Prototype 420 kv Power Transformer Using Natural Ester Dielectric Fluid. Ronny Fritsche, Uwe Rimmele, Frank Trautmann, and Dr.

Transcription

1 Prototype 420 kv Power Transformer Using Natural Ester Dielectric Fluid By Ronny Fritsche, Uwe Rimmele, Frank Trautmann, and Dr. Michael Schäfer

2 PROTOTYPE 420 KV POWER TRANSFORMER USING NATURAL ESTER DIELECTRIC FLUID Ronny Fritsche *, Uwe Rimmele *, Frank Trautmann *, and Michael Schäfer ** * Siemens AG TR Nuremburg ** TransnetBW GmbH Germany Abstract First introduced in the early 21st century, the use of alternative liquids, especially synthetic esters, is a well-established practice with distribution transformers. Similarly, retrofit transformers that are refilled with alternative liquids are also an accepted practice. The story of alternative liquids began in 2004 when a medium power transformer with a rated voltage of 238 kv and rated power of 135 MVA was filled and tested successfully with a synthetic ester for the first time at Siemens. Then, in 2008, a medium power transformer with rated voltage of 110 kv and rated power of 40 MVA was filled and tested successfully for the first time with a natural ester. In April 2013, the world s first large power transformer filled with natural ester was successfully tested. This transformer was developed and built by Siemens at their power transformer factory in Nuremberg, Germany. The transformer is rated at 420 kv, uses new design criteria, and is the largest unit using a renewable resource liquid. The rated power of this transformer is 300 MVA with an overload condition of up to 400 MVA. Introduction The increasing demand for electric energy leads to huge investments to develop new primary resources and techniques for electric energy generation and transmission. Simultaneously, the demand for environmentally friendly energy generation and supply becomes more important for all of us. Sustainability, biodegradability, and renewable energy are keywords in today s global energy sector. New markets, as well as new challenges for the entire energy supply system, are already announced and upcoming. Power transformers are one of the most important components of energy grid systems. These components enable the most efficient transport of electric energy from the location where the energy is generated to the location where the energy is needed. However, power transformers are conventional pieces of electric equipment, due to physics concepts and materials developed decades ago. This makes it critical to implement new materials based on renewable resources to achieve sustainability, reliability, and safety. It is also becoming increasingly important to develop and install electrical equipment that provides a high level of environmental safety and reliability. Especially in regions of great infrastructure density, where newly interlinked systems become an ideal solution, this is a major target. A high degree of health and environmental safety will enable the installation of power equipment in areas of high population density without increasing risk level. 2

3 In the case of power transformers, insulating oil is the most critical conventional material. Consequently, the focus is to replace transformer oil with alternative liquids that enable a higher level of sustainability and environmental safety. This paper will review the existing solutions for alternative liquids and will spotlight the introduction of such materials to large power transformers. State of the Art Insulating Liquids The insulation liquid most commonly used in power transformers is based on mineral oil. It has been used for decades in electrical equipment for insulation and cooling purposes because the technical parameters, as well as the physical behavior of mineral-based oil, are well known. Mineral oil is a non-renewable fossil resource that may be exhausted in the upcoming decades. Consequently, there is a strong demand to replace that kind of insulation liquid with any kind of alternative solution that enables a higher degree of sustainability. Several types of alternative liquids are available for the market of electrical equipment. The alternative liquids can be divided into three basic types: natural ester-based liquid, synthetic ester-based liquid, and silicone oil. Because the properties of these types of alternative liquids are different from those of mineral oil, as well as among themselves, it has to be clearly investigated for which purpose these liquids will be used. The following sections will describe the composition of various liquids and will outline some specific characteristics of each. Mineral Oil Conventionally applied transformer oil consists mainly of carbon and hydrogen in molecules of different structures. There are three basic transformer oil structures: paraffinic, naphthenic, and aromatic. Paraffinic molecules have lower thermal stability than naphthenic molecules. The most important group is the aromatic structure group, which has a major influence on the grade of the oil. While monoaromatics are always alkalized and generally have good electrical properties, polyaromatics exist naturally in the mineral oil and may have a negative impact on electrical properties (e.g., charging tendency and impulse breakdown withstand strength). Additional molecules, such as nitrogen, oxygen, and sulfur, also naturally exist in mineral oil. The final properties and grade of the transformer oil are dependent on the composition of the different components. The composition of hydrocarbons in oil varies for each particular crude, and the final amount and composition of hydrocarbons is dependent on the particular refinement process and number of steps. Because of its excellent thermal and insulating properties, transformer oil has been used in power transformers for many decades. Consequently, the characteristic behavior and parameters of transformer oil are well known. The electrical characteristics are particularly well reviewed and are approved for the majority of large power transformer applications. There are a lot of parameters, investigation results, and design rules published that enable the insulation design of mineral oil filled electrical equipment. Additionally, the cooling characteristics and behavior of flowing fluid have been thoroughly investigated and fixed in several rules and standards. 3

4 Finally, the service characteristics of transformer oil are well known and published in several standards. Gas-to-Liquid Insulating Oil Recently, a second fossil source has entered the market of electrical equipment for use as a cooling and insulation liquid. The so-called gas-to-liquid (GTL) is a fossil gas converted to liquid form. This type of mineral oil was introduced by Shell and approved for use in power transformers. Natural gas is converted into an oil form (GTL) using a three step process. This GTL is suitable for making transformer oil. In the first step, methane is reacted with oxygen to create synthesis gas. Next, the synthesis gas is converted to liquid waxy hydrocarbons. Finally, the waxy hydrocarbons are upgraded, in a process known as hydrocracking, and then distilled into a wide range of products. Base oil is the primary product, but transport fuels and feedstocks for the chemical industry are also produced. During the third step of the process, the final properties of the liquid are adjusted by the degree of distillations. Because GTL is produced from synthesis gas, no impurities, such as sulfur, nitrogen, or other heterocyclic species, are included. Therefore, GTL is easy to adjust to the final required properties of the insulation liquid. In comparison to mineral oil-based transformer oil, GTL is highly resistant to degradation and oxidation. Consequently, the aging behavior of GTL is better than that of mineral oil. The properties of GTL must be thoroughly evaluated and compared to mineral oilbased transformer oil before it can be used in power transformers. Silicone Fluid Other alternative liquids to transformer oil are silicone fluids, which are specially developed for transformer applications and are fully synthetic coolants and insulation fluids. Due to their high ignition temperature and self-extinguishing behavior, these liquids present a lower fire hazard than mineral oil. Thermal stability, even in the presence of air, is better with silicone fluids than that of other liquids. However, its high viscosity at higher temperatures and poor lubrication properties are disadvantages in transformer applications. The very low biodegradability and the formation of jelly-like bridges of silicone-oxide in the presence of arcing are also disadvantages. Furthermore, silicone fluids are expensive when compared to other insulation liquids. Silicone fluids are mainly used in traction transformers and will not be considered in detail within this paper. Ester Fluids In regard to the requirements for increased biodegradability and sustainability, the most important alternative liquids are ester based. These liquids are the only alternative liquids that are classified to be fully biodegradable. Of course, this is still dependent on the composition of the esters, the volume of the disposed liquid, and the time allowed for biodegradation. Esters are divided into two main classes: synthetic and natural. Synthetic esters are derived from crude oil-based chemicals. These esters are usually the product of a polyol combined with synthetic or natural carboxylic acids to create structures where several acid groups are bonded to a central polyol structure. Polyol is a molecule with more than one alcohol functional group. 4

5 The acids used are usually saturated in the chain, giving the synthetic esters a very stable chemical structure. The properties of synthetic esters lead to benefits for transformer insulation, especially with respect to the thermal and environmental behavior of the equipment. Natural esters are derived from vegetable oils. Consequently, they are the only kinds of liquids that are primarily derived from renewable sources. Natural ester fluids are based on saturated as well as single, double, and triple unsaturated fatty acids. Saturated fatty acids are chemically stable but have a high viscosity. Triple unsaturated fatty acids have a lower viscosity but are very unstable in oxidation. To reach an acceptable value of oxidation stability for natural esters, it is necessary to add suitable antioxidants. In addition to DBPC-specific antioxidants (2.6-ditert-butyl-p-cresol) that use complex phenols, there are also amines in operation. To avoid an unacceptable increase in liquid conductivity, the total amount of antioxidant is limited to one percent or less. Ester fluids with a high percentage of single unsaturated fatty acids have proven to be suitable. Depending on the natural plant and the composition of the natural ester, the properties will differ among various types of natural esters. Therefore, it is important to accurately define the final specification of the liquid to be used so that the composition can be adjusted to meet transformer operational requirements. Characteristics of the Liquid and Design Impacts Insulation liquids within electrical power equipment have to meet several standards. The ideal liquid represents the optimum electrical insulation and cooling performance and will be resistant to oxidation and aging. Aside from these important features, there are a lot of other factors that must be considered irrespective of liquid type. An evaluation will need to be completed to determine which liquid characteristics will be sufficient for the type of electrical equipment being used. The following is a partial list of challenges that need to be considered for use in power transformers: Electrical insulation Cooling and transport of thermal energy Impregnation of solid insulations material (most likely based on cellulose) Oxidation stability in case of thermal service stresses Aging resistivity Material compatibility and low chemical reactivity Gassing tendency Flash point and reaction to fire Corrosion protection Depending on customer requests, additional issues may need to be considered (e.g., biodegradability, low temperature characteristics, sustainability, and gas absorption characteristics). None of the existing liquids can be considered the ideal solution for all of the given challenges. Therefore, it must be clearly determined which challenges have to be tackled and which weak points of the liquid shall be balanced. Characteristics with Impact to Thermal Design 5

6 The thermal design of a power transformer depends strongly on the physical characteristics of the insulation liquid. The kinematic viscosity, thermal capacity, and thermal conductivity are mainly influencing the heat transport characteristic of the insulation liquid. The hydraulic resistance of the whole transformer cooling circuit, inner and outer cooling, is proportional to the kinematic viscosity. In a comparison of the previously mentioned liquid alternatives, there are essential differences in the behavior of the kinematic viscosity depending on the temperature of the considered liquid. There are significant differences among liquid types, especially at low temperatures. This will be a key criterion for the design of power transformers for low temperature environments. In some cases, additional measures should be taken into account to ensure a reliable low temperature function of the liquid. The kinematic viscosity is almost equal along the whole temperature range for mineral oil and GTL-based transformer oil. There is no impact on the design of the transformer using GTL or mineral oil. However, the kinematic viscosities of natural and synthetic esters are approximately four times higher than mineral oil at typical operating temperatures. Figure 1 illustrates the behavior of the kinematic viscosity in relation to liquid temperature for esters, mineral oils, and GTL-based transformer oil. All given values are mean values for the different liquid types. The graphic is illustrated in logarithmic scale because of the strong increase in viscosity at low temperatures. Along the whole temperature range, the esters kinematic viscosities are higher than those of mineral oil. This needs to be considered in the thermal design of the transformer, especially in the cooling design of the inner cooling circuit. In particular, winding configuration and winding insulation arrangement need to be modified for ester use. In case of the esters higher kinematic viscosities, pressure drop along the winding will increase significantly if oil duct width is equal to the design for mineral oil application. Furthermore, in case of forced cooling, the strength of pumps in relation to pipe cross section and required cooling performance will need to be increased. Besides kinematic viscosity, the specific heat capacity of alternative liquids must be considered for thermal design of the transformer. In comparison to mineral oil, the most significant difference in specific heat capacity can be observed in natural ester liquids. For natural esters, the increase of specific heat with increasing temperature is stronger than those of the other liquids. In contrast, this parameter is almost equal when comparing GTL-based transformer oil to mineral oil. Figure 2 illustrates the specific heat capacity and the thermal conductivity of different liquids. Because GTL-based transformer oil is almost equal to mineral oil, a single chart for transformer oil is shown for both types. Note that thermal conductivity of natural esters is higher compared to mineral oil over the whole temperature range. 6

7 Kinematic Viscosity [mm²/s] Synthetic Ester (Mean) Naturel Ester (Mean) Mineral Oil (Mean) GTL based transformer oil Liquid temperature [ C] Figure 1 Kinematic Viscosity as a Function of Temperature ,2500 Specific Heat Capacity [WS/(kg * K] Natural Ester (Mean) Synthetic Ester (Mean) Thermal Conductivity [W/(m * K)] 0,2000 0,1500 0,1000 0,0500 Natural Ester (Mean) Synthetic Ester (Mean) Transformer oil, mineral and GTL based (Mean) Transformer oil, mineral and GTL based (Mean) Liquid temperature [ C] 0, Liquid temperature [ C] Figure 2 Specific Heat Capacity (Left) and Thermal Conductivity (Right) as a Function of Temperature Higher kinematic viscosity may lead to the conclusion that cooling performance of esters is worse than that of mineral oils, but this is not necessarily true. Because of the higher specific heat capacity and higher thermal conductivity, natural ester is able to transport a higher amount of energy. Therefore, the cooling performance over the lifetime of the transformer can be an advantage if the cooling design is adjusted to the natural ester characteristics. Aging tests can be used to infer the better thermal characteristic of esters (especially natural esters) when compared to mineral or GTL-based transformer oils. These tests investigate the degree of polymerization (DP) of paper insulation material immersed in different liquids. Figure 3 illustrates an example of such an aging test, with typical results for a large number of aging 7

8 tests. The samples are aged for up to a month at 140 C (284 F), in both sealed and non-sealed (open to environment) conditions. It can be observed that papers immersed in esters have a better aging performance than mineral oil when considered with the same boundary conditions. The drop in degree of polymerization is lower for ester than for mineral oil. This leads to increased potential life span of an ester-insulated transformer of similar load conditions compared to a mineral oil-insulated transformer with equal cooling performances. In summary, for thermal and aging performance, it will be beneficial to use esters as alternative fluids, but thermal design of the power transformer must be modified according to the specific characteristics of the esters Sealed condition 1000 Open condition Degree of polymerization NEW Cellulose Mineral oil Natural Ester Synthetic Ester Figure 3 Results of Aging Tests of Different Impregnated Papers Characteristics with Impact to Insulation Design Following the cooling task, the dielectric performance of the liquid is next most important for the design of the electrical equipment. During the last decade, a lot of investigations were done to characterize the dielectric performance of alternative liquids. Especially in the case of esters, a large number of investigations were done to determine the dielectric withstand strength in relation to different configurations and stresses. The dielectric withstand strength of GTL-based transformer oil is almost equal to mineral oil. In some configurations, it seems that GTL-based transformer oil will have slightly higher withstand strength than mineral oil. However, because it has not been investigated in detail, it is still recommended that the same design rules are used for GTL insulation design as for mineral oil insulation design. There is no significant difference among the insulation liquids. In contrast, the dielectric behavior of ester fluids differs significantly from mineral oils. The withstand strength of ester liquids is strongly dependent on the applied test voltage, the test 8

9 voltage setup, as well as the kind of test voltage. Verification of the dielectric strength for the different voltage stresses alternating current (AC), lightning impulse (LI), and switching impulse (SI) is essential as criteria for proper insulation design of power transformers. Unfortunately, there are still no standards available that determine testing conditions to verify the essential dielectric properties of liquids for all test purposes and for their use in transformer equipment. Standards like IEC test the oil breakdown on a short distance, but not across composite insulation. Consequently, new testing had to be developed to verify the dielectric withstand strength for ester liquid. Impulse Stress for Alternative Liquids The most significant difference in dielectric withstand strength of esters compared to mineral oil occurs at impulse stresses and with inhomogeneous test arrangements. When test voltages are increased, the ratio of withstand strength between esters and mineral oil also increases. Figure 4 illustrates breakdown voltage behavior for an inhomogeneous point-to-plate arrangement in different kinds of insulating liquid. It is shown that with increasing electrode distance, the ratio between related breakdown voltage of mineral oil and esters increases. Especially in the case of natural esters, the ratio of related breakdown voltage compared to mineral oil increases significantly. At higher impulse stresses of greater than 650 kv, the ratio also becomes significant for slight inhomogeneous arrangements, and this disparity has to be considered within the insulation design of transformers. Further test results of strong inhomogeneous arrangements show that the streamer propagation is much faster in insulation arrangements with ester. Fortunately, streamer propagation mode for strong inhomogeneous arrangements is not of essential significance for the insulation design of transformers. Rather, strong inhomogeneous arrangements can be avoided in power transformers themselves (although this statement does not apply to the inhomogeneous arrangements required for tap changer internal configurations). The dielectric strength as a function of insulation arrangement and type of dielectric stress is of much more interest for the power transformer s design. Table 1 documents the most critical design areas in power transformers and how to estimate the reduction of dielectric withstand strength in ester liquids. 9

10 3,5 UBD / UBD0 (Lightning Impuls Stresses) 3 2,5 2 1,5 1 0,5 0 Mineral oil (Mean) Synthetic Ester (Mean) Natural Ester (Mean) Electrode distance [mm] Figure 4 Related Breakdown Voltage of a Inhomogeneous Point-to-Plate Electrode Arrangement in Different Insulation Liquids Table 1 Insulation Arrangements in Power Transformers and Related Significant Stress for Design Kind of Arrangement Insulation structures inside Windings Slight inhomogeneous insulation arrangement Inhomogeneous arrangements Critical Stress Type LI and AC service stresses LI and AC stresses All kinds of stresses Ratio of Oil to Ester Dielectric Strength (Approach) about 1.4 for LI stresses , dependent on stress type , dependent on degree of homogeneity AC Stress and Partial Discharge for Alternative Liquids In addition to the impulse stresses, the partial discharge behavior and the withstand strength at AC stresses was also investigated for ester fluids. It was found that in the case of partial discharges, the energy input of the discharges is much higher for ester liquids. Therefore, inhomogeneous arrangements and non-insulated metal elements on ground potential should be avoided if they are close to high voltage windings or leads. In particular, the areas around esterinsulated lead arrangements have to be analyzed in detail, and lead arrangements will need to be modified compared to mineral oil design. At AC stresses, the dielectric strength of large ester liquid gaps is reduced compared to mineral oil gaps. Consequently, large oil gaps have to be avoided in the design of transformers with ester 10

11 application. This has to be considered mainly for lead arrangement design. Especially at high AC voltage tests (395 kv and above), this is important for lead design. In some cases, the lead design for ester application is comparable to those of High Voltage Direct Current (HVDC) transformers. In summary, different dielectric characteristics of esters need to be considered for insulation design. In particular, distances between windings and grounded elements need to be adjusted accordingly. The insulation designs of oil ducts within barriers at winding end and lead arrangement also have to be modified, in comparison to mineral oil insulation systems. Depending on the power level and the insulation level of the transformer, this may lead to an increase in the physical dimensions of the transformer by up to 10 percent. Specifically in large power transformers, the dielectric characteristics of ester fluids will have a significant influence on the insulation structure. Characteristics with Impact to Mechanical Design and Processes With respect to insulating liquids, the most important issues for mechanical design and processing are oxidation behavior of the liquid, kinematic viscosity, expansion coefficient, material compatibility, and pour point of the liquid. Impregnation Issues for Alternative Liquids The kinematic viscosity strongly influences the impregnation of the solid insulation material based on cellulose. The viscosity of ester liquids is four times higher than that of mineral oil in the range of normal processing temperatures (about 70 C (158 F)). Therefore, the mechanical design of large insulation structures as well the transformer impregnation process itself has to be adjusted. For example, design of large wooden and pressboard elements needs to be reconsidered when using esters. Installation of drying and impregnation holes is suggested to speed up the impregnation process. Impregnation tests of pressboard imply that it is necessary to extend impregnation time if the same boundary conditions (such as liquid temperature) are maintained. Figure 5 illustrates the impregnation behavior of laminated pressboard LB3.1.A in different liquids and as a function of time. The impregnation depth related to impregnation volume is shown. It can clearly be observed that it takes at least two times longer to totally impregnate the laminated board with ester. The liquid temperature for both specimens is equal. Natural and synthetic esters behave similarly because of equal kinematic viscosity behavior. In order to speed up the process of impregnation, the ester liquid should be processed at slightly higher temperatures. However, liquid temperature has to be limited depending on the composition of the ester liquid. Especially in the case of natural esters, additives will be removed at higher temperatures in vacuum condition. To balance the process for successful transformer application, it will be useful to extend the time of impregnation and to increase ester liquid temperature slightly. This combination will achieve a sufficient impregnation of solid insulation material. 11

12 1,2 Standardised impregnation volume of specimen 1,0 0,8 0,6 0,4 0,2 Mineral oil at 60 C Ester liquid at 60 C 0, Impregnation time [h] Figure 5 Impregnation Behavior of Pressboard Specimen in Different Liquids; Impregnation Depth is Related to Volume as a Function of Impregnation Time As described previously, the kinematic viscosity must be considered in the design of the inner and outer cooling circuit. Specifically, pumps have to be able to carry a higher viscose liquid, and this will result also in higher power consumption of the pump. Consequently, the whole control system has to be adjusted. Oxidation Stability The oxidation stability is a weak point of natural esters. In the case of higher temperatures and liquid exposure to environmental air, the oxidation of esters increases significantly. Measures in transformer design have to be implemented to avoid exposure to the environment. Different options are available, such as hermetic (or sealed-tank) design of the transformer with nitrogen cushion for the liquid expansion. An additional option is to accommodate expansion with a conservator that is sealed by a special rubber bag. The amount of oil and the mechanical construction of the unit will determine the best final solution. It is important that the ester liquid s different expansion coefficient is considered for the conservator design. Material Compatibility Because esters have different chemical characteristics, it is important to check material compatibility of the liquid to solid materials used in transformers, including metal parts. The tests have to be conducted according to existing standards, both internal and external. In particular, materials used for sealing purposes, like rubber, need to be investigated. Furthermore, all components need to be checked by the supplier or by the manufacturer for compatibility for ester application. Bushings, cable joints, and tap changers have to be approved for ester 12

13 application, not only because of material compatibility, but also because these components need to be qualified for dielectric strength and cooling purpose in ester liquid. Low Temperature Operation In some global regions, there are special environmental conditions, especially with respect to temperature profile. In the case of natural esters, low temperatures application can be critical because esters have a higher pour point than mineral oil (about -21 C. or -5 F). However, solidification of esters begins at approximately -10 C (14 F), so it is important to consider this in the design of the transformer. Many transformer specifications request lowest ambient temperatures of lower than -25 C (-13 F). In this case, the evaluation of the yearly temperature profile will be necessary. If the time period of low temperatures is rather short, some simple measures can be taken to avoid solidification of the fluid. A broad variance of alternatives is available to master the challenge of natural ester solidification, but it has to be determined which alternative will be the optimum from case to case. For example, operating regulations can be established for the transformer service condition in case of ambient low temperature (for example, short-term low ambient temperature can be mastered by control of the outer cooling circuit.) There are also mechanical alternatives that will enable a re-heating of the liquid, if the ambient temperature is below the pour point for a longer period. Characteristics with Impact to Environment and Facilities The most important characteristics of alternative fluids are the environmental properties. Biodegradability is one of the most significant benefits of natural, as well as synthetic, esters. Natural esters are known to be up to 100 percent biodegradable, depending on the composition of the natural esters. Synthetic esters are up to 90 percent biodegradable. Mineral oil is nondegradable within an acceptable time period, and just 10 percent of the composition of mineral oil will be naturally defragmented. Table 2 shows the most important characteristics of the liquids in regard to environmental health and safety. Because GTL-based transformer oil is specified according to IEC 60296, its properties are almost identical to those of mineral oil. Consequently, no extra data is given for GTL in Table 2. The high degree of biodegradability of esters enables a financial benefit for facility investment, because no separate containment facility needs to be installed to prevent pollution of the environment in the event of leakage. 13

14 Property (Mean Values) Table 2 Typical Environmental Characteristics of Liquids Mineral Oil (IEC 60296) Synthetic Ester (IEC ) Natural Ester Fire point ( C) C Flash point ( C) C Pour Point ( C) C Biodegradability - Non biodegradability * % Up to 100% Water hazard class - Yes None None Raw material - Crude oil Chemical Intermediate Good Renewable Resource *1 Non biodegradability within an adequate time period, which should be below several decades With respect to safety, it is critical to examine the flash point and fire point of liquids. According to Table 2, esters have a great advantage compared to mineral oil. To illustrate the benefit of esters in regard to flash and fire points, a rough estimation for energy consumption in the case of heating the liquid from 70 C (158 F) to fire point was done. Because the result is only a rough calculation, the absolute values are not of interest, but rather the ratio between mineral oil and ester liquid is considered. With Equation 1, energy consumption was calculated by consideration of the liquid density (ρ), the volume (V), the specific heat capacity (c), the fire point (T Fire ), and the starting temperature (T liquid ) of the liquid. For a one-liter liquid volume, approximately 2.9 times energy consumption was estimated for ester as compared to mineral oil. This result is influenced by the heat capacity of the liquid as well as the fire point. Consequently, the flash and fire point characteristics of esters could potentially have a huge impact on the fire protection classification of the facility where the transformer is installed. It may be possible for requirements of the fire extinguishing system to be reduced, which would decrease the overall cost of the facility. W = ρ (1) c V ( T Fire Tliquid ) Furthermore, this higher energy consumption lowers the risk of fire and explosion in the event of an electrical equipment failure. Because there is a higher energy consumption needed for ester fluid combustion in the event of an electrical failure, it will take longer for the ester-filled equipment to start burning. This combustion time may be long enough to permit de-energization 14

15 of the failure by the protection devices. This may make ester-filled transformers preferable in highly populated locations where safety is of particular importance. The Practice of Alternative Liquid Application in Power Transformers First introduced in the early 21st century, the use of alternative liquids, especially synthetic esters, is a well-established practice with distribution transformers. Similarly, retrofit transformers that are refilled with alternative liquids are also an accepted practice. The story of alternative liquids began in 2004 when a medium power transformer with a rated voltage of 238 kv and a rated power of 135 MVA was filled and tested successfully with a synthetic ester for the first time at Siemens. Several projects followed throughout the years. Then, in 2008, a medium power transformer with a rated voltage of 110 kv and a rated power of 40 MVA was filled and tested successfully for the first time with a natural ester. The first power transformers with GTL-based insulation fluid were introduced in This liquid is approved for power transformers up to 600 MVA rated power and 500 kv rated insulation voltage. In April 2013, the world s first large power transformer filled with natural ester was successfully tested. This transformer was developed and built by Siemens at their power transformer factory in Nuremberg, Germany. This transformer used new design criteria and is the largest unit using a renewable resource liquid. Table 3 documents the important specifications of that transformer. Figure 6 is a picture of the unit in the test bay of the transformer factory. The rated power of this transformer is 300 MVA, with overload condition up to 400 MVA. For the cooling type, forced cooling for the inner and outer cooling circuit was used, but it was also specified to run for 180 MVA load conditions with non-forced cooling. The cooling performance was designed according to the limits given in standard IEC The potential of the esters to experience limiting top oil temperatures was not encountered in this design. Therefore, this unit will have a longer expected life span than a similar mineral oil unit. This transformer will operate in the highest voltage level in the power grid of Germany, so the rated voltage was specified to 380 kv, and the insulation levels were specified according to standard IEC As previously discussed, the design rules for insulation design had to be modified. Several experiments were done to determine the spread between mineral oil composite insulation arrangement and ester composite insulation arrangement. Existing design rules for homogeneous and slightly inhomogeneous arrangements, as well as design rules for different kinds of stresses (e.g., AC, LI, and SI), were adjusted to the empirically evaluated withstand strength of natural esters. All of the insulation modifications confirmed the success of this extra high voltage (EHV) transformer. Also, the special partial discharge requirement given in transformer specification was fulfilled. A partial discharge level of below 10 pico coulombs (pc) at 1.1 of rated voltage was requested and successfully achieved in a factory test. Of course, the mechanical design of this transformer is also special in some respects. In particular, sealing equipment materials based on fluororubber were used. All components were 15

16 qualified to operate in natural ester liquid. The bushings for medium voltage and wye connection were specially designed by the supplier and type tested in the Research and Testing Laboratory of the transformer factory in Nuremberg by the end of For the low voltage connection, cable joints are used with special coupling capacitors. These capacitors were also specially designed by the supplier and type tested in the transformer factory in Nuremberg. For processing of the transformer, special treatment equipment was necessary. Therefore, an existing treatment device was renewed and updated with materials compatible to ester oil. Additionally, the heating device for the ester was upgraded to reach a liquid temperature greater than 70 C (158 F). This equipment was used for factory and on-site treatment. After commissioning, the equipment was decontaminated and used again for mineral oil. The impregnation time was extended at least two times longer, as compared to mineral oil. The customer and the manufacturer agreed to perform a joint monitoring of the ester-filled EHV transformer in the upcoming months and years. The results will be used to increase knowledge with respect to ester liquid behavior because state-of-the-art operational knowledge of esters is presently limited to results available from smaller transformers. The operation of this unit will guide further development and testing and is a pioneering feat for the future of large transformer development. Table 3 Specific Data of 420 kv Natural Ester Transformer Character Rated Voltage Rated Power Ratio Cooling Type Liquid Insulation Level Liquid Weight Rate 420 kv 300 MVA 405 ± 11%/115/22 kv KDAF/KNAN Natural Ester 630 kv AC/1425 kv LI/1050 kv SI ~95 tons 16

17 Figure kv Natural Ester Transformer in Test Bay, Nuremberg

18 References [1] Fritsche Ronny, Pukel Georg, Siemens AG; The Green Machine: Renewable materials for transformers, 2014, Electricity Today, India, January [2] Nynas Naphthenics AB; Transformatoroil handbook ; 2010, Nynas Naphthenics AG, [3] Moser H.P., Dahinden V., Weidmann AG; Transformer Board II, 1987, Weidmann AG, Rapperswil, [4] IEC , Power Transformers Part 2: Temperature rise for liquid immersed transformers, 2011, IEC Edition 3.0, [5] Peter Smith, Shell Global Solutions; The benefits of inhibited transformer oil using the latest gas-to-liquid based technology, 2013; 20th European TechCon, Glasgow, [6] Cooper Power Systems, Envirotemp FR3 Fluid, 2005, Cooper Power Systems, Bulletin B Product Information, [7] Siemens AG Transformers, Internal Report on Transformer Design with respect to Alternative Liquids, 2013, Internal Report, Siemens AG, [8] Pukel G.J., Schwarz R., Baumann F., Muhr H.M., Eberhardt R., Wieder B., Chu D., Power transformers with environmentally friendly and low flammability ester liquids, 2012, CIGRE Session, SC A2-201 Paris, France, [9] Wang Z.D., Jarmen P., Dyer P., Perrot F., Walker D., Lashbrook M., Noakhes J.; Ester Insulating Liquids for Power Transformers, 2012, Cigre Session, SC A2-209 Paris, France, [10] IEC , Laminated pressboard for electrical purposes Part 1: Definitions, classification and general requirement ; 2010, IEC Edition 2.0, [11] IEC , Power Transformers Part 3: Insulation levels, dielectric tests and clearances in air ; 2013, IEC Edition 3.0,