In construction of power transformers, depending on the voltage class, it is desirable to suppress the water content of cellulosic-based materials to <0.5 % and even as low as 0.1 % (by weight) prior to shipment. Cellulosic Kraft materials such as high density pressboard, papers, tapes, crepe tubes and spacers are used in the paper/oil insulation system of the transformer. These are used to support windings and leads as well as to maintain dielectric clearances, these usually contain 8 – 10% of moisture by weight at ambient temperature. This is dangerous to the health of the transformer because it reduces the electric strength, resistivity and accelerates deterioration of solid insulation. Therefore, it is required to remove moisture from insulation to maintain its insulating properties. Conventional drying has not remained efficient and effective in terms of energy savings, cost of operation and achieving process parameter.
Complete Dry out of the core and coil assembly begin by placing the entire assembly inside vapor phase drying chamber. Vapor Phase Drying (VPD) also applies vacuum but the method of heating is not through air. In this method, the carrier of heat is Vapor of a low viscosity solvent like kerosene with a higher flash point. Vapor is heated in an evaporator by use of electric heaters and are passed over the core and coil assembly placed inside for drying. The solvent vapor condenses when encountering the lower temperature job.
The condensed kerosene is collected and subjected to many rounds of heating and evaporation. The job temperature rises and evaporated water is pumped off to attain thorough drying. Vacuum maintained during the process is very low (as low as < 0.5 Torr).
Dew point reading dictates end of cycle and thus unit is ready for oil spray process. Unit is now sprayed with transformer oil within the chamber to cover the pores of cellulosic material (insulation) to inhibit moisture absorption.
At this point the unit is taken out from vapor phase vacuum chamber and proceeds to a process called servicing. Unit is retightened to compensate any shrinkage. With minimum exposure time unit is tanked immediately and covered, vacuum is applied prior to oil filling for final impregnation, after appropriate “under vacuum” time, testing of transformer begins in accordance to customer specification and standards.
In summary, the use of vacuum allows for drying of the components without the need of high heat that would be detrimental to the insulation material.
Each of our factories are equipped with VPD ovens. We have three independent 430.5 sqft (13345 cubic foot) VPD ovens for the drying of transformer active parts and accessories at our large and extra-large power transformer manufacturing plant in Rincon, GA. Each oven is PLC operated to allow for precise temperature, vacuum and time control and allows for adjustable Isostatic pressing of transformer winding to ensure proper calibration and sizing of the coils.
If a transformer is too large for VPD, then Hot Oil Spray (HOS) method is used for drying. In this method, transformer is placed under vacuum, and sprayed with heated transformer oil to drive the moisture away. HOS is continued until the desired moisture levels are achieved.