GENERAL DISCUSSION: CONVENTIONAL AND NOVEL DRYING CONCEPTS Need for Advanced Drying Technologies Classification and Selection Criteria: Conventional versus Novel Technologies Innovation and Trends in Drying Technologies SELECTED ADVANCED DRYING TECHNOLOGIES Drying on Inert Particles Impinging Stream Drying Drying in Pulsed Fluid Beds Superheated Steam Drying Airless Drying Drying in Mobilized Beds Drying with Shock Waves Vacu Jet Drying System Contact-Sorption Drying Sonic Drying Pulse Combustion Drying Heat Pump Drying Fry-Drying SELECTED TECHNIQUES FOR DRYING AND DEWATERING Mechanical Thermal Expression Displacement Drying Vapor Drying Slush Drying Atmospheric Freeze-Drying Spray-Freeze-Drying Refractance Window Carver-Greenfield Process HYBRID DRYING TECHNOLOGIES Microwave-Convective Drying with Cogeneration Microwave-Vacuum Drying Filtermat Drying Spray-Fluid Bed-Vibrated Fluid Bed Dryer Combined Filtration and Drying Radio-Frequency Drying with 50 O Technology Radio Frequency-Assisted Heat Pump Drying Radio Frequency-Vacuum Drying Miscellaneous Hybrid Technologies OTHER TECHNIQUES Special Drying Technologies Index

Advanced drying technologies

This paper presents a brief discussion on the development of electrical grade paper/pressboard for transformer use from the raw materials, improvements made, and particularly the use of thermal upgrading agents to extend the useful life of transformers

Cellulose insulation in oil-filled power transformers: Part I - history and development

Developments in transformer insulation, leading to increased use of aramid insulation in oil-filled transformers, are briefly summarized. A series of tests designed to gather data on the dielectric strength of aramid paper in oil used as turn-to-turn insulation is presented. The test setup, sample configuration and preparation, and test procedures are described. Turn-to-turn and coil-to-coil results are presented and discussed. It is shown that, turn-to-turn, the aramid paper is stronger dielectrically than the kraft paper by 25% using a full-wave impulse and 20% using a 1 mu s chopped wave. There is little or no difference in the dielectric coil-to-coil strength between the aramid and kraft papers. Thus, it is possible to improve the turn-to-turn impulse strength of liquid-filled power transformers by using aramid paper in place of kraft paper. The aramid also has improved mechanical strength and thermal aging characteristics. >

Conductor insulation tests in oil: aramid vs. kraft

For many years traditional design of liquid-immersed transformers has been based on the combination of cellulose solid insulation and mineral oil. As a result, average temperature rise for windings is normally limited to 65 K, and top oil rise to 60 K as given in IEC 60076-2. Sometimes space or weight limitations or high ambient temperatures force designers to look for new solutions such as those with higher temperature rises using high temperature materials. Using aramid paper with a much higher thermal capability than Kraft cellulose paper can allow operating the equipment at a higher winding temperature rise without any negative impact on the insulation life.

Aging of high temperature insulation systems with alternative fluids

Power transformer (PT), essential equipment in electrical power system proceeds complicated manufacturing process to meet high standard design. During manufacturing process of PT, the drying stage is of importance to remove excessive moisture contaminant on the solid insulation. This study investigated the effects of drying duration on partial discharge test of 83.3 MVA 275/160 kV PT. The partial discharge test measurements were carried out using IEC 60270 standards comprising of the capacitive network model based on the main quantity apparent charge concept. We found that the lowest partial discharge of 21 pC was achieved at 68 hours drying duration, while the highest value of 60 pC was reached at 81 hours drying time. The longer drying time may lead to initial degradation of cellulose insulation due to excessive heating. The amount of water extracted during drying effect to partial discharge also investigated. We conclude that the drying time at a higher and lower than 68 hours significantly contributes to the increased partial discharge values of PT.

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The effects of drying time during manufacturing process on partial discharge of 83.3 mva 275/160 kv power transformer

The processing procedure described above has already resulted in several important benefits of interest to both the manufacturers and the users of power transformers: 1 Insulation structures which formerly would have been impossible or impractical to properly dry and impregnate can now be easily processed in a relatively short time This allows the transformer designed to use design practices on high rating units which would not have been permissible formerly It also extends upward the maximum ratings which can be built 2 The electrical, magnetic and mechanical parts of transformers of all ratings can be designed and built to much closer tolerances because of the consistently thorough drying and dimensional stabilization of the insulation obtained by this process For example, the magnetic core can now be designed to have a closer fit with the coils, thus reducing the amount of wasted space without applying damaging physical stresses to co re material during assembly 3 Because of the more dependable electrical properties of the insulation resulting from the more rigorous drying, the required insulation clearances can be more accurately calculated with more assurance that the transformer will stand up on test and in service 4 The transformer will never be any drier in service than at the time the core-coil assembly is put together and assembled onto the tank, so that the coils will not loosen up in service Movement of the coil conductors and leads under short-circuit conditions should be effectively prevented for the entire life of the transformer 5 The time required to manufacture a power transformer has been reduced by from a day to a week or more because of the shortened processing time This is of assistance both to the manufacturer and the customer, making it easier to meet short delivery dates

Drying and oil impregnation of power transformer insulation

1. The 60 Hertz and impulse electrical strength of oil impregnated polyamide paper is as good as that of high quality electrical grade kraft paper and better than that of rope-kraft paper used as conductor insulated. 2. The 60 Hertz dissipation factor of polyamide paper in oil increases moderately with increasing temperature. 3. Based on burst strength and tensile strength retention, the thermal stability of the polyamide paper is substantially superior to that of cellulosic paper when aged in oil or askarel at 150°C and 175°C. 4. Polyamide paper showed little or no degradation after extended life tests in distribution transformers. 5. Polyamide paper with its superior mechanical properties can better withstand manufacturing operations in production than can cellulosic papers. 6. Based on life test results the polyamide paper will permit the design of transformers with higher operating temperatures and greater overload capacities than can be obtained using cellulosic paper.

Application of polyamide papers in liquid-cooled transformers

The problem of designing the coils for a large power transformer is one of great complexity. In addition to the obvious parameters of operating voltage and current, impedance and losses, the designer must consider many other requirements, including the ability to withstand various types of impulse voltage waves and the mechanical strength to resist stresses resulting from short-circuit conditions. A requirement which is always present is that of maximum reliability. At the same time it is essential that the size, weight and cost of the finished transformer be as small as possible.

http://ieeexplore.ieee.org/iel7/7461806/7461807/07461859.pdf

L. E. Feather

Papers

Drying and oil impregnation of power transformer insulation

Application of polyamide papers in liquid-cooled transformers

A new insuldur∗ turn insulation for large power transformers