21, rue d Artois, F-758 PARIS CIGRE 212 http : //www.cigre.org D1-111 Thermal Aging Study of Cellulosic Materials in Natural Ester Liquid for Hybrid Insulation Systems J-C DUART * L. C. BATES R. ASANO JR L. CHEIM E. W. KEY D. B. CHERRY C. C. CLAIBORNE DuPont DuPont ABB ABB Switzerland USA Brazil USA SUMMARY This paper presents the joint efforts to combine new materials in a technically and economically advantageous alternatives to the electrical utilities facing challenges to keep high standards of reliability in scenarios of increasing demand, aged infrastructure confined by the urbanization or environmental or fire hazard risks. A hybrid transformer takes advantages of the high temperature materials such as NOMEX in the hottest parts, reducing the thermal ageing rate of the transformer while the remaining insulation structure is made of traditional cellulose materials. However, when immersed in mineral oil, the oil temperature limit has shown to be a very restrictive factor in the transformer design. Recently, the development and good performance of high oleic natural esters such as BIOTEMP for application in power transformers transformer has opened a new horizon to the use of hybrid systems at higher liquid temperatures due to the fluid higher thermal capability and stability. Therefore, in order to unlock the potential of this combination, a survey study has been initiated to better understand the influence of the higher temperatures in the transformer design and materials. This paper focused on sealed tube testing of thick board-like structures made of cellulose. The study undertaken examines aging of both cellulosic board and Kraft paper under three temperatures in a natural ester liquid and under one temperature in mineral oil as a reference. Aging of Kraft paper was utilized as a reference as there are several published studies on aging of paper but few aging studies of cellulosic board have been found in the public record. The results indicate that the Kraft paper ages at a faster rate than the cellulosic board in either liquid. However aging of cellulosic board as well as the Kraft paper is slower in the natural ester liquid studied than in mineral oil. This aging information on cellulosic board should be helpful in thermally optimizing hybrid insulation system designs. KEYWORDS Ageing, cellulose paper, vegetable oil, hybrid insulation, high oleic, natural ester, Kraft, pressboard * Jean-claude.duart@dupont.com 1
INTRODUCTION In hybrid insulation systems, high-temperature solid insulation is used next to the hot conductors and cellulosic materials used for cylinders and angle rings and the mineral oil became the thermally limiting material. With increasing use of less flammable fluids, such as natural esters, in power transformers, the potential of removing the mineral oil limitation became evident. However, in order to thermally optimize the use of all insulation materials, it is necessary to understand the thermal aging characteristics of the thermally limiting solid insulation. In the absence of standardized procedure to assess the thermal capabilities of a solid insulation immersed in liquids, a survey study was undertaken to examine the thermal aging of such cellulosic board materials in mineral oil and in natural ester liquid for the purpose of understanding its thermal limitations in hybrid insulation systems. In addition to the higher temperature capabilities of the natural ester fluids, results of this study indicate that the cellulosic board have a higher thermal capability in the natural ester liquid studied than in mineral oil [1], enabling the use of hybrid systems at its full potential with natural esters [2]. However, it is important to recognize that the current standard has a gap as it does not foresee the combination of ester liquids with high-temperature insulation material and cellulose in hybrid insulation systems. With those important results for the industry, this paper will present the test setup used to assess the thermal capabilities, the ageing methodology adopted and data generated will be compared to existing information available. BACKGROUND Historically, thermal aging techniques have included both transformer model tests as well as laboratory based tests. Transformer model tests such as the Lockie test, described in IEEE C57.1-1999, include thermal cycling of transformers followed by a proof test at specified time intervals, looking for failure of the test sample. Through this kind of testing, the time to failure can be estimated based on the specified test times. This testing is useful as long as the test material of interest is the first material to fail during aging and the subsequent proof test. Laboratory-based tests such as sealed tube and dual temperature tests are more specific to a material. The dual temperature test is designed for aging a thin solid insulation at a temperature that may be very different from the insulating liquid temperature, thus reducing the impact of by-products from the aging of the insulating liquid on aging of the solid insulation. In sealed tube aging, all materials are aged in one vessel and therefore one cannot separate out the potential influence of aging of the insulating liquid from aging of the solid insulation. However, besides very simple, sealed tube aging allows for aging of a wide range of solid materials, not only thin materials but also thick, board-like structures. In the case of the boards used in transformers barriers and angle rings, the surrounding oil transport the heat as they are not in contact with the hot metal parts, condition that is well simulated with the sealed tube test. More details of the test setup are presented in the next sections of this paper. 2
METHODOLOGY The life of a transformer has, for decades, been taken by the life of the solid insulation. However controversial this statement may be it is true, however, that the weakest link in the electrical insulation of the windings is the paper at the hot-spot location. This is the most unfavoured region, thermally speaking, where the insulating paper is expected to degrade faster. A much more complex discussion, though, follows when considering the actual meaning of aging, which is knowingly based on multiple parameters such as Oxygen, Moisture, Acidity and Temperature, in addition to the discussion of end of life of the transformer when compared to the end of life of the solid insulation. Following the standards and guides in the industry, this current discussion of aging is also based on temperature as the single aging parameter, as the other parameters are expected to be controlled by the good maintenance practices. Thermal degradation of the paper has been the subject of hundreds of investigations and publications over the last 4 or 5 decades. The IEC 676-7:25 [3] loading guide proposes, in accordance to Montsinger s equation, that the ageing of a transformer solid insulation is doubled for every increase of 6 C of the hot-spot temperature in the range 8 C - 14 C, for normal Kraft paper, whereas the IEEE C57.91:1995 [4] loading guide proposes another model for thermally upgraded Kraft paper (Figure 1). In the IEC guide (normal Kraft paper) the paper is expected to rate at a nominal rate at a constant temperature of 98 C. In the IEEE guide (upgraded insulation) the nominal aging rate is achieved at a constant operating temperature of 11 C. Figure 1 Aging rates as found in the IEC676-7 and IEEE C57.91 loading guides The IEEE guide proposes the Arrhenius model to calculate the per-unit life of the solid insulation based on the following equation (graph shown in Figure 2): Per unit life = 9.8 1-18 exp(15,/( h + 273)) 3
Figure 2 Per unit life of solid insulation based on the Arrhenius model, as in the IEEE C57.91:1995 loading guide Notice that neither curve in Figure 1 nor Figure 2 indicates time to reach end of life, based on a given temperature. The IEC guide (item 6.4) refers to the IEEE guide which in turn proposes 4 possible criteria to determine insulation end of life, as below (upgraded insulation). Table 1: Normal insulation life of a well dried, oxygen free, thermally upgraded insulation system, at the reference temperature 11 o C (Table 3 in IEEE C57.91:1995) As seen from Table 1 above the IEEE guide does not select a given aging criterion but rather leave it to the user to choose the preferred end of life mode. It does not cover other type of insulating fluids, such as natural esters, neither it covers non-upgraded insulation. For decades the 5% retained tensile strength concept has been applied as an indicator of minimum requirement before end of life of the paper insulation. Later it has been utilized alternatively with the degree of polymerization of the cellulosic chains which form the paper. Figure 3 below shows a correlation between retained tensile strength and DP [5]. Figure 3 - Typical correlation between tensile strength and DP value for Kraft paper 4
Considering that all guides and standards mentioned above are related to paper aging in mineral oil and that there is not proposed methodology to evaluate time to reach end of life, particularly to non-upgraded Kraft paper, thus this research work proposes a methodology to assess paper aging aspects in mineral oil and natural ester fluid using the following criteria: a) Consider the mechanical property tensile strength as the aging parameter b) End of life criterion assumed to be below 5% retained tensile strength c) Compare results for dry insulation (.4% moisture) and wet insulation (.9% moisture) d) Compare aging characteristics of paper in mineral oil and natural ester fluid e) Analyze aging of both cellulosic board and Kraft paper under three temperatures in a natural ester liquid and under one temperature in mineral oil as a reference. f) Plot results against the Arrhenius model described above TEST SET-UP This investigation of ageing insulation with alternative fluids has focused on single temperature ageing to study the solid in fluid ageing characteristics and has been previously described [1]. All materials to be evaluated were placed into a stainless steel vessel (Figure 4) and put into a forced air oven for a designated period of time. Various oven temperatures and times were evaluated. Variables included fluid type, ageing temperature, and ageing time. Each test vessel contained mineral oil or a natural ester fluid, Kraft Paper, Low Density (LD) board, High Density (HD) board, copper, and core steel. The samples were prepared using a standard impregnation process following ASTM D2413. This resulted in dry samples with moisture content of ~.4%. The wet samples were prepared in the same way as the lower moisture samples, but with.5% de-ionized water, by weight of the insulation, added to the oil of the cell ending up at a moisture level of ~.9%. The moisture was then allowed to distribute itself to the insulation materials in the cell during ageing. Figure 4: Single temperature vessels used for ageing of Kraft Insulating Material All of the test cells were tested under a nitrogen headspace initially at.2 MPa. A subset of cells was allowed to vent at.5 MPa, rather than allow them to build pressure like the closed 5
cells would during ageing. This was to determine if pressure factored into the results. Tensile properties were measured on the Kraft Paper. The total insulation present in the vessel is a higher ratio between the volumes of solid insulation versus fluid than would typically be found in a transformer application due to the amount of solid insulation necessary for our various evaluations. The volume of the test cell limited the amount of fluid that could be used. The copper and core steel were present to represent realistic ratios of metal to fluid in liquid-immersed transformer applications. The 2 liters of fluid in each vessel filled approximately 2/3 of the volume of the vessel. The volume ratio between headspace and fluid is larger than would be typically be found in a transformer. By testing under nitrogen, this would be the least susceptible case to oxidative degradation and is consistent with the recommended operating condition of transformers using natural ester fluids. TEST RESULTS Following are some of the results using the criteria described in the previous section we obtained from our experiments. Presence of water does have an impact on the ageing of cellulose whenever immersed in any of the two fluids (Figure 5 and 6). 35 3 25 MD tensile strength (lb / in) 2 15 1 15 dry - MD 15 wet - MD 5 5 1 15 2 25 3 35 4 Time (hrs) Figure 5: Tensile strength of non-upgraded Kraft paper aged with and without added water in mineral oil. 6
35 3 MD tensile strength (lb / in) 25 2 15 1 18 dry - MD 18 wet - MD 16 dry - MD 16 wet - MD 15 dry - MD 15 wet - MD 5 1 2 3 4 5 6 7 8 9 1 Time (hrs) Figure 6: Tensile strength of non-upgraded Kraft paper aged with and without added water in BIOTEMP. Aging of insulation in mineral oil is shown to be more rapid than the same insulation aged in the natural ester fluid under both the dry and wet conditions (Figure 7 and 8). 3 25 MD tensile strength (lb / in) 2 15 1 BIOTEMP 16 dry - MD BIOTEMP 15 dry - MD Mineral Oil 15 dry - MD 5 1 2 3 4 5 6 7 8 9 1 Time (hrs) Figure 7: Tensile strength of Kraft paper aged without added water in BIOTEMP or mineral oil. 7
3 25 MD tensile strength (lb / in) 2 15 1 BIOTEMP 16 wet - MD BIOTEMP 15 wet - MD Mineral Oil 15 wet - MD 5 1 2 3 4 5 6 7 8 9 1 Time (hrs) Figure 8: Tensile strength of Kraft paper aged with added water in BIOTEMP or mineral oil. Arrhenius curves can be drawn based on using 5% retained tensile as the end-of-life criteria for both aging with and without added moisture (Figure 9 and Figure 1). Because aging in mineral oil was only performed at 1 test temperature, the slope of the mineral oil Arrhenius from IEEE C57.91-1995 for non-thermally upgraded Kraft paper was used. Based on this data and this assumption, the calculated temperature at which end-of-life is reached in 65 hours is 114 C for non-thermally upgraded paper in high oleic natural ester fluid vs. 98 C in mineral oil. This difference of 16 C is smaller than the 23 C differential calculated in [2] on the low density board. The difference may be due to the density difference between a board structure versus a paper structure, as high density materials may age at a slower aging rate. In addition, the thickness of a board means that more cellulose molecules are intertwined with themselves than in the paper so that the lifetime of a board is enhanced by chain entanglements. The presence of increased moisture appears to lessen this effect slightly for the mineral oil matrix at the lower temperature where the moisture in the mineral oil is at a significantly higher relative humidity than in the natural ester. 4.5.4 % moisture data with 95% Confidence Interval log [time (h) to 5% retained tensile strength (lb/in)] 4 3.5 3 2.5 2 1.5 1.5 natural ester - 5% mineral oil - 5% Linear (natural ester - 5%) Linear (mineral oil projection - 5%) y = 6972.1x - 13.954 R 2 = 1 T @ 65 h = 99 C y = 9343.6x - 19.32 R 2 = 1 T @ 65h = 114.21.215.22.225.23.235.24 1/T (1/K) Figure 9: Arrhenius graph for Kraft paper aged in BIOTEMP or mineral oil at.4% moisture. 8
4.5.9 % moisture data with 95% Confidence Interval log [time (h) to 5% retained tensile strength (lb/in)] 4 3.5 3 2.5 2 1.5 1.5 natural ester - 5% mineral oil - 5% Linear (natural ester - 5%) y = 856x - 17.986 R 2 =.9911.218.22.222.224.226.228.23.232.234.236.238 1/T (1/K) Figure 1: Arrhenius graph for Kraft paper aged in BIOTEMP or mineral oil at.9% moisture. Table 2: Parameters for curve fit of tensile vs.log [time (h)] shown in Fig. 9 and Fig. 1 Insulating Liquid Moisture content (wt%) End-of-life Criteria Slope Intercept Temp at 65 h as End of Life ( C) Natural ester fluid.4 5% retained tensile 9344-19.32 114 Mineral oil.4 5% retained tensile 6972-13.95 98 Natural ester fluid.9 5% retained tensile 856-17.986 1 FLUID TESTING OBSERVATIONS While not the focus of this study, samples of the fluids were tested at the intervals where samples of the solid insulation materials were evaluated. Testing was conducted for dissipation factor (power factor), acid neutralization values and color measurements. Changes in the high oleic natural ester fluid with aging are observed particularly with dissipation (power) factor. Both temperature and moisture content appeared to directly affect the measured values in the natural ester. The water soluble acids formed during the aging of mineral oil are known to be a good indicator of fluid degradation. This is due to the generation of strong acids that can attack the internal components of the transformer. While significantly higher neutralization numbers are measured in aged fluid, the acids formed are not the aggressive type as seen in mineral oil. The presence of additional moisture does increase the degradation of both fluids as can be seen with comparing the dry and wet aged samples. The color value of the fluid changes more dramatically than the mineral oil under the same aging conditions. Again this may not be as significant an indicator of the degradation of the high oleic natural ester fluid as would be in mineral oil. The fluid testing results performed on aged samples can be a valuable indicator when diagnosing the health of a transformer. More involved studies will be necessary in order to gain a full understanding of the significance of these test values in aged fluid. 9
CONCLUSION In spite of the lack of standardized methods or standards, this paper has presented a methodology to identify the differences in the ageing behaviour of cellulose paper and board immersed in natural ester fluid and mineral oil that demonstrate to be appropriate. The temperature limit obtained with the 65 h lifetime criteria for normal cellulose Kraft paper in mineral oil shows good fit with historical data. The results also confirm that, relative to its initial strength, pressboard preserves better mechanical properties than Kraft paper when aged under the same conditions in both fluids. When compared with the performance in the different fluids, in addition to the better thermal capabilities and stability of the high oleic ester, the Kraft paper and the board presented a slower rate of ageing when immersed in the ester than in mineral oil. With the 65 h lifetime criteria, this difference could be translated to a 23K higher temperature for the pressboard. However it is important to note that, similar to the mineral oil, this advantage is lost when the water content increases, thus requiring the same maintenance care. This validates the advantage cellulose board exhibits which allows, in transformers with hybrid insulation, the manufacturer and the user to take full advantage of the vegetable oil and high temperature solid insulation capabilities as discussed in previous papers. Ongoing work such as this supported by existing and future work by other researchers may well lead to revised or new standards by such organizations as IEC or IEEE. BIBLIOGRAPHY [1] R. Asano, Jr., L. Cheim, D. B. Cherry, C. C. Claiborne, L. C. Bates, J.-C. Duart, E. W. Key. Thermal Evaluation of Cellulosic Board in Natural Ester Fluid for Hybrid Insulation Systems. (78 th Annual International Conference of Doble Clients, March 211). [2] R. Asano, Jr., L. Cheim, D. B. Cherry, C. C. Claiborne, L. C. Bates, J.-C. Duart, E. W. Key. Development of New Hybrid Insulation System Using Natural Ester. (CIGRE SC A2 & D1 Joint Colloquium 211, Kyoto, Japan) [3] IEC 676-7:25 Loading guide for oil immersed power transformers [4] IEEE C57.91:1995 Guide for loading mineral oil immersed transformers [5] Cigre brochure 323:27 - Aging of Cellulose in mineral oil insulated transformers 1