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Investigation on effects of different types of nanoparticles on critical parameters of nano-liquid insulation systems
Journal of Molecular Liquids 230 (2017) 437–444
Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Investigation on effects of different types of nanoparticles on critical parameters of nano-liquid insulation systems Madavan R. a,⁎, Sujatha Balaraman b a b
Department of EEE, P.S.R. Engineering College, Sivakasi, 626 140, India Department of EEE, Government College of Technology, Coimbatore, 641 013, India
a r t i c l e
i n f o
Article history: Received 12 December 2016 Accepted 17 January 2017 Available online 19 January 2017 Keywords: Natural esters Mineral oil Nanoparticles Nano-oil Electrical characteristics
a b s t r a c t The effects of insulative, semi-conductive and conductive nanostructured particles at various concentrations on critical parameters of Nano-Liquid Insulation (NLI) systems are experimentally investigated and the results are discussed in detail. In this study, liquid insulations such as Sunflower Oil (SO) and Rapeseed Oil (RO) as Natural Esters and conventional Mineral Oil (MO) as petroleum derived oil have been utilized as base fluid. Here, various nanostructured Magnetite (Fe3O4) as conductive nanoparticle, Zinc Oxide (ZnO) as semi-conductive nanoparticle and Silica (SiO2) as insulating nanoparticle are employed to investigate the influence of concentration and different types of nanoparticles in critical characteristics of base fluids. Experimental findings reveal that, the concentration and the types of nanoparticles make an impact in the performance of NLIs. The conductive nanoparticles based NLIs with higher concentration offer higher enhancement in performance characteristics compared to semi-conductive and insulating nanoparticles based NLIs. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Over a century, power industries have grown in many aspects particularly in transformer technology to meet the needs of customers and be a part in growth of a country. The reliability of transformer is ruled by a multifaceted insulation structure as paper and pressboard surrounded conductors immersed in liquid insulation. Liquid insulation is the heart of transformer, as it provides electrical insulation to the live parts, transfers the heat released from conductors and diagnosis support to assess the condition of the transformer [1]. In transformers, two types of petroleum derived Mineral Oil (MO) is used as insulating fluid for the past 100 years. One is paraffinic oil and other is napthanic oil. The major anxiety over MO is environmental unfriendliness and depletion in availability of resources. Due to serious environmental regulations and liability risks involving MO replacement, transformer industries have begun to use natural esters as alternative insulating fluid for MO due to its environmental friendliness. Commercially available natural ester based liquid insulations are BIOTEMP and Envirotemp [2,3]. From the Failure data of transformers, it's noticeable that the average service life time of transformer gets reduced by 17.8 years due to the problems in insulation systems. This is far away from the expected life time of 35 to 40 years. In power transformers, 75% of failures occur due to the insulation problems alone [4,5]. Operational life time and ⁎ Corresponding author. E-mail address: [email protected] (M. R.).
http://dx.doi.org/10.1016/j.molliq.2017.01.057 0167-7322/© 2017 Elsevier B.V. All rights reserved.
reliability of transformers depends on the condition of liquid – solid insulation system used in it [6,7]. Therefore, there is a need to improve the properties of transformer insulation system to avoid premature failure of transformer. Due to the smarter physical properties of nanomaterials, it attracts many researchers to do more research works in various field includes liquid insulation too. The major focus on present day research is to enhance the electrical properties of liquid insulation system without negotiating its thermal and physical properties. A liquid with the dispersion of nanoparticle is entitled as nanofluid, a term nanofluid is conferred by Choi at Argonne National Lab in 1995 [8]. In 1990s, researchers prepared conductive nanoparticles suspended nanofluids, dielectric and thermal properties have been studied. From the results, it's confirmed that dielectric and thermal properties of the conductive nanoparticles suspended nanofluids gets improved excellently [9,10]. Basically, the conductive nanoparticles are able to trap the free fast moving electrons which contribute for steamer development and convert those fast moving electrons into slow moving negatively charged particles. The above phenomenon depends on the relaxation time constant of the nanoparticle. If the relaxation time constant of a nanoparticle is short relative to the time scale of streamer growth, then it modifies electrodynamics of oil notably and it increases the dielectric breakdown voltage of the transformer oil [10–13]. Else, there is a slight change in the electrodynamics of oil. Some researchers prepared semi-conductive and non-conductive Titanium oxide, Zinc oxide, Zirconium oxide, Copper oxide, Aluminum oxide, etc. … suspended nanofluids because of its superior stability compared to the pure metals [14–20] and carbon nanotubes [21]. Also,
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M. R., S. Balaraman / Journal of Molecular Liquids 230 (2017) 437–444
Table 1 Base values of liquid insulation systems. Properties at RTPa
MO
SO
RO
Density (kg/m3) Breakdown strength (kV/cm) Flash point (°C) Fire point (°C) Viscosity (cst)
870 28 137 146 17
920 38 270 282 46
921 32 263 274 54
a
(RO) and three different nanoparticles namely Magnetite (Fe3O4), Zinc Oxide (ZnO) and Silica (SiO2) have been utilized to investigate the effects of different nanoparticles at various concentrations from 0% to 0.25% on the performance of NLIs. 2. Materials 2.1. Liquid insulations
RTP – Room temperature and pressure.
Table 2 Base values of nanoparticles [10]. Parameters
Fe3O4
ZnO
SiO2
Density (g/m3) Electrical conductivity (S/m)
5.17 1 × 104– 1 × 105 80 7.47 × 10−14 Conductor
5.61 10–1 × 103
2.20 1.4 × 10−9
7.4–8.9 1.05 × 10−11 Semiconductor
3.8 5.12 × 10−2 Insulator
Relative dielectric constant Relaxation time (s) Material type
these nanoparticles are cheaper and can be manufactured easily. These nanofluids can improve the dielectric breakdown voltage of the transformer oil. Here, the electron trapping is in the process of streamer development in transformer liquid insulation [22]. As of now, in natural esters, there is an absence of detailed study on effects of different categories of nanoparticles at various concentrations. The present work reports the effects of concentration and types of nanoparticles on the Nano Liquid Insulation system (NLIs). Three different liquid insulations namely MO, Sunflower Oil (SO) and Rapeseed Oil
The liquid insulations (MO, SO and RO) are utilized in the present study have been purchased from commercial oil companies. After that, the insulating liquids are thermally treated at a temperature of 100 °C to remove the moisture content present in the oil. Thermally treated liquids are allowed to cool down to room temperature and filtered with Whatman filter paper to remove the impurities present in the liquids. Before the suspension of nanoparticles with liquids, the moisture content of MO, SO and RO liquids are reduced to 5.7 ppm, 17.2 ppm and 18.4 ppm respectively. Since, the moisture content and impurities present in liquid insulations affects the performance of the liquids. The base values of liquid insulations are given in Table 1. 2.2. Nanomaterials The nanomaterials utilized for the present investigation are purchased from Sigma-Aldrich. Before dispersion of nanoparticles into liquid insulations, preprocessing has been done to remove the organic and inorganic impurities present in the nanoparticles. Some of the basic properties of nanoparticles are given in Table 2. Scanning Electron Microscopy images of Fe3O4, ZnO and SiO2 nanoparticles are shown in Fig. 1(a–c). It is
Fig. 1. SEM images of (a) Fe3O4, (b) ZnO and (c) SiO2.
M. R., S. Balaraman / Journal of Molecular Liquids 230 (2017) 437–444
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Table 3 Description of samples. Sample no
Concentration
MO1 MO2 MO3 MO4 MO5 MO6 MO7 MO8 MO9 MO10 MO11 MO12 MO13 MO14 MO15
MO MO MO MO MO MO MO MO MO MO MO MO MO MO MO
+ + + + + + + + + + + + + + +
0.05% of Fe3O4 0.1% of Fe3O4 0.15% of Fe3O4 0.2% of Fe3O4 0.25% of Fe3O4 0.05% of ZnO 0.1% of ZnO 0.15% of ZnO 0.2% of ZnO 0.25% of ZnO 0.05% of SiO2 0.1% of SiO2 0.15% of SiO2 0.2% of SiO2 0.25% of SiO2
Sample no
Concentration
Sample no
Concentration
SO1 SO2 SO3 SO4 SO5 SO6 SO7 SO8 SO9 SO10 SO11 SO12 SO13 SO14 SO15
SO SO SO SO SO SO SO SO SO SO SO SO SO SO SO
RO1 RO2 RO3 RO4 RO5 RO6 RO7 RO8 RO9 RO10 RO11 RO12 RO13 RO14 RO15
RO RO RO RO RO RO RO RO RO RO RO RO RO RO RO
evident that the particles are in spherical shape and the average size of nanoparticles utilized for the present study is between 50 nm–80 nm. 3. Preparation of nano-liquid insulation system NLIs are prepared by dispersion of different categories of nanoparticles (Fe3O4, ZnO and SiO2) in a concentration range of 0%–0.25% volume fraction with liquid insulations. After the magnetic stirring process for 1 h, the NLI sample is placed in the sonicator for 2 h to agitate the agglomeration of nanoparticles and make the dispersion as uniform and stable. Smaller amount of oleic acid is utilized as a restricting agent of agglomeration
+ + + + + + + + + + + + + + +
0.05% of Fe3O4 0.1% of Fe3O4 0.15% of Fe3O4 0.2% of Fe3O4 0.25% of Fe3O4 0.05% of ZnO 0.1% of ZnO 0.15% of ZnO 0.2% of ZnO 0.25% of ZnO 0.05% of SiO2 0.1% of SiO2 0.15% of SiO2 0.2% of SiO2 0.25% of SiO2
+ + + + + + + + + + + + + + +
0.05% of Fe3O4 0.1% of Fe3O4 0.15% of Fe3O4 0.2% of Fe3O4 0.25% of Fe3O4 0.05% of ZnO 0.1% of ZnO 0.15% of ZnO 0.2% of ZnO 0.25% of ZnO 0.05% of SiO2 0.1% of SiO2 0.15% of SiO2 0.2% of SiO2 0.25% of SiO2
and improve the stability of the dispersion. It is noticed from the experiments that the existence of restricting agents in NLIs has no or very weak impact on the performance of liquid insulations. The prepared NLIs samples are numbered as given in Table 3. 4. Experimental procedure and standards 4.1. Breakdown voltage A fully automatic Megger oil breakdown tester is used to measure AC breakdown voltage of liquid insulations as per IEC 60156 standard,
Fig. 2. Breakdown strength of NLI samples (a) MO, (b) SO and (c) RO.
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Fig. 3. Permittivity of NLI samples (a) MO, (b) SO and (c) RO.
using spherical electrode with spacing of 2.5 mm. A step input voltage is applied on the electrodes at a uniform rate of 2 kV/s and the average of six successive measurements is taken as breakdown voltage of oil. The time delay between successive measurements is 2 min.
furnace for increasing the temperature of the sample and measurement is carried out at 90 °C.
4.2. Moisture
The flash point and fire point of NLIs samples are measured as per ASTM D93 standard using Pensky Martin Flash point measurement kit at room temperature. The measurement apparatus consists of closed brass test cup, the sample to be tested is filled in the test cup and temperature of the sample is increased by external regulator setup. The state at which the vapour formed within closed test cup combined with air to kindle a short-term fire on the surface of oil for less than 1 s is called flash point temperature. Similarly, occurrence of continual fire on the surface of oil is called fire point temperature.
NLIs moisture content is measured as per ASTM D1533 standard by using Karl-Fischer titration. Before starting the measurements, the instrument is calibrated with reference sample. Consecutively, the measuring sample is taken in syringe and weight of the syringe with sample is measured in high accurate digital weighing machine. Then, the samples are subjected to titration process. After that, the weight of the empty syringe is measured. These two measured are utilized to calculate the presence of moisture content in liquid insulations in parts per mole (ppm). The above said measurements are carried out at room temperature. 4.3. Electrical conductivity and permittivity The conductivity and permittivity of the NLIs samples are measured at 90 °C as per IEC 60247 standards using MLO-1D apparatus. The apparatus consists of test cell, electrodes and furnaces. One of the prime concerns over the apparatus is cleanliness of the test cell. Since, presence of minute impurities in the test cell influences the conductivity measurement. Therefore, the test cell is properly cleaned and dried at 110 °C for 1 h. Subsequently, the test cell is filled with NLI samples and the electrodes are dipped into the test cell. Then, the test cell is placed into the
4.4. Flash point and fire point
4.5. Viscosity The viscosity of the NLIs samples are measured as per ASTM D445 standard using Redwood viscometer apparatus at 90 °C. The apparatus consists of silver plated test cup with orifice. The sample to be tested is filled in the test cup; by the opening of orifice the time required for collection of 50 ml of oil at the bottom of the apparatus is measured to find the kinematic viscosity of the sample. 5. Experimental results and discussions Breakdown voltage performance of NLIs has been experimentally performed for various volume concentrations and the results are
M. R., S. Balaraman / Journal of Molecular Liquids 230 (2017) 437–444
shown in Fig. 2(a–c). It is interesting to notice that the breakdown voltage of the NLIs is high compared to the pure (i.e. non-modified liquid insulation) liquid insulation. With the increase in volume concentration, breakdown voltage of the NLIs gets enhanced linearly for all the three categories of nanoparticles. The presence of Fe3O4, ZnO and SiO2 nanoparticles enhances the breakdown voltage of MO by 50%, 44% and 40% respectively. For SO, enhancement in breakdown voltage is 51%, 46% and 42% respectively and for RO, enhancement in breakdown voltage is 52.5%, 47.5% and 42.5% respectively. This confirms that Fe3O4, ZnO and SiO2 nanoparticles effectively trap the free electrons and enhances the breakdown voltage. The breakdown strength of nanofluids mainly depends on relaxation time constant, size of the nanoparticle and shape of the nanoparticle. It is confirmed from the Fig. 1, all the three (Fe3O4, ZnO and SiO2) dispersed nanoparticles are in spherical shape. Hence, the breakdown strength of nanofluids gets enhanced. Fe3O4 nanoparticles have lesser relaxation time constant (7.4 ∗ 10−14) followed by ZnO (1.05 ∗ 10−11) and then by SiO2 (5.12 ∗ 10−2) nanoparticles [10] thereby Fe3O4 based nanofluids have higher breakdown strength than that of ZnO and SiO2 dispersed nanofluids. This further reduces streamer development in Fe3O4 dispersed nanofluids and also modifies the electrodynamics property of oil. Hence, the rate of enhancement in breakdown strength of Fe3O4 dispersed nanofluids is higher when compared to ZnO and SiO2 dispersed nanofluids. Moreover, while considering size of the nanoparticle, Fe3O4 nanoparticle size is less (50 nm) compared to ZnO (60 nm) and SiO2 (80 nm). Density of the small size nanoparticle is higher than the large size nanoparticle for the same concentration. This enhances the probability of scavenging free charges from the streamer and leads to higher breakdown strength. Moreover,
441
comparing the dielectric properties of liquid insulations, SO and RO have higher breakdown strength than MO and this confirms that, natural esters are better alternatives for MO. The possible fact behind the enhancement of breakdown voltage of NLIs is the addition of nanoparticles causes changes in electrodynamics of NLIs when it's subjected to high voltage stress. The enhancement in breakdown voltage of NLIs is depends on morphed electrodynamics of oil due to introduction of nanoparticles. Generally, the electric arc bridges the gap between two electrodes during the occurrence of breakdown in NLIs at typical voltage. The development of streamer propagates towards each other to bridge the electrode gap and is having the direct relationship with applied voltage and breakdown voltage of liquid insulation [23]. According to the model proposed by Hwang et al. [10] uniformly and homogenously dispersed, polarized/charged nanoparticles in liquid insulation are responsible for trapping of fast moving electrons which causes reduction in velocity of positive streamer and thus breakdown voltage of NLIs gets enhanced. Introduction of nanoparticles in liquid insulation leads to effective scavenging of free electrons in the streamer and thus reduction in concentration of free electrons in the streamer. Thereby, growth of streamer at a slower manner and breakdown in nanofluids gets delayed. The relative permittivity of NLIs is higher than that of pure liquid insulations as shown in Fig. 3(a–c). With increase in volume concentration, the permittivity of NLIs is excellently improved. The dispersed Fe3O4, ZnO and SiO2 nanoparticles enhance the permittivity of the MO by 54.04%, 45.95% and 37.83% respectively, for SO, the enhancement is 21.5%, 19.45% and 15.01% respectively and for RO, the enhancement is 20.25%, 19.45% and 13.92% respectively. The enhancement rate of
Fig. 4. Electrical conductivity of NLI samples (a) MO, (b) SO and (c) RO.
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permittivity of MO is high compared to SO and RO. The relative permittivity of liquid insulation is fairly low compared to oil-impregnated pressboard. This phenomenon is considered to be a beneficial cause to have more uniform field distribution in the oil – paper insulation system [24]. Fig. 4(a–c) shows the experimental results of electrical conductivity of NLIs versus volume concentration. In Fig. 4(a–c) linear dependence relationship between electrical conductivity and volume concentration have been reported. The presence of Fe3O4, ZnO and SiO2 nanoparticles augment the conductivity of NLIs and in addition to this, the conductivity of the NLIs is increased with increase in volume concentration. While considering the conductivity of liquid insulations, SO and RO have lesser conductivity compared to MO. Moreover, the Fe3O4 nanoparticle dispersed liquid insulation has lesser enhancement in conductivity compared to other nanoparticles dispersed in liquid insulations. Since, the conductivity is having inversely proportional relationship with permittivity, i.e. if the permittivity is high, then the conductivity ought to be less. Thereby, the breakdown voltage will be high. The conductivity of NLIs not only influenced by physical properties of liquid insulation and category of nanoparticles, but also influenced by electro-chemical properties, particle size and aggregation of nanoparticles. After the dispersion of nanoparticles in liquid insulation, distance between the nanoparticles exists and the nanofluid is treated as scattered system. Therefore, zeta potential and the mobility of the Brownian motion changes with change in the distance between nanoparticle i.e. for example, if the distance between the dispersed nanoparticles is large, the zeta potential and Brownian motion gets increased. Thus the
electrical conductivity of the NLIs gets changed. If the volume concentration on NLIs increases, the aggregation of nanoparticles cannot be neglected and it leads to amplification of the particle size. This factor may have an effect on the Brownian motion and electrophoresis of nanoparticles, which affects the electrical conductivity of the NLIs [25]. Viscosity expresses the internal resistance to flow of a liquid insulation and it is a prime concern for all the thermal applications which involves liquid as cooling agents. It's clear from Fig. 5(a–c), in all three nanoparticles; viscosity of liquid insulations gets increased with increase in volume concentrations. The dispersed Fe3O4, ZnO and SiO2 nanoparticles increases the viscosity of MO by 61%, 69% and 78% respectively, for SO, increment in viscosity is 30%, 47% and 52% respectively and for RO increment in viscosity is 28%, 34% and 39% respectively. Moreover, comparing the viscosity of liquid insulations, viscosity of SO and RO is high as compared to MO. The viscosity of NLIs affected by particle volume fraction, material type, size of the particle and agglomeration of dispersed particles in the liquid insulation. When the dispersed particles in liquid insulations tend to arrange into a structure, the neighboring particles in liquid gets less mobilized which in turn the liquid insulation becomes less viscous [26]. The enhancement in viscosity weakens the potential benefits of NLIs. Therefore, it is very important to do more studies on the viscosity of NLIs. The viscosity has a greater influence in both electrical and thermal properties of liquid insulation system. Generally, viscosity is inversely proportional to temperature and breakdown strength of oil. When the temperature increases, it decreases the viscosity of the oil thereby breakdown strength of the oil gets increased. Moreover, viscosity of oil
Fig. 5. Viscosity of NLI samples (a) MO, (b) SO and (c) RO.
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Fig. 6. Flashpoint of NLI samples (a) MO, (b) SO and (c) RO.
gets affected by formation of sludges and pour point. Sludges are formed due to oxidation during the aging process and do not have immediate impact. Generally, pour point is the lowest temperature at which the oil starts flow. The pour point of the oil is determined by measuring the temperature at which the oil does not move in the transformer tank. When oil becomes solid at lower temperature, it directly affects the viscosity of the oil. Thereby, increase in viscosity at lower temperature weakens the rate of heat convection of oil. The SO and RO have somewhat higher (around −10 °C) pour point than MO (−45 °C) [3]. In the countries like India there is no need to consider pour point due to its warm climatic condition. The flash point and fire point values of NLIs are increasing with increase in volume concentration as shown in Figs. 6(a–c) and 7(a–c). It confirms that thermal properties of NLIs show excellent improvement. The presence of Fe3 O4 , ZnO and SiO2 nanoparticles augment the flash point and fire point properties of MO with an utmost augmentation of 13.87%, 10.95% and 8.76% and 9.59%, 7.53% and 6.16% respectively. When comparing the thermal properties of liquid insulations, SO and RO have higher flash point and fire point values compared to MO. This factor proves that, natural esters as excellent alternative for MO and its considered to be a promising one. Thermal properties of NLIs affected by agglomeration of nanoparticles dispersed in liquid insulation, shape of the material, size, category, Brownian motion of the nanoparticle and heat transfer ability of nanoparticle. Nanoparticles which are present in liquid insulation moves constantly and irregularly caused by Brownian can enhance the thermal properties of NLIs [26].
6. Conclusion The study on the effects of nanoparticles in MO, SO and RO have been experimentally analyzed. A comprehensive method for various properties of NLIs observed as a function of various types of nanoparticles and its concentration has been reported. The nanoparticles infused in liquid insulations properties acquire maximum level of enhancement. In particular, the breakdown strength of Fe3O4 nanoparticle dispersed liquid insulations recorded maximum enhancement compared to other nanoparticles. In addition to that SO and RO liquid insulations breakdown strength are higher when compared with MO. By looking over the permittivity and electrical conductivity of NLIs, the Fe3O4 nanoparticle dispersed liquid insulations have registered superior performance compared to ZnO and SiO2. By considering the thermal properties, NLIs have higher flash point and fire point enhancement rate with increase in volume concentration. Here, SO and RO liquid insulations with nanoparticles recorded excellent performance compared to MO. By concern over viscosity, the SO and RO samples viscosity is high compared with MO and at the same time, the viscosity of the NLIs increases with increase in volume concentration. When comparing all the above reported experimental results, natural esters (SO and RO) based nanofluids are having superior performance then MO and they may be considered as a promising alternative to MO based nanofluids. Apart from this, there must be a detailed study carried out for improving the viscosity of natural esters liquid insulations.
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Fig. 7. Firepoint of NLI samples (a) MO, (b) SO and (c) RO.
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Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Investigation on effects of different types of nanoparticles on critical parameters of nano-liquid insulation systems Madavan R. a,⁎, Sujatha Balaraman b a b
Department of EEE, P.S.R. Engineering College, Sivakasi, 626 140, India Department of EEE, Government College of Technology, Coimbatore, 641 013, India
a r t i c l e
i n f o
Article history: Received 12 December 2016 Accepted 17 January 2017 Available online 19 January 2017 Keywords: Natural esters Mineral oil Nanoparticles Nano-oil Electrical characteristics
a b s t r a c t The effects of insulative, semi-conductive and conductive nanostructured particles at various concentrations on critical parameters of Nano-Liquid Insulation (NLI) systems are experimentally investigated and the results are discussed in detail. In this study, liquid insulations such as Sunflower Oil (SO) and Rapeseed Oil (RO) as Natural Esters and conventional Mineral Oil (MO) as petroleum derived oil have been utilized as base fluid. Here, various nanostructured Magnetite (Fe3O4) as conductive nanoparticle, Zinc Oxide (ZnO) as semi-conductive nanoparticle and Silica (SiO2) as insulating nanoparticle are employed to investigate the influence of concentration and different types of nanoparticles in critical characteristics of base fluids. Experimental findings reveal that, the concentration and the types of nanoparticles make an impact in the performance of NLIs. The conductive nanoparticles based NLIs with higher concentration offer higher enhancement in performance characteristics compared to semi-conductive and insulating nanoparticles based NLIs. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Over a century, power industries have grown in many aspects particularly in transformer technology to meet the needs of customers and be a part in growth of a country. The reliability of transformer is ruled by a multifaceted insulation structure as paper and pressboard surrounded conductors immersed in liquid insulation. Liquid insulation is the heart of transformer, as it provides electrical insulation to the live parts, transfers the heat released from conductors and diagnosis support to assess the condition of the transformer [1]. In transformers, two types of petroleum derived Mineral Oil (MO) is used as insulating fluid for the past 100 years. One is paraffinic oil and other is napthanic oil. The major anxiety over MO is environmental unfriendliness and depletion in availability of resources. Due to serious environmental regulations and liability risks involving MO replacement, transformer industries have begun to use natural esters as alternative insulating fluid for MO due to its environmental friendliness. Commercially available natural ester based liquid insulations are BIOTEMP and Envirotemp [2,3]. From the Failure data of transformers, it's noticeable that the average service life time of transformer gets reduced by 17.8 years due to the problems in insulation systems. This is far away from the expected life time of 35 to 40 years. In power transformers, 75% of failures occur due to the insulation problems alone [4,5]. Operational life time and ⁎ Corresponding author. E-mail address: [email protected] (M. R.).
http://dx.doi.org/10.1016/j.molliq.2017.01.057 0167-7322/© 2017 Elsevier B.V. All rights reserved.
reliability of transformers depends on the condition of liquid – solid insulation system used in it [6,7]. Therefore, there is a need to improve the properties of transformer insulation system to avoid premature failure of transformer. Due to the smarter physical properties of nanomaterials, it attracts many researchers to do more research works in various field includes liquid insulation too. The major focus on present day research is to enhance the electrical properties of liquid insulation system without negotiating its thermal and physical properties. A liquid with the dispersion of nanoparticle is entitled as nanofluid, a term nanofluid is conferred by Choi at Argonne National Lab in 1995 [8]. In 1990s, researchers prepared conductive nanoparticles suspended nanofluids, dielectric and thermal properties have been studied. From the results, it's confirmed that dielectric and thermal properties of the conductive nanoparticles suspended nanofluids gets improved excellently [9,10]. Basically, the conductive nanoparticles are able to trap the free fast moving electrons which contribute for steamer development and convert those fast moving electrons into slow moving negatively charged particles. The above phenomenon depends on the relaxation time constant of the nanoparticle. If the relaxation time constant of a nanoparticle is short relative to the time scale of streamer growth, then it modifies electrodynamics of oil notably and it increases the dielectric breakdown voltage of the transformer oil [10–13]. Else, there is a slight change in the electrodynamics of oil. Some researchers prepared semi-conductive and non-conductive Titanium oxide, Zinc oxide, Zirconium oxide, Copper oxide, Aluminum oxide, etc. … suspended nanofluids because of its superior stability compared to the pure metals [14–20] and carbon nanotubes [21]. Also,
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Table 1 Base values of liquid insulation systems. Properties at RTPa
MO
SO
RO
Density (kg/m3) Breakdown strength (kV/cm) Flash point (°C) Fire point (°C) Viscosity (cst)
870 28 137 146 17
920 38 270 282 46
921 32 263 274 54
a
(RO) and three different nanoparticles namely Magnetite (Fe3O4), Zinc Oxide (ZnO) and Silica (SiO2) have been utilized to investigate the effects of different nanoparticles at various concentrations from 0% to 0.25% on the performance of NLIs. 2. Materials 2.1. Liquid insulations
RTP – Room temperature and pressure.
Table 2 Base values of nanoparticles [10]. Parameters
Fe3O4
ZnO
SiO2
Density (g/m3) Electrical conductivity (S/m)
5.17 1 × 104– 1 × 105 80 7.47 × 10−14 Conductor
5.61 10–1 × 103
2.20 1.4 × 10−9
7.4–8.9 1.05 × 10−11 Semiconductor
3.8 5.12 × 10−2 Insulator
Relative dielectric constant Relaxation time (s) Material type
these nanoparticles are cheaper and can be manufactured easily. These nanofluids can improve the dielectric breakdown voltage of the transformer oil. Here, the electron trapping is in the process of streamer development in transformer liquid insulation [22]. As of now, in natural esters, there is an absence of detailed study on effects of different categories of nanoparticles at various concentrations. The present work reports the effects of concentration and types of nanoparticles on the Nano Liquid Insulation system (NLIs). Three different liquid insulations namely MO, Sunflower Oil (SO) and Rapeseed Oil
The liquid insulations (MO, SO and RO) are utilized in the present study have been purchased from commercial oil companies. After that, the insulating liquids are thermally treated at a temperature of 100 °C to remove the moisture content present in the oil. Thermally treated liquids are allowed to cool down to room temperature and filtered with Whatman filter paper to remove the impurities present in the liquids. Before the suspension of nanoparticles with liquids, the moisture content of MO, SO and RO liquids are reduced to 5.7 ppm, 17.2 ppm and 18.4 ppm respectively. Since, the moisture content and impurities present in liquid insulations affects the performance of the liquids. The base values of liquid insulations are given in Table 1. 2.2. Nanomaterials The nanomaterials utilized for the present investigation are purchased from Sigma-Aldrich. Before dispersion of nanoparticles into liquid insulations, preprocessing has been done to remove the organic and inorganic impurities present in the nanoparticles. Some of the basic properties of nanoparticles are given in Table 2. Scanning Electron Microscopy images of Fe3O4, ZnO and SiO2 nanoparticles are shown in Fig. 1(a–c). It is
Fig. 1. SEM images of (a) Fe3O4, (b) ZnO and (c) SiO2.
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Table 3 Description of samples. Sample no
Concentration
MO1 MO2 MO3 MO4 MO5 MO6 MO7 MO8 MO9 MO10 MO11 MO12 MO13 MO14 MO15
MO MO MO MO MO MO MO MO MO MO MO MO MO MO MO
+ + + + + + + + + + + + + + +
0.05% of Fe3O4 0.1% of Fe3O4 0.15% of Fe3O4 0.2% of Fe3O4 0.25% of Fe3O4 0.05% of ZnO 0.1% of ZnO 0.15% of ZnO 0.2% of ZnO 0.25% of ZnO 0.05% of SiO2 0.1% of SiO2 0.15% of SiO2 0.2% of SiO2 0.25% of SiO2
Sample no
Concentration
Sample no
Concentration
SO1 SO2 SO3 SO4 SO5 SO6 SO7 SO8 SO9 SO10 SO11 SO12 SO13 SO14 SO15
SO SO SO SO SO SO SO SO SO SO SO SO SO SO SO
RO1 RO2 RO3 RO4 RO5 RO6 RO7 RO8 RO9 RO10 RO11 RO12 RO13 RO14 RO15
RO RO RO RO RO RO RO RO RO RO RO RO RO RO RO
evident that the particles are in spherical shape and the average size of nanoparticles utilized for the present study is between 50 nm–80 nm. 3. Preparation of nano-liquid insulation system NLIs are prepared by dispersion of different categories of nanoparticles (Fe3O4, ZnO and SiO2) in a concentration range of 0%–0.25% volume fraction with liquid insulations. After the magnetic stirring process for 1 h, the NLI sample is placed in the sonicator for 2 h to agitate the agglomeration of nanoparticles and make the dispersion as uniform and stable. Smaller amount of oleic acid is utilized as a restricting agent of agglomeration
+ + + + + + + + + + + + + + +
0.05% of Fe3O4 0.1% of Fe3O4 0.15% of Fe3O4 0.2% of Fe3O4 0.25% of Fe3O4 0.05% of ZnO 0.1% of ZnO 0.15% of ZnO 0.2% of ZnO 0.25% of ZnO 0.05% of SiO2 0.1% of SiO2 0.15% of SiO2 0.2% of SiO2 0.25% of SiO2
+ + + + + + + + + + + + + + +
0.05% of Fe3O4 0.1% of Fe3O4 0.15% of Fe3O4 0.2% of Fe3O4 0.25% of Fe3O4 0.05% of ZnO 0.1% of ZnO 0.15% of ZnO 0.2% of ZnO 0.25% of ZnO 0.05% of SiO2 0.1% of SiO2 0.15% of SiO2 0.2% of SiO2 0.25% of SiO2
and improve the stability of the dispersion. It is noticed from the experiments that the existence of restricting agents in NLIs has no or very weak impact on the performance of liquid insulations. The prepared NLIs samples are numbered as given in Table 3. 4. Experimental procedure and standards 4.1. Breakdown voltage A fully automatic Megger oil breakdown tester is used to measure AC breakdown voltage of liquid insulations as per IEC 60156 standard,
Fig. 2. Breakdown strength of NLI samples (a) MO, (b) SO and (c) RO.
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Fig. 3. Permittivity of NLI samples (a) MO, (b) SO and (c) RO.
using spherical electrode with spacing of 2.5 mm. A step input voltage is applied on the electrodes at a uniform rate of 2 kV/s and the average of six successive measurements is taken as breakdown voltage of oil. The time delay between successive measurements is 2 min.
furnace for increasing the temperature of the sample and measurement is carried out at 90 °C.
4.2. Moisture
The flash point and fire point of NLIs samples are measured as per ASTM D93 standard using Pensky Martin Flash point measurement kit at room temperature. The measurement apparatus consists of closed brass test cup, the sample to be tested is filled in the test cup and temperature of the sample is increased by external regulator setup. The state at which the vapour formed within closed test cup combined with air to kindle a short-term fire on the surface of oil for less than 1 s is called flash point temperature. Similarly, occurrence of continual fire on the surface of oil is called fire point temperature.
NLIs moisture content is measured as per ASTM D1533 standard by using Karl-Fischer titration. Before starting the measurements, the instrument is calibrated with reference sample. Consecutively, the measuring sample is taken in syringe and weight of the syringe with sample is measured in high accurate digital weighing machine. Then, the samples are subjected to titration process. After that, the weight of the empty syringe is measured. These two measured are utilized to calculate the presence of moisture content in liquid insulations in parts per mole (ppm). The above said measurements are carried out at room temperature. 4.3. Electrical conductivity and permittivity The conductivity and permittivity of the NLIs samples are measured at 90 °C as per IEC 60247 standards using MLO-1D apparatus. The apparatus consists of test cell, electrodes and furnaces. One of the prime concerns over the apparatus is cleanliness of the test cell. Since, presence of minute impurities in the test cell influences the conductivity measurement. Therefore, the test cell is properly cleaned and dried at 110 °C for 1 h. Subsequently, the test cell is filled with NLI samples and the electrodes are dipped into the test cell. Then, the test cell is placed into the
4.4. Flash point and fire point
4.5. Viscosity The viscosity of the NLIs samples are measured as per ASTM D445 standard using Redwood viscometer apparatus at 90 °C. The apparatus consists of silver plated test cup with orifice. The sample to be tested is filled in the test cup; by the opening of orifice the time required for collection of 50 ml of oil at the bottom of the apparatus is measured to find the kinematic viscosity of the sample. 5. Experimental results and discussions Breakdown voltage performance of NLIs has been experimentally performed for various volume concentrations and the results are
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shown in Fig. 2(a–c). It is interesting to notice that the breakdown voltage of the NLIs is high compared to the pure (i.e. non-modified liquid insulation) liquid insulation. With the increase in volume concentration, breakdown voltage of the NLIs gets enhanced linearly for all the three categories of nanoparticles. The presence of Fe3O4, ZnO and SiO2 nanoparticles enhances the breakdown voltage of MO by 50%, 44% and 40% respectively. For SO, enhancement in breakdown voltage is 51%, 46% and 42% respectively and for RO, enhancement in breakdown voltage is 52.5%, 47.5% and 42.5% respectively. This confirms that Fe3O4, ZnO and SiO2 nanoparticles effectively trap the free electrons and enhances the breakdown voltage. The breakdown strength of nanofluids mainly depends on relaxation time constant, size of the nanoparticle and shape of the nanoparticle. It is confirmed from the Fig. 1, all the three (Fe3O4, ZnO and SiO2) dispersed nanoparticles are in spherical shape. Hence, the breakdown strength of nanofluids gets enhanced. Fe3O4 nanoparticles have lesser relaxation time constant (7.4 ∗ 10−14) followed by ZnO (1.05 ∗ 10−11) and then by SiO2 (5.12 ∗ 10−2) nanoparticles [10] thereby Fe3O4 based nanofluids have higher breakdown strength than that of ZnO and SiO2 dispersed nanofluids. This further reduces streamer development in Fe3O4 dispersed nanofluids and also modifies the electrodynamics property of oil. Hence, the rate of enhancement in breakdown strength of Fe3O4 dispersed nanofluids is higher when compared to ZnO and SiO2 dispersed nanofluids. Moreover, while considering size of the nanoparticle, Fe3O4 nanoparticle size is less (50 nm) compared to ZnO (60 nm) and SiO2 (80 nm). Density of the small size nanoparticle is higher than the large size nanoparticle for the same concentration. This enhances the probability of scavenging free charges from the streamer and leads to higher breakdown strength. Moreover,
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comparing the dielectric properties of liquid insulations, SO and RO have higher breakdown strength than MO and this confirms that, natural esters are better alternatives for MO. The possible fact behind the enhancement of breakdown voltage of NLIs is the addition of nanoparticles causes changes in electrodynamics of NLIs when it's subjected to high voltage stress. The enhancement in breakdown voltage of NLIs is depends on morphed electrodynamics of oil due to introduction of nanoparticles. Generally, the electric arc bridges the gap between two electrodes during the occurrence of breakdown in NLIs at typical voltage. The development of streamer propagates towards each other to bridge the electrode gap and is having the direct relationship with applied voltage and breakdown voltage of liquid insulation [23]. According to the model proposed by Hwang et al. [10] uniformly and homogenously dispersed, polarized/charged nanoparticles in liquid insulation are responsible for trapping of fast moving electrons which causes reduction in velocity of positive streamer and thus breakdown voltage of NLIs gets enhanced. Introduction of nanoparticles in liquid insulation leads to effective scavenging of free electrons in the streamer and thus reduction in concentration of free electrons in the streamer. Thereby, growth of streamer at a slower manner and breakdown in nanofluids gets delayed. The relative permittivity of NLIs is higher than that of pure liquid insulations as shown in Fig. 3(a–c). With increase in volume concentration, the permittivity of NLIs is excellently improved. The dispersed Fe3O4, ZnO and SiO2 nanoparticles enhance the permittivity of the MO by 54.04%, 45.95% and 37.83% respectively, for SO, the enhancement is 21.5%, 19.45% and 15.01% respectively and for RO, the enhancement is 20.25%, 19.45% and 13.92% respectively. The enhancement rate of
Fig. 4. Electrical conductivity of NLI samples (a) MO, (b) SO and (c) RO.
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permittivity of MO is high compared to SO and RO. The relative permittivity of liquid insulation is fairly low compared to oil-impregnated pressboard. This phenomenon is considered to be a beneficial cause to have more uniform field distribution in the oil – paper insulation system [24]. Fig. 4(a–c) shows the experimental results of electrical conductivity of NLIs versus volume concentration. In Fig. 4(a–c) linear dependence relationship between electrical conductivity and volume concentration have been reported. The presence of Fe3O4, ZnO and SiO2 nanoparticles augment the conductivity of NLIs and in addition to this, the conductivity of the NLIs is increased with increase in volume concentration. While considering the conductivity of liquid insulations, SO and RO have lesser conductivity compared to MO. Moreover, the Fe3O4 nanoparticle dispersed liquid insulation has lesser enhancement in conductivity compared to other nanoparticles dispersed in liquid insulations. Since, the conductivity is having inversely proportional relationship with permittivity, i.e. if the permittivity is high, then the conductivity ought to be less. Thereby, the breakdown voltage will be high. The conductivity of NLIs not only influenced by physical properties of liquid insulation and category of nanoparticles, but also influenced by electro-chemical properties, particle size and aggregation of nanoparticles. After the dispersion of nanoparticles in liquid insulation, distance between the nanoparticles exists and the nanofluid is treated as scattered system. Therefore, zeta potential and the mobility of the Brownian motion changes with change in the distance between nanoparticle i.e. for example, if the distance between the dispersed nanoparticles is large, the zeta potential and Brownian motion gets increased. Thus the
electrical conductivity of the NLIs gets changed. If the volume concentration on NLIs increases, the aggregation of nanoparticles cannot be neglected and it leads to amplification of the particle size. This factor may have an effect on the Brownian motion and electrophoresis of nanoparticles, which affects the electrical conductivity of the NLIs [25]. Viscosity expresses the internal resistance to flow of a liquid insulation and it is a prime concern for all the thermal applications which involves liquid as cooling agents. It's clear from Fig. 5(a–c), in all three nanoparticles; viscosity of liquid insulations gets increased with increase in volume concentrations. The dispersed Fe3O4, ZnO and SiO2 nanoparticles increases the viscosity of MO by 61%, 69% and 78% respectively, for SO, increment in viscosity is 30%, 47% and 52% respectively and for RO increment in viscosity is 28%, 34% and 39% respectively. Moreover, comparing the viscosity of liquid insulations, viscosity of SO and RO is high as compared to MO. The viscosity of NLIs affected by particle volume fraction, material type, size of the particle and agglomeration of dispersed particles in the liquid insulation. When the dispersed particles in liquid insulations tend to arrange into a structure, the neighboring particles in liquid gets less mobilized which in turn the liquid insulation becomes less viscous [26]. The enhancement in viscosity weakens the potential benefits of NLIs. Therefore, it is very important to do more studies on the viscosity of NLIs. The viscosity has a greater influence in both electrical and thermal properties of liquid insulation system. Generally, viscosity is inversely proportional to temperature and breakdown strength of oil. When the temperature increases, it decreases the viscosity of the oil thereby breakdown strength of the oil gets increased. Moreover, viscosity of oil
Fig. 5. Viscosity of NLI samples (a) MO, (b) SO and (c) RO.
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Fig. 6. Flashpoint of NLI samples (a) MO, (b) SO and (c) RO.
gets affected by formation of sludges and pour point. Sludges are formed due to oxidation during the aging process and do not have immediate impact. Generally, pour point is the lowest temperature at which the oil starts flow. The pour point of the oil is determined by measuring the temperature at which the oil does not move in the transformer tank. When oil becomes solid at lower temperature, it directly affects the viscosity of the oil. Thereby, increase in viscosity at lower temperature weakens the rate of heat convection of oil. The SO and RO have somewhat higher (around −10 °C) pour point than MO (−45 °C) [3]. In the countries like India there is no need to consider pour point due to its warm climatic condition. The flash point and fire point values of NLIs are increasing with increase in volume concentration as shown in Figs. 6(a–c) and 7(a–c). It confirms that thermal properties of NLIs show excellent improvement. The presence of Fe3 O4 , ZnO and SiO2 nanoparticles augment the flash point and fire point properties of MO with an utmost augmentation of 13.87%, 10.95% and 8.76% and 9.59%, 7.53% and 6.16% respectively. When comparing the thermal properties of liquid insulations, SO and RO have higher flash point and fire point values compared to MO. This factor proves that, natural esters as excellent alternative for MO and its considered to be a promising one. Thermal properties of NLIs affected by agglomeration of nanoparticles dispersed in liquid insulation, shape of the material, size, category, Brownian motion of the nanoparticle and heat transfer ability of nanoparticle. Nanoparticles which are present in liquid insulation moves constantly and irregularly caused by Brownian can enhance the thermal properties of NLIs [26].
6. Conclusion The study on the effects of nanoparticles in MO, SO and RO have been experimentally analyzed. A comprehensive method for various properties of NLIs observed as a function of various types of nanoparticles and its concentration has been reported. The nanoparticles infused in liquid insulations properties acquire maximum level of enhancement. In particular, the breakdown strength of Fe3O4 nanoparticle dispersed liquid insulations recorded maximum enhancement compared to other nanoparticles. In addition to that SO and RO liquid insulations breakdown strength are higher when compared with MO. By looking over the permittivity and electrical conductivity of NLIs, the Fe3O4 nanoparticle dispersed liquid insulations have registered superior performance compared to ZnO and SiO2. By considering the thermal properties, NLIs have higher flash point and fire point enhancement rate with increase in volume concentration. Here, SO and RO liquid insulations with nanoparticles recorded excellent performance compared to MO. By concern over viscosity, the SO and RO samples viscosity is high compared with MO and at the same time, the viscosity of the NLIs increases with increase in volume concentration. When comparing all the above reported experimental results, natural esters (SO and RO) based nanofluids are having superior performance then MO and they may be considered as a promising alternative to MO based nanofluids. Apart from this, there must be a detailed study carried out for improving the viscosity of natural esters liquid insulations.
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Fig. 7. Firepoint of NLI samples (a) MO, (b) SO and (c) RO.
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