Life Extension of the Transformer Insulation with an Innovative Online Drying and Filtering systemH. Borsi, E. Gockenbach, V. Wasserberg University of Hannover, Germany Institute of Electric Power Systems, Division of High Voltage Engineering Schering-Institute Callinstrasse 25A D-30167 Hannover, Germany Keywords: Life Extension, Transformer Insulation, Transformer Drying Abstract Water and Oxygen are the most important enemies of the insulation of oil filed transformers leading to an accelerated ageing of the insulation. Thus drying procedures and reducing the influence of oxygen are required to extend lifetime and operation reliability. In this contribution an innovative procedure for the dehydration of mineral insulating oils and insulating paper in transformers is presented, working online during transformer operation. This procedure not only performs a complete desiccation, it also purifies the liquid without influencing the dissolved gas analysis (DGA) or any other physical property. Furthermore a Zeolite filled air filtering is introduced which not only prevents water molecules from penetrating the transformer but also oxygen. Herewith presented investigations on the operation parameters like temperature and flow rate have shown, that the procedure is beneficial for insulating liquids as well as for solid insulation immersed therein. oxygen and especially water. Due to cost aspects a combination of cellulosic paper and mineral oil has shown to be the favorite choice. Moreover the cellulosic paper and pressboard insulation collects the water out of the liquid which leads to a severely accelerated aging [1]. Even small contents of water in the mineral oil reduce the electrical strength of the insulation. In consequence every transformer operator must intend to increase the dependability of his devices. Parameters affecting the aging Water may diffuse slowly inside the transformer during service (for breathing transformers) or may be generated inside the device due to the depolymerization of the cellulose (Figure 1). With most of the actually available systems only a drying of the liquid insulation is performed . the presence of catalysts. Conventional Drying of the insulating system Due to the costs of repair a destroyed paper insulation represents in most cases an unrecoverable failure which leads to the retirement of the transformer. oxygen and heat. The degree of polymerization (DP) decreases until the material becomes brittle and a safe transformer operation is jeopardized. H 2O H2COOH C H CH OH C H H C OH CH O O OH C H C CH H CH H COOH OH 2 C H O Figure 1: Water generation by the scission of cellulose molecules. The trustiness of a power transformer is especially limited by the reliability of the winding insulation system which additionally has to operate as a heat transfer medium moving the losses out of the core and the windings.-2- Introduction Transformers are one of the key components of electric power distribution and transmission systems. This statement is especially valid for power transformers. To reduce the insulation aging it is therefore essential to avoid or to remove the mentioned influences. where an outage possibly endangers the reliability of electrical energy supply and always affords unpredictable expenses. The aging of the cellulose can be accelerated by water. although the lifetime of this insulation depends on the operating conditions. An effective drying of the cellulosic insulation before it has already been completely destroyed therefore enlarges the transformer operation lifetime and reliability. Even today the most frequently used insulation systems in these devices are therefore the liquid immersed paper and pressboard insulations. Regarded over the time of its application such a procedure is also more effective than a short time treatment with heat and vacuum. It has been shown by many investigations. thus any measure. beside the water. thus the cellulose fibers are covered with insulating liquid which impedes the extraction of the water from the cellulose. The influence of oxygen should be prevented or reduced and overheating of the cellulose should be avoided. that affects this tool. With the novel system presented in this contribution the mentioned disadvantages are avoided while the transformer operation management is not affected at all. But there is a difference between new and aged transformers: the solid insulation is already impregnated. thus it should be clean and dry. An alternative thereto is the continuous extraction of the water covered by the insulating liquid. Due to the heating of the insulation above the normal operating temperature such a treatment also causes an extended "lifetime consumption". Actual systems. The new drying and filtering system Water and impurities in the liquid significantly reduce the electrical capabilities of the liquid. that are used for DGA. The insulating liquid acts as a “water transfer medium”. For a safe transformer operation optimal properties of the liquid are required. As the combination of a cellulosic paper and . By heating the cellulosic material with Low Frequency Heating (LFH) or Vapor Phase technologies like they are very often applied for new. what is even more important. The advantages of such a cure are. non impregnated transformers it is tried to remove the water. The basis of the conceived procedure for the drying system is the water solubility in the insulating liquid and the equilibrium between the water content in the insulating liquid and the solid insulation immersed therein. thus influencing the mechanical and electrical properties of the paper and resulting in a reduction of the life expectancy of the insulation. extract the water either by vacuum or with the application of a hygroscopic material. that they affect the Dissolved Gas Analysis (DGA) because they extract or affect. also the indicating gases. that it needs no transformer outage and. working according to this principle. that water in the liquid and cellulose insulation is the main cause for the reduction of electrical properties and acceleration of the aging. If only the liquid insulation is dried or replaced most of the collected water is still inside the paper [2]. DGA is actually the most important tool for the surveillance of the insulation condition. For an optimal lifetime therefore the paper should be kept dry during operation. All these parameters can cause depolymerization of the paper insulation.within short times. also has impact on the monitoring of the transformer insulation condition. The vital disadvantage of these systems is. The life expectancy of the paper insulation is influenced by water. When the water saturation level of the fluid decreases due to the drying thereof water is extracted from the solid insulation. oxygen and heat. that the transformer has to be drawn out of service. An additional drying of the paper insulation affords normally. that it does not endanger the solid insulation during drying. Therefore one of the main tasks of the new system is to keep the insulating system dry. . This behavior can be used for a continuous drying of the paper insulation in a liquid immersed insulation system. This effect. For breathing transformers additionally water can penetrate from outside the transformer into the vessel. The insulating oil of any transformer in operation. that for high temperatures the water content of the mineral oil is relatively high while the water content of the paper insulation is low. which always has a higher temperature than its environment due to the different losses inside the vessel. The equilibrium between the moisture contained in the paper and the moisture content in the surrounding mineral oil is depicted in figure 2. K = 1. the water content in the paper is high while the water in the oil is low.807 * 103. 11 % Water Content in the Paper 9 8 7 6 5 4 3 2 1 0 0 10 20 °C 30 °C 40 °C 50 °C 60 °C 80 °C 100°C 20 30 40 50 60 70 ppm 90 Water Content in the Oil Figure 2: Dependency between the water content in mineral oil and cellulosic paper [5]. The water solubility of an insulating liquid can be expressed by the following formula [3] − H T WS = K * e Where K and H are constants specific for each insulating liquid and T the temperature. The equilibrium between the water content in the mineral oil and the cellulosic paper is diverse for different temperatures. which has been reported by several authors like [4]. This diagram shows.-4- pressboard immersed in a mineral based insulating oil is the most common combination of materials the operating principle is illustrated for these materials.918 * 107 and H = 3. For low temperatures this relation is reverse. can be used to dry the insulation of liquid and paper insulated HV apparatuses.g. therefore intends to collect water. For a very common mineral oil these constants are e. The water solubility therefore increases exponentially with the temperature. The warm and "wet" oil inside the transformer vessel is slowly drained from the transformer and pressed through a cooled cellulosic filter in a circular flow. For a sealed transformer the only source for water is the paper and pressboard insulation. For sealed transformers oxygen usually is not in contact with the transformer insulation and therefore the influence of oxygen by the aging is negligible. . coming in contact with the transformer insulation from the environment. With this not only the amount of oxygen. Technical realization Basic investigations on mineral oil as well as on ester liquid have shown. About between 1/8 and 1/7 of this weight is the mass of the solid insulation. With an oil humidity of 15 ppm and an operation temperature of 60 °C the oil dissolves 130 g of water while the solid insulation envelopes 2 % of its weight. which can be installed preceding the silicagel container for breathing air desiccation. is reduced but also the amount of water to be adsorbed by the silicagel as Zeolite is an effective desiccant. The speed of the water transfer out of the core of the solid insulation is thus limited by the transfer speed through the impregnated material. As this speed is quite low short term procedures can only be effective for the outer insulation layers close to the surface as long as the material is still impregnated. Additionally to the drying the filter purifies the oil and removes particles of dirt and impurities out of the liquid. The described procedure is actually protected by German and international patents. As such particles are possible sources for partial discharges or even breakdowns of the liquid insulation the operation reliability is additionally enhanced. too. The mineral oil has previously been cooled down to a temperature of about 3 °C when it enters the cartridge. thus it is in almost any cases supersaturated with water and therefore dispenses it easily. For the appraisal presented here a weight of 1200 kg is therefore rated.In this filter a very porous cartridge offers a large surface for the oil to transfer its water to the cellulose fibers. The new system is directly applied to the transformer in operation as depicted in figure 3. The basic characteristic of online treatments is their profitableness over long time periods. For breathing transformers the influence of the oxygen is reduced by the installation of a vessel filled with special Zeolite (a synthetic mineral). that the innovative procedure is applicable for drying transformer insulating liquids and therewith impregnated solid insulations [6]. thus being 24 kg (values gathered from figure 2). Although power transformers contain several tons of liquid and solid insulating materials the drying of small volumes of insulating liquid is therefore sufficient. To estimate the water inside a transformer the following example is given: Inside a 10 MVA transformer for voltages up to 123 kV 8600 kg of insulating oil are contained [7]. Therewith the vital problem of any short time treatment becomes obvious. If the insulating liquid is removed and vacuum is applied the water originating from the inner parts of the insulation has to pass the surrounding cellulose material. thus still being impregnated with the insulating liquid. The water covered by the solid insulation is captured all over the material. As the most interesting parameter the maximum attainable drying has been tested at first. As the filter cartridge is a flow resistance this part of the system is continuously under a pressure of about 1 bar plus the hydrostatic pressure originating from oil level of the transformer. but as the transformer model is sealed it is time consuming and implies the risk of leakage. wet oil Cooling circuit Upgraded insulating liquid Figure 3: Schematic of the application of the presented system on a transformer in operation.6 ppm and the water content of the solid insulation from about 4. Additionally the oil is not thermally stressed during the desiccation process.-6Cellulose filter cartridge Transformer vessel Cooler Pump Hot. Primary tests For the tests performed with this facility a transformer model has been used. The water content of insulating liquid samples extracted via a cannula into the barrel and from the outlet of the filter has been measured using a Karl-FischerCoulometer 684KF manufactured by Metrohm. Afterwards the oil is injected into the cycle stream through the filter vessel and the cooling unit where it is cooled down to 3 °C before it enters the filter cartridge. The samples had previously been aerated with water. In comparison to the conventional method where the oil is spread over large surfaces under vacuum and high temperatures this is a significant improvement.8 % respectively.7 to 22. For a practical application on power transformers there is no need for such an extreme dryness but it may be interesting for laboratory use. During the test time of 12 days the water content of the transformer model liquid has been reduced from 56. It consists of a heatable oil barrel containing 200 liters of mineral oil plus 30 kg of therewith impregnated paper and pressboard samples. The water content of the mineral oil leaving this setup has continuously been around and below 1 ppm. . After passing the filter cartridge the dried and purified mineral oil is refilled into the transformer vessel via another oil sample extraction tube or directly into the conservator. thus the oil humidity at the start of the experiment was about 60 ppm at 60 °C. The insulating liquid removed from the transformer via the oil sample extraction tubes passes at first the pump that transports the fluid through the system. Therefore the filter cartridge has been cooled down to -4 °C and the oil flow has been reduced to 4 liters per hour. The temperature of the transformer model and of the filter cartridge has been controlled using a Pt 100 thermometer.5 % to 2. A direct measurement of the paper samples water content is possible. For these reasons the filter cartridge used in the presented system has been specified for single use. The drying of the impregnated cellulose of the filter cartridge is a time consuming process during which the continuous drying has to stop. Each drying cycle worsens the condition of the filter due to the material burden. by destination. Alternatively a drying of the filter cartridge inside the filter vessel is possible but comprises several disadvantages. . remain on the surface of the filter and hamper the subsequent drying by vacuum. And finally the transformer gas balance and therewith also the DGA may be influenced by the application of vacuum. For this purpose the resistance of the impregnated filter material is measured as schematically depicted in figure 4. The filter cartridge consists of several layers of filter material that are glued together. The system flow-rate for a power transformer is actually rated to 240 liters per day but the basic limit for this value is the capacity of the cooling system as transformer insulating liquids are. To nevertheless assimilate large amounts of water until a replacement is necessary the filter dimensions have been enlarged.For transformers the flux of dried insulating liquid is more important than the maximum attainable dryness as the volume of insulating liquid to be dried is very large and with a higher throughput of the filter unit the drying is accelerated. removed from the insulating liquid. For an application on power transformers it is essential to detect the saturation state of the filter as in the case of a saturated purifier the drying stops and the cartridge has to be replaced. The particles. good thermal transfer media carrying a lot of thermal energy that has to be removed. The actually implanted cellulose filter cartridge is capable for covering up to 15 liters of water. Flow direction Water content measurement Electrodes Filter cartridge Figure 4 – Schematic view of the filter cartridge saturation detector. The liquid flows in radial direction through this cartridge. thus a measurement of the resistance between the two electrodes covering one of the filter slices is representative for the whole filter condition. Conclusions The innovative method for the lifetime extension of oil filled transformers is based on drying the solid and liquid insulation. Another possibility for increasing the drying performance would be the addition of a hygroscopic insulating liquid like ester liquids. Actually a similar setup is applied to a transformer operating at temperatures around 80 °C. analyses of the oil before and after the application of the setup have shown. The main tasks of the system are as follows: . Due to the lower temperature the equilibrium between the water in the cellulosic insulation and the water dissolved in the oil moves to the cellulose side (see Figure 2) and the insulating oil contains relatively small amounts of water. Even if this water is almost completely removed from the liquid during desiccation the poor supply with water by the transformer insulating liquid limits the drying speed. The water content of the transformer has been reduced by 300 g although the efficiency of the drying has been limited by the relatively low temperature of the transformer. The transformer is used as a backup. As the drying has just started no values concerning the amount of removed water are available yet. filtering of the oil and reducing the influence of oxygen . Even the addition of 20 % ester increases the water solubility of the liquid by a factor of 10. Further information about this method can be found in [6]. During the application for 3 months the results of the DGA have not been changed as Drying setup in water-proof housing Figure 5: Drying setup applied to a transformer on site. which is connected to the high voltage net but transmits energy only if the consumer needs extra power. Therefore only the losses during idle operation led to a warming of the transformer The ordinary operation temperature has been around 30 °C while usual power transformers operate at temperatures above 60 °C.-8- Application on site The system has been applied to a 30 MVA transformer as it is depicted by Figure 5. thesis.C. .04. Germany (info@kh. gentle and efficient drying of the insulating system and thereby increases the operation reliability and residual lifetime. Acknowlegements The authors gratefully acknowledge the Minna and James Heinemann foundation for their grant to Mr. B. Zahn.-21. 1 January/February 1999 [5] J. Lesieutre. Krause. U. pp. . .R. W. IEEE Electrical Insulation Magazine Vol. . Du. Lindgren: "Moisture Equilibrium in Transformer Paper-Oil Systems". France. Paris.1998 [2] F.502 [3] M. thus the Dissolved Gas Analysis (DGA) is not affected. Fabre and A. Munich. 1960 International Conference on Large High Voltage Electric Systems (CIGRE). H. K.purifies the insulating liquid by filtering out solid contaminants and therewith increases the breakdown and partial discharge inception voltage of the liquid. pp. The dryness attainable with this method is below 1 ppm. Sidler: "Effects of Moisture in Transformerboard Insulation and the Mechanism of Oil Impregnation of Voids". Flottmeyer. No. Zaengl: " . 496 . The insulating liquid is not stressed during this procedure. 15.does not influence the gas balance of the transformer. Pichon: "Deteriorating Processes and Products of Paper in Oil. 8 .is efficient over long time periods and can be applied on new and aged apparatus without the need for long time outages. Elektrizitätswirtschaft Jahrgang 95 (1996). M.D. Springer-Verlag 1986.P. . transform´98 Forum der Technik. Mamishev and S. Hochspannungstechnik – Theoretische und praktische Grundlagen für die Anwendung". A. Wasserberg in supporting his Ph. 145 .. References [1] C.148 [4] Y. paper 137Huser and A. Beyer. J.de) for their interest in the performed work and the support of a technical realization. Boeck.Reduces the influence of oxygen Actually the system is in the beginning of a serial production and is already applied on sample transformers throughout Germany.V. Möllmann: “Alterungsverhalten von Transformatorisolierölen”.allows a continuous. Application to Transformers". The experiences gained during these final tests will lead to a “fine tuning” of the setup and finish the development. Gasser. Germany.filter.allows an almost complete desiccation of mineral insulating oils. Hamburg. 20. No. Sundermann and A. Also special thanks to the Karberg & Hennemann CJC Feinfilter corporation. Möller and W. Borsi. 13th Intern. E. Symposium on Electrical Insulation (ISEI 2000). Anaheim. p.N. Gockenbach: “A New Method for Drying the Paper Insulation of Power Transformers during Service”.10 - [6] V.. H. paper 9-2 [7] N. Publication of the Siemens AG. USA.: “Power Engineering Guide – Transmission and Distribution”. Wasserberg. 4/19 .