International Conference on Dielectric Liquids (ICDL), pp. 1-4, 2014. M. Ryadi, A. Tanguy, J. Jalbert, and C. Rajotte, "Alcohol based ageing chemical markers for the diagnosis of transformer cellulosic insulation," CIGRE A2 and D1 Joint Colloquium, Kyoto, Japan, 2011. Y. Bertrand, M. dahlund, d. pablo, and et.al, "CIGRE Brochure 526: Oxidation stability of insuating fluids," 2013. H. Z. Ding and Z. D. Wang, "On the degradation evolution equations of cellulose," Cellulose, vol. 15, pp. 205-224, 2008.
