Drying of high voltage power transformers in the field with a mobile vapour phase drying equipment Dr.-Ing. Stefan Zabeschek, Hedrich vacuum systems, Ehringshausen, Germany Dipl.- Ing. Helmut Strzala, Hedrich vacuum systems, Ehringshausen, Germany Abstract To extend the life time of high voltage power transformers, it is necessary to extract the water moisture from the paper insulation. After a short description of the state-of-the-art drying technologies, a new drying process is presented. This new development allows to dry high voltage transformers in the field with the advanced vapour phase drying technology. Until now, this technology has only been used for the drying of active parts during the production process of new power transformers and repair transformers. The repair transformer is dried in its own housing. Residual moistures similar to new transformers can be reached. Residues of cracked transformer oil can be removed from the surface of the insulation material. The cost-intensive transport of the transformer to a repair workshop is no longer necessary. Life time of repair transformers can be extended. Introduction For the background of increasing delivery time and purchase price of high voltage power transformers, extension of the life time of existing transformers got into the spot light for the users of transformers. One important reason for the failure of transformers is the aging of the insulation material. About 10 % of the weight of a power transformer consists of paper, pressboard and wood. One decomposition product in the cellulose de-polymerization of these materials is water which is stored in the insulation and also partly in the transformer oil. Further sources of water moisture are leakages in the transformer tank and missing functions of the air dehumidifier. By increasing moisture, the breakdown voltage declines and the tan delta factor describing the dielectric quality of the insulation increases. Figure 1 shows this correlation. Breakdown voltage U d 120 [%] 120 x 10-3 tan δ 100 80 60 40 20 U d 100 80 60 40 20 Dissipation factor tan δ at 90 C 0 0,1 1 10 Moisture content 0 [%] Fig. 1: Electric characteristics o f oil-impregnated paper as a function of the moisture content. 1
Additionally, the increasing moisture content of the insulation accelerates de-polymerization of the insulation paper (Fig. 2). One percent more water content in the paper results in the increase of the de-polymerization speed of about 100 %. relative de poymerization speed _. Fig. 2: 1000 100 10 1 80 C 100 C 120 C 0 1 2 3 4 Moisture content of insulation paper [%] Comparison of the relative de-polymerization speed as a function of the moisture content and temperature of insulation paper. [Transformerboar d II. Weidmann, CH- 8640, Rapperswill, 1987] A moisture content of approximately more than 2 to 4 % depending on working voltage, field strength, load factor and working temperature of the transformer recommends initiating a drying process. The ideal drying process for repair transformers has to fulfil several parameters like an uniform heating up of the insulation to a maximum temperature of 125 C and an average residual moisture of less than 0.3%. The drying process should happen under vacuum to reduce depolymerization of the insulation. The aged decomposition of the transformer oil covers the surface of the insulation material and should also be removed by the drying process. Nevertheless, the whole process should take place on site during a fix period to guaranty clear costs. Current state of technology for transformer drying in the field The well-established conventional drying processes for repair transformers are the processes: Hot Air, Oil Circulation and Oil Spray. Hot Air Hot air with a maximum temperature of 120 C is used for heating-up the active parts of the transformer. A maximum insulation temperature of 105 C is recommended. Provided that the transformer tank is vacuum-tight, the drying process can be optimized by evacuating the transformer. This process is inefficient, due to the fact that the active parts are non-uniformly heated up under atmospheric conditions. Residues of transformer oil cannot be removed out of the insulation. 2
Oil Circulation Transformer oil is circulating through an oil purification equipment. After filtering the oil to remove the particles, the oil is heated up to maximum 80 C. In a so-called vacuum degassing unit, all residual gases are removed and the moisture evaporates. Residual moisture contents of less than 5 ppm can be reached. Examples of an oil purification units manufactured by Hedrich Vacuum systems show Figure 3. Fig. 3: Single stage oil purification units of Hedrich with a capacity of 2000 l/h and 6000 l/h. Oil purification is a necessary maintenance process for oil-filled power transformers, but not very efficient for removing moisture. The reason is the indirect drying of the insulation which takes a lot of time. The equilibrium of moisture in oil and moisture in insulation paper is demonstrated in Figure 4. Fig. 4: Moisture equilibrium of paper-oil insulation as a function of the oil temperature. (Source: Fabre, J. Pichon, A.: " Deteriorating Processes and Products of paper in Oil. Application to Transformers" CIGRE 1960 Paris, Nr.137) 3
Heating-up of the transformer oil shifts the moisture equilibrium from paper to the oil side. The main fault made by the operators is to stop the drying process after reaching an oil moisture of less than 5 ppm or 5 gh 2 O/t. After a while, when again stationary conditions exist in the transformer, the moisture content of the oil distinctly increases. It is necessary to dry the transformer oil continuously throughout a long period. One optimization of the process is to intermediately evacuate the transformer after discharging the transformer oil out of the tank. Exposition of the insulation surface to vacuum improves evaporation of the moisture. This process has to be repeated several times. The long heating and evacuation steps do not significantly increase efficiency of this drying process. Oil Spray After removing the transformer oil from the tank, spray nozzles are assembled at the man holes and flanges of the transformer tank. The tank is evacuated by a vacuum pump unit to a pressure range of 5 mbar. Oil is pumped through filter devices and a heating unit to the top of the autoclave. Via nozzles, the oil is sprayed onto the surface of the active parts. The active parts are heated up and moisture evaporates. The vacuum pump unit continuously sucks the water vapour out of the transformer tank. The absence of oxygen allows the increase of the oil temperature to 120 C. The oil spray drying process is the most common process for drying power transformers in the field. It fulfils most demands to achieve an ideal drying process. The drying times are still very long, the insulation is non-uniformly heated up and removal of oil residues is not optimally realized. A new drying technology for transformers in the field The most advanced drying technology for new power transformers is the vapour phase drying technology. Until now, it was not possible to use this technology in the field because of the dimensions of standard vapour phase equipment. Based on more than 25 years of experience in designing and developing vapour phase drying equipment, the company Hedrich has developed a very compact technology allowing to install it on a lorry. The so-called mobile vapour phase equipment enables to carry it directly to the transformer in the field. Disassembly and transportation of the transformer to a repair workshop with a stationary vapour phase drying equipment are no longer required. Some physical basic features of vapour phase drying The vapour phase technology makes use of the physical effect of the very good heat transfer by film condensation of the heat-transferring vapour. The effective heat transfer is also characterized by the fact that the heat-transferring vapour condenses most intensively at the colder spots of the drying object. So, a nearly uniform heating-up of the active parts can be reached. In comparison to Hot Air, the drying time can be reduced by more than 50 % and is especially very effective for large transformer up to 500 MVA. A typical drying time for a transformer of this size, when using vapour phase technology, is one week in comparison of several weeks when using the oil spray process. The whole process is effected under vacuum to reduce the depolymerization factor of the insulation. For the heat-transfer, a solvent is used which is chemically related to the transformer oil, such as white spirit. The removal of cracked transformer oil residues placed on the surface of the insulation material is very advantageous which can be described as a "dry cleaner" effect. 4
Process description of the mobile vapour phase drying Preparation on site: The transformer is shut down and the high and low voltage bushings are disassembled. Transformer oil is discharged into a storage vessel. The transformer tank has to be well heat insulated. A heating tent or mobile house has to be placed over the transformer tank. The characteristic feature of the solvent vapour to condense in favour on the coldest spots of the transformer makes good transformer tank insulation absolutely necessary. The mobile vapour phase is connected to the transformer tank via flexible stainless steel pipes. The main components of the mobile vapour phase drying equipment shows Figure 5. Fig. 5: Principle design of the mobile vapour phase drying equipment. Evacuation: The transformer tank is evacuated to less than 10 mbar by means of vacuum pumping unit (item 4 in fig. 5). Heating unit (item 2.1) is switched on. Leakage pump (item 4.1) evacuates condensation system (item 3.1 and 3.8). Heating-up: The solvent is heated up and pumped to the expansion valves under pressure. The solvent expands after passing the nozzles and evaporates partly immediately. This technology is known as flash or expansion evaporation. Vapour condenses at the coldest spots of the active parts and deeply penetrates into the insulation material. The unit to be dried is quickly heated up, especially by the latent heat of the condensing vapour. The moisture from the insulation is transported by means of the water-solvent vapour mixture circuit, through main condenser (item 3.1) and water-solvent condensate circuit to collecting and separating vessel (item 3.8). In the separating vessel, water and solvent are separated. 5
For transformer containing great amounts of insulation and those having an unfavourable surface factor (ratio of surface to volume), it is advisable to apply one or several intermediate pressure reductions for faster removal of moisture. Moreover, the heat transfer to cavities, where moisture is accumulated, can be intensified. Pressure reduction After the limit temperature of the insulation has been reached and most of the moisture has been removed, the vapour supply is interrupted and the solvent evaporates from the insulation by continuous pressure reduction up to 20 mbar. Heat losses are avoided by means of the tank insulation. Fine Vacuum All connections with evaporation and condensation systems are closed; the vacuum pumping unit (item 4) is started directly. The fine vacuum phase needs approximately the same time like the heating up of the transformer. This phase of the drying process is stopped after reaching a steady end pressure of 0.1 to 0.2 mbar. At this point the drying process is normally completed. Additionally a partial water pressure gauge for measuring the amount of water vapour in the pumped out flow can be used to evaluate the residual moisture in the insulation. During the drying of the active parts of new transformer residual moisture less than 0.3 % can be reached. After the drying process the transformer tank is charged again under vacuum with transformer oil. During the fine vacuum process the solvent is regenerated by a distillation process. The residuals of old transformer oil get removed and the solvent can be used for further drying processes. Experiences The first tests with a prototype have been started in the year 2003. Based on this first test the design was improved. Fig. 6: A compact vapour phase dr ying equipment with flash evaporation system designed and manufactured by Hedrich vacuum system, Germany. 6
Figure 6 shows an example of semi-mobile compact vapour phase drying equipment with the flash evaporation system designed and manufactured in a Hedrich work shop. The customer is placed in Europe and has started successfully the drying process of transformers in summer 2007. Additional drying equipment has been ordered in the meantime. Conclusions The mobile compact vapour phase drying equipment with the flash evaporation system fulfils all demands of an ideal drying process of transformers. Drying times and residual moisture contents of insulation materials are similar to the common vapour phase drying technology in transformer production factories. The residuals of used transformer oil can be removed very efficiently. Therefore the drying of transformers in their own housing in the field or in repair work shops can be done successfully with the new system. In the meantime this technology is requested for the drying of new ultra high voltage power transformers, which have to be assembled in the field substituting the hot air drying technology. On the other hand this technology opens the market for the cost efficient drying for small power transformers as well as distribution power transformers with vapour phase drying technology. October 2007 Visit us on our homepage in the Internet under www.hedrich.com 7