Chemist’s Perspective Laboratory Testing of Natural Ester Dielectric Liquids A previous article presented information on a new breed of liquid dielectrics, namely natural esters. This article provides information on the tests that can be performed to evaluate the chemical, physical, and electrical properties of natural ester oils. After the last article, I became aware that Dielectric Systems Incorporated also manufactures a natural ester dielectric liquid called ECO Fluid which is based on a high-oleic acid content canola oil (see Sundin). Information concerning ECO Fluid is presented in this article along with the other natural esters. In addition, two oils have been removed from the market, BITORANS 1000 and 5000. Throughout this article, many comparisons are made to the oil quality test results of transformer mineral oil, as this is something we are all familiar with. It must be emphasized, however, that the properties of a natural ester cannot be correlated directly to that of a mineral oil as the chemistries of the two types of liquids are very different. They are so different, in fact, that ASTM has produced a new specification just for natural esters, referenced as ASTM D 6871 “Specification for Natural (Vegetable Oil) Ester Fluids Used in Electrical Apparatus.” For the most part, the same tests used to evaluate mineral oil are used to evaluate natural esters. Results and the meaning of the tests are different between the two oils. Doble evaluates natural esters using five categories identified as follows: • • • • • Physical tests Electrical tests Chemical tests Accelerated oxidation tests (discussed in the next article) Dissolved gas analysis (discussed in the next article) Winter 2004-2005 by Lance R. Lewand Doble Engineering Company The purpose of these tests is to evaluate the quality of new oils and then, once they are in use, the extent of degradation due to thermal, oxidative, and hydrolytic actions and contamination. As oils age while in service, acidic and polar compounds are formed and ultimately develop into a sludge or polymerized material if remedial action is not taken. The oil can be replaced or rejuvenated by clay treatment or possibly other processes such as activated alumina to remove the acidic and polar contaminants before they induce excessive aging of the solid insulation and corrosion of metal structures or result in sludge formation. The generation of sludge and polymerized material can hamper cooling in the transformer, resulting in an increased rate of aging of the insulation. 1 Physical Tests Below are the results of testing of new natural esters. Where Doble did not perform the testing, the manufacturer’s data is provided, if available. Table 1 – Physical Tests of Natural Ester Dielectric Liquids ASTM TEST Mineral Oil BIOTEMP Coconut Oil ECO Fluid Envirotemp FR3 Doble Producer Producer Doble 1.0 L0.5 (Green) Typical Values Data Source Aniline Point, D 611, °C ≤ 80 25.0 Color, D 1500 ≤ 0.5 L0.5 Flash Point, D 92, °C > 145 328 321 326 Fire Point, D 92, °C 180a 358 341 362 Interfacial Tension, D 971, mN/m > 40 26 26 24 Pour Point, D 97, °C ≤ (-40) -12a 20 -12 -21 Relative Density, D 1298, 60/60 ≤ 0.910 0.919 0.917 0.920 b 0.923 Viscosity at 40°C, D 445 ≤ 11.0 41.4 29 92c 33.8 Refractive Index at 20°C, D 1807 1.4808 1.4708 1.46 1.4750 Clear/Bright Clear/Bright Clear Clear/Bright Visual, D 1524 2.4 225 a Without pour point depressants added b Corrected to 68°F Performed by ASTM Method D 88 c As shown in Table 1, significant differences exist between natural esters and typical mineral oil – especially aniline point, flash point, interfacial tension, pour point, and viscosity. Certain tests such as aniline point and interfacial tension may not have as much applicability for natural esters as mineral oils. For example, aniline point is affected by the aromatics present in an oil of which natural esters have none as refined. Because of the high hydrophilic nature of the liquid, the interfacial tension test may not be very sensitive to changes in oil quality, as the highest values are in the mid-20s mN/m. Another distinct difference but a major advantage of natural esters is the more elevated flash and fire points. A distinct margin of safety is attributed to dielectric liquids with more elevated flash and fire points. The fire point may affect where certain liquid-filled transformers can be installed with or without additional fire resistance protection, especially in indoor applications or by commercial buildings. In the United States, indoor installations of less-flammable liquid-filled transformers are significantly less regulated than their mineral oil filled counterparts. The NEC in the United States specifies that the liquid in a less-flammable transformer must have a minimum fire point of 300°C. 2 In general, the viscosity and pour point values of natural esters are much more elevated than those of typical mineral oil. These physical attributes are very important and cannot be disregarded. The viscosity is important to the cooling functions and, therefore, to the transformer. A lower viscosity liquid allows a more efficient dissipation of heat. Large differences in viscosity such as those attributed to natural esters can be accounted for in the design of the transformer and, thus, can minimize any adverse effects and optimize its operation. However, retrofilling of a mineral oil transformer with a natural ester may require special considerations and may not be intended as a direct replacement for transformer mineral oil, although these types of retrofills have taken place (see Cooper). The pour point is the lowest temperature at which oil will flow. A low pour point is important, particularly in cold climates, to ensure that the oil will circulate and serve its purpose as an insulating and cooling medium. Even with added pour point depressants, the pour point of a natural ester cannot match that of a typical transformer mineral oil. It has been documented that natural esters will solidify in a transformer once the pour point has been reached and, thus, must function as a solid impregnant. As long as no voids exist in the solidified mass, it seems to function well as no incidents have been reported. NETA WORLD Chemical Tests The results of the chemical tests performed are shown in Table 2 and are compared to specification values for transformer mineral oil. Table 2 – Chemical Tests of Natural Ester Dielectric Liquids ASTM TEST Mineral Oil BIOTEMP Coconut Oil ECO Fluid Envirotemp FR3 Doble Producer Producer Doble Noncorrosive Noncorrosive Typical Values Data Source Corrosive Sulfur D 1275 Water Content, D 1533, ppm Neutralization Number, D 974, mg KOH/g Noncorrosive Noncorrosive ≤ 30 <80 110 <80 ≤ 0.015 0.02 0.05 0.02 0.15 Oxidation Inhibitor, D 2668, % ≤ 0.30 ND Furanic Compounds, D 5837, µg/L all < 5 all < 5 Benzotriazole, Doble, µg/L ≤ 10a ≤ 10 <2 <2 Polychlorinated Biphenyls, D 4059, ppm 1.0 ND 654 HMF ≤ 10 <2 <2 ND = None Detected a Typical value for mineral oils from North America The chemical tests for natural esters, as shown in Table 2, are all very similar to typical mineral oil except for water content, which is higher than that normally found in mineral oil. This is to be expected, due to the chemical composition of the liquid causing it to be hydrophilic in nature. When used in electric apparatus with paper insulation of the same water content, natural esters have a water content higher than that of mineral oil. This does not mean that the dielectric strength is adversely affected since dielectric strength is a function of the relative saturation of water in the dielectric liquid, not the absolute concentration. Natural esters actually provide a benefit in this respect. Because they are able to hold more water, insulating paper tends to be slightly drier in those systems with natural esters as compared to mineral oils, all other things being equal. In fact, some have considered using natural esters as the water conducting medium during transformer dryout processes since it can hold so much more water than mineral oil. For example, at 65°C the maximum solubility of water in oil is 285 parts per million whereas for natural esters it is around 2100 parts per million. Commonly used inhibitors such as DBPC and DBP were not detected in the liquids tested. It is already known that all four of the natural esters listed in the above table contain additive packages with oxidation inhibitors, metal deactivators, and pour point depressants. Winter 2004-2005 For furanic compound and benzotriazole analysis, interferences present in the extract of the solid phase extraction made the chromatograms difficult to read when compared to a typical mineral oil. This may present problems when trying to determine the aging of the solid insulation as the transformer ages, using this type of analysis. In one case, Envirotemp® FR3™, the analysis did reveal the presence of an appreciable amount of 5-hydroxymethyl-2-furfural or an interfering compound detected at the exact retention time of the standard used. It cannot be determined whether or not this peak represented 5-hydroxymethyl-2furfural or was an interference. It is conceivable that the peak does represent 5-hydroxymethyl-2-furfural. This is because Envirotemp® FR3™ is an agriculturally based product which naturally contains fructose and glucose, both of which are used to synthesize 5hydroxymethyl-2-furfural (see Budavari). It may be a byproduct of the refining process. Cooper strongly believes that this detected peak is an interference and not 5-hydroxymethyl-2-furfural. Cooper has indicated it believes solid phase extraction is a viable technique for determination of at least four of the five furanic compounds and is presently working on a method to remove the interference. In all fairness, this analysis was not conducted on the coconut oil or the ECO Fluid, as those liquids were not available. 3 Electrical Tests The results of the electrical tests performed are shown in Table 3. Table 3 – Electrical Tests of Natural Ester Dielectric Liquids ASTM TEST Mineral Oil Typical Values BIOTEMP ECO Fluid Envirotemp FR3 Doble Producer Doble Dielectric Breakdown, D 877, kV ≥ 30 38 45 51 Dielectric Breakdown, D 1816 kV, 1 mm gap ≥ 20 33 61c 36 Power Factor at 20-25°C, D 924, % ≤ 0.05 0.0093 0.09 0.0610 Power Factor at 100°C, D 924, % ≤ 0.30 0.590 0.30 1.850 2.2a 3.1 2.86 3.1 ≥ 145 134b 148 Insufficient sample negative -52.7 +22.0 -80.5 Data Source Dielectric Constant at 25°C, D 924 Impulse Breakdown, D 3300, kV Gassing Tendency, D 2300, µL/min a Typical of a mineral oil Test performed using a negative polarity point. ABB reports that impulse breakdown is slightly higher when a positive polarity point is used. c Testing performed using a 2mm gap instead of a 1mm gap b The results for coconut oil are not listed as the producer did not provide any data. Electrical test results for natural esters are similar to that for mineral oil. Exceptions are the very low gassing tendency and the higher dielectric constant. The low impulse breakdown strength is close to the minimum value for mineral oil. The extremely low gassing tendency of some of the natural esters is most likely due to the amount of unsaturated, nonaromatic molecules as compared to mineral oils. This attribute may be beneficial for applications in liquid-filled cable systems where a negative gassing tendency is a sought-after property. In addition, the biodegradability of natural ester liquids is an added benefit if leaks should occur – especially in environmentally sensitive areas. The higher power factor results for some of the natural esters at 100°C are most likely due to the higher polar content compared to that of mineral oil. The inhibitor packages used are also partially responsible for the increased power factor. Conclusions The physical, chemical, and electrical tests performed indicate similarities and some significant differences between natural esters and typical mineral oil. This is not to say that mineral oil is superior to natural esters. Each liquid type has its advantages and disadvantages and must be fitted to the application. The comparison is made because mineral oil is by far the more commonly used dielectric liquid. Many natural esters contain additive packages consisting of chemicals to reduce the pour point and aid in oxygen stability, and, in some cases they have an antimicrobial agent or copper deactivators. This contrasts with mineral oil which has either no additives or merely oxidation inhibitors. It is not known if any adverse characteristics exist when natural esters are used in transformers over a long period. 4 Several routinely performed tests such as water content, furanic compounds, and PCB analysis required modifications to yield accurate results for natural esters. Because of their chemistry, in-service natural esters will not yield the same results when compared to mineral oil, especially for tests such as water content, neutralization number, interfacial tension, and power factor. Instead, they must be judged based on their inservice criteria. Unfortunately, because natural esters are relatively new, not enough in-service data has been collected to make those informed decisions. References 1. David Sundin, 2001, “Vegetable Seed Insulating Fluid,” United States Patent 6,280,659. 2. Cooper Power Systems, 2000, “Recommended Retrofill Procedures for the Replacement of Oil with Envirotemp® FR3 Fluid,” Bulletin 00046, Waukesha, WI. 3. Budavari, Susan, ed., 1989, “4764. 5-(Hydroxymethyl)-2furaldehyde,” Merck Index 11th Edition, Merck & Co., Inc., Rahway, N.J. Lance Lewand received his Bachelor of Science degree from St. Mary’s College of Maryland in 1980. He has been employed by the Doble Engineering Company since 1992 and is currently the Laboratory Manager for the Doble Materials Laboratory and Product Manager for the DOMINO. product line. Prior to his present position at Doble, he was Manager of the Transformer Fluid Test Laboratory and PCB and Oil Services at MET Electrical Testing in Baltimore, MD. Mr. Lewand is a member of ASTM Committee D 27. NETA WORLD
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