testing. In the present paper, thermal stability of thermally upgraded Kraft (TUK) and Nomex-910 impregnated mineral transformer oils has been investigated at ...
Life Assessment of TUK and Nomex-910 Impregnated Mineral Transformer Oils using Raman Spectroscopy Chilaka Ranga
Ashwani Kumar Chandel
Department of Electrical Engineering National Institute of Technology Hamirpur Himachal Pradesh, 177005, India
Department of Electrical Engineering National Institute of Technology Hamirpur Himachal Pradesh, 177005, India
Abstract—Concentration of furfural (Furan-2carbaldehyde) in transformer oils is a well-known indicator for thermal degradation in oil-paper insulation. The application of Raman spectroscopy to determine thermal deterioration as a function of Furfural concentration can overcome many disadvantages of traditional detection methods and accelerated testing. In the present paper, thermal stability of thermally upgraded Kraft (TUK) and Nomex-910 impregnated mineral transformer oils has been investigated at higher accelerated thermal stresses. Samples having a standard mass ratio of oil and paper insulations in accordance with IEEE Std. C57.1542012 have been studied. These samples were thermally stressed at 1200C and 1500C. Subsequently, the aged samples were analyzed using Raman spectroscopy test. Furfural was characterized by Raman signal at 1705.69 cm-1, where no spectral interferences caused by oil-derived Raman signals occur. It has been observed from the test results that the oil sample consisting of TUK has higher life than that of the Nomex-910 immersed oil sample. It indicates that the Furfural formation in TUK test samples is more as compared to its Nomex-910 counterpart. Hence the degradation rate of TUK oil samples is more at higher temperatures. Consequently, Nomex-910 has been found more suitable for high temperature applications than TUK. Keywords—transformer; insulation; condition monitoring; Raman spectroscopy; mineral oil.
I. INTRODUCTION Transformer is one of the most important equipments of electrical power transmission and distribution systems [1, 2]. Reliability and long operational life of power transformer are highly desirable. Both reliability and life of the transformer are determined by its health condition and degradation rate of its insulation. Degradation of insulation in a transformer caused due to the various stress present in the transformer such as electrical, mechanical, chemical and thermal stress. These stresses mainly influence the degradation rate of the transformers. In general, mineral oil and thermally upgraded Kraft are used as tradition insulation inside the transformers. Mineral oil is a petroleum based product which is used in power transformer for more than 100 years, and it has long
successful history [3]. Nominal temperature of mineral transformer oil is 1200C. Despite its long term success mineral oil is facing criticism because of its low fire point, flash point, 2-FAL, low breakdown voltage and higher degree of polymerization (DP) and degradation rate. DP value for highly degraded oil sample is less than 250 whereas it is in between 1000 to 1200 for fresh mineral oil. Mineral oil is not biodegradable spillage of oil in the event of fault or leakage create major environmental hazard [4]. Also with the increase in environmental concern we are looking for eco-friendlier alternative. Moreover, limited stock of petroleum product will limit its use in future as insulating liquid in transformer [5]. During the past few decades, researchers across the world are working in this area in order to investigate an alternative liquid dielectric for the transformers [6]. Some of the researchers proposed that the combination of mineral oil with synthetic ester oil offer better performance than that of mineral and synthetic ester oils separately. Also it is found that a blend of mineral and synthetic ester oils in 80% and 20 % proportion is suitable for both technical and economical point of view [79]. With the advancement in technology and increase in population, the need of compact substation is ineludible. Thus, there is a great need for the equipment which occupies less space and high safety factor. Loading and size of the transformer is heavily dependent on the losses occurring in the transformer [10]. To reduce the volume of power transformers and enhance their long-term operational reliability, it is desirable to develop a new kind of oil-paper insulation system which has higher dielectric strength, higher thermal conductivity and lower dielectric losses as compared to conventional insulation systems. In this paper, an effort has been made to find the remnant life of transformer with alternative solid dielectrics. To find an alternative solid dielectric, a new paper is considered which is designed and manufactured by Du-Pont company in 2014 i.e. Nomex [11] which have higher thermal stability than thermally upgraded Kraft. Nomex have higher tensile and breakdown strength than thermally upgraded Kraft. Further, an accelerated thermal aging is performed at different temperatures. In order to analyze the insulation performance, Raman spectroscopy test has been performed. From the test
results and visual examination, it is observed that Nomex have higher operating life than that of thermally upgraded Kraft. II. RAMAN SPECTROSCOPY Raman scattering refers to inelastic scattering of photons. Raman scattering gives information regarding the molecular structure and properties on account of their transitions to different vibrational states when excited by a source of light. Raman scattering is a two-photon event. The polarizibility of the molecule changes with respect to its vibrational transitions. An induced dipole moment is created as a result of interaction of polarizibility and incoming photon [11, 12]. The light scattered as a result of induced dipole of the molecule consists of both Rayleigh scattering and Raman scattering. Rayleigh scattering represents the scattering in which frequency of incoming light and outgoing light is same thus no net energy transfer takes place. If there is gain in energy then photons are scattered at lower wavelength and such a shift is known as stokes shift. Similarly if photons are shifted to lower energy level at higher energy level then the shift is known as anti stokes shift. Fig. 1 shows the energy diagram for infrared absorption and stokes Raman scattering for a vibrational transition from g0 to g1. The scattering photo energy, hw, is shifted from the incident laser radiation energy by the infrared vibrational energy, hw, gained by the molecule. Fig. 2 shows Raman spectroscopy test set up of NIT Hamirpur. The Raman spectroscopy test was conducted as per the procedure given in Ref. [9] and [10].
Fig.1 Energy Diagram.
hours. Later the flasks were cooled down to room temperature. Samples in the present work were prepared with both Nomex and TUK insulations, in a standard mass ratio (i.e. 20% ester oil and 80% mineral oil) as per IEEE Std C57.154-2012 [13].
Fig. 3 Samples placed in an air-circulated temperature oven. Conical flasks are filled with 200 ml of mineral oil. These flasks were kept in oven, and subjected to a temperature of 100oC for 24 hours so as to remove the moisture from the oil. Further, the Nomex (3 mill) and TUK (3 mill) paper put in two separate flask filled with mix which were put into oven at 100oC. Sample also contains a copper strip (35mm x 20mm x 3mm) to replicate the real operating condition of power transformer. Likewise, a total of eight samples were thermally aged. Finally, all the samples were tightly sealed with rubber corks and covered by aluminum foil. B. Aging Procedure Initially, four samples of each TUK and Nomex were kept in temperature oven and thermally aged at 120 oC. After 24 hours, one set of Nomex and TUK is removed from oven. Similarly, other sets of samples are removed from oven after 48, 72 and 96 hours. After the completion of aging at 120 oC, 8 samples are collected whereas 4 samples correspond to Nomex and the other four are having TUK, aged for 24, 48, 72 and 96 hours. Same accelerated thermal aging procedure is performed for 150oC and 180oC temperatures. Finally, two sets each with 8 samples aged at 120oC and 150oC. Subsequently, Raman spectroscopy test has been performed on each of all the test samples. C. Relation between 2-FAL and DP Chendong Method was updated by Stebbins method which was suitable for thermally upgraded version of Kraft [14]. Equation (2) gives the relation between DP and 2-FAL.
Fig. 2 RAMAN Spectroscopy test set-up in Energy Centre of Material Science and Engineering at NIT Hamirpur. III. EXPERIMENTAL WORK A. Preparation of Samples Initially, the conical flasks were neatly cleaned with water and then dried out by PID controller based air-circulated temperature oven (500±2)oC) which is set at 100oC for 24
2
DP
log10 [2 - FALppm .0.88] - 4.51 -0.0035
(1)
IV. RESULTS AND DISCUSSION It is necessary to point out that Raman Spectroscopy will be used to detect the presence of 2-FAL by presence of a peak at 1705.69cm-1. This is nothing but qualitative analysis using
Raman Spectroscopy. This test will also be used to quantify the amount of 2-FAL in order to calculate DP value and thus finding remnant life of insulation. Raman spectra of various samples are obtained with the help of Raman Spectrometer and control set. These spectra are shown in Fig. 4 and Fig. 5. These spectra are at different temperature and time duration thus showing different levels of deterioration. These spectra clearly indicate the presence of peak at 1705.69cm-1. The presence of peak at this particular wave number indicates the presence of 2-FAL. The plot is between wave number and intensity ratio. The value of intensity will be later utilized to quantify 2-FAL. Table 1 details peak intensity of Raman Spectra at 1705.69cm-1.
ageing. This clearly suggests that fall in degree of polymerization value for Nomex-910 will be less than TUK for same ageing. It can be concluded from above statement that insulation life of Nomex-910 will be more than its TUK counterpart. A. Quantitative Analysis using Raman spectroscopy The mathematical formula correlating the intensity ratio and concentration of 2-FAL is given in Equation (1). The analysis at 3 sets of temperature is carried out on the basis of above mentioned equations and is presented in form of tables and graphs. The remnant life calculation of TUK at 120oC is given in Tables 2 and 3. Fig. 6 shows the elapsed life as per the aging factor. Table 2. Remnant life calculation of TUK at 120OC Time duration
Intensity ratio
2-FAL concentration
DP
Expected life
Life used
24 48 72 96
2.25× 106 2.85×106 3.94×106 4.93×106
0.0113 0.0144 0.0199 0.0249
1002 973 933 905
30 30 30 30
1.9 2.51 3.36 3.99
Remn ant life 28.1 27.49 26.64 26.01
Table 3. Remnant life calculation of Nomex-910 at 120OC
Fig. 4 Raman Spectra of TUK at 120OC, 24hrs
Time duration
Intensity ratio
2-FAL concentration
DP
Expected life
Life used
24 48 72 96
2.07×106 2.55×106 3.31×106 4.11×106
0.0104 0.0129 0.0167 0.0208
1013 987 954 927
35 35 35 35
1.69 2.22 2.9 3.49
Rem nant life 33.31 32.78 32.1 31.51
Fig. 5 Raman Spectra of Nomex-910 at 120OC, 24hrs Table 1. Peak Intensity of Raman Spectra at 1705.69cm-1 Hours
Temperature (1800C) 6998.79 6158.94 8029.56 6664.54 9319.24 7362.20 11154.45 8365.83
Fig. 6 Elapsed life vs aging factor curve of TUK and Nomex-910 at 120OC.
Table 1 clearly suggests that for TUK, intensity of peak is increasing with the increase in time duration of ageing. Also a significant increase in peak intensity can be observed with increase in temperature. Similar assessment can be made for Nomex-910 where also one can see that intensity of peak to be a function of temperature and time. This table also suggests that the intensity of peak of Nomex-910 sample for a given temperature and time is less as compared to TUK for same temperature and time. This clearly states that concentration of 2- FAL in Nomex-910 is less as compared to TUK for same
DP value goes on decreasing with the increase in temperature and time duration of aging. This suggests loss of tensile strength. The rate of loss of DP value is higher in case of TUK than in Nomex-910, which in turn suggests the loss of strength is rapid in TUK as in comparison to Nomex-910. Life lapsed which is a function of DP value is more in case of TUK as in case of Nomex-910. This suggests that for same temperature and time remnant life of Nomex-910 is more than that of TUK. The graph between life elapsed and aging factor clearly indicates that amount of life lost for same aging is less in nomex-910 as compared to its TUK counterpart. During the
24 48 72 96
Temperature (1200C) 103.53 95.25 175.49 154.43 393.56 366.66 699.17 559.33
Temperature (1500C) 1582.56 1424.31 2695.69 2293.88 3938.37 3230.58 5823.57 4542.38
experimental work it is found that Raman Spectroscopy is a very strong tool for degradation analysis of solid insulation. Table 4. Remnant life calculation of TUK at 150OC Time durati on 24 48
4.09×106 1.05×106
2-FAL concentra tion 0.0207 0.0759
72
3.39×106
0.1713
666
30
96
5.50×106
0.2782
606
30
Intensity ratio
DP
Expecte d life
Life used
928 767
30 30
3.48 7.39 10.2 8 12.2 2
Rem nant life 26.52 22.61 19.72 17.88
Table 5. Remnant life calculation of Nomex-910 at 150OC Time duratio n
Intensit y ratio 3.54×10
24
6
1.15×10
48
6
2.57×10
72
6
3.94×10
96
6
2-FAL concentratio n 0.0178 0.058 0.130 0.198
DP 94 6 79 8 70 0 64 7
Expecte d life
Life used
Remnan t life
35
3.08
31.92
35
6.58
28.42
35
9.26
25.74
35
10.8 6
24.14
Fig.8 Colour variation with ageing temperature for TUK and Nomex-910 in mix oil after 72 hours. For more clearly visual observation, paper is brought out from the flask. It is observed that the reduction in thickness in Nomex paper is less than that of TUK [19]. It was also observed that Nomex papers were physically stronger than TUK. This was evident from minute fragments which started to come out from TUK paper, when it was kept at higher thermal stress for more time period. Minute fragments settle down at bottom of the ampoules. This increases the darkness, and increases the viscosity of the oil. In Actual practice with the increase in viscosity, fragment coming out from paper and sludge formation hinder the flow of oil in cooling tube. Thus, the use of Nomex paper not only increases the oxidation stability but also improves the cooling performances of transformer oil. It has better thermal performance at higher temperatures. VI. CONCLUSION
Fig. 7 Elapsed Life vs Aging Factor of TUK and Nomex-910 at 150OC
V. VISUAL INSPECTION As oil is subjected to thermal stress the colour of the oil becomes darker and its viscosity increases. Colour of oil changes with an increase in temperature and duration of thermal aging [16, 17]. Change in colour and physical appearance is critical observation to determine the condition of transformer from insulating oil. When sample is subjected under thermal degradation, both oil and paper get degrade. The degradation products from oil and paper will dissolve in to the oil [18]. Thus, diagnosis of oil gives indication about the overall degradation of transformer. From Figure 5, it is evident that Nomex samples are less dark for same accelerated aging as compared to its TUK counterpart.
4
In the present paper, the operating performance of thermally upgraded Kraft (TUK) and Nomex-910 insulated samples has been investigated at higher accelerated thermal stresses. Samples having a standard mass ratio of oil and paper insulations in accordance with IEEE Std. C57.154-2012 have been studied. These samples were stressed at higher temperatures between 120oC, 150oC and 180oC. Subsequently, the aged samples were analyzed using Raman spectroscopy test. Furfural was characterized by Raman signal at 1705.69 cm-1, where no spectral interferences caused by oil-derived Raman signals occur. Further, the remnant life of samples has been evaluated. It has been determined from the test results that the oil sample consisting of TUK has higher life than that of the Nomex-910 immersed oil sample. Acknowledgment The authors are thankful to the authorities of Centre for Material Science & Engineering and TEQIP-II of NIT Hamirpur India for providing the necessary facilities and financial aid wide grant number NIT/HMR/TEQIPII/Research & Develpoment-19/2015/2157-63 to perform the experiments and carrying out the present research. Authors are also thankful to the Himachal Pradesh State Electricity Board (HPSEB), India for providing the transformer oil samples.
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Chilaka Ranga (S’16) received B. Tech. degree in Electrical and Electronics Engineering from Bapatla Engineering College, Bapatla (AP), India in 2010. He received his M.Tech. degree from National Institute of Technology (NIT), Hamirpur (HP), India, in 2012. Presently he is pursuing his Ph.D. from EED, NIT Hamirpur. His areas of interest are performance evaluation and health assessment of power transformers. He is the IEEE Student Branch Chair of NIT Hamirpur, India.
Ashwani Kumar (S’05–M’15) received his Ph.D. degree from Indian Institute of Technology Roorkee, India in 2005. Dr. Chandel joined the Department of Electrical Engineering, NIT, Hamirpur, India, as Lecturer in 1991, where presently he is working as a Professor. His research areas are harmonic estimation and elimination, condition monitoring of transformers. He is a Fellow of IETE, Member IEEE and Life Member of ISTE.
