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9. PROCESSING AND DRY OUT
The paper insulation and pressboard material, which make up a significant proportion by volume of transformer windings, have the capacity to absorb large amounts of moisture from the atmosphere. The presence of this moisture brings about a reduction in the dielectric strength of the material and also an increase in its volume. The increase in volume is such that, on a large transformer, until the windings have been given an initial dry out, it is impossible to reduce their length sufficiently to fit them on to the leg of the core and to fit the top yoke in place.
The final drying-out is commenced either when the core and windings are placed in an autoclave or when they are fitted into their tank, all main connections made, and the tank placed in an oven and connected to the drying sys tem. The tapping switch may be fitted at this stage, or later, depending on the ability of the tapping switch components to withstand the drying process.
Traditional methods of drying-out involve heating the windings and insulation to between 85ºC and 120ºC, by circulating heated dry air and finally applying a vacuum to complete the removal of water vapor and air from the interstices of the paper before admitting transformer oil to cover the windings.
For a small transformer operating at up to, say, 11 kV, this heating could be carried out by placing the complete unit in a steam or gas-heated oven. For a large transformer the process could take several days, or even weeks, so that nowadays the preference is to use a vapor-phase heating system in which a liquid, such as white spirit, is heated and admitted to the transformer tank under low pressure as vapor. This condenses on the core and windings, and as it does so, it releases its latent heat of vaporization, thus causing the tank internals to be rapidly heated. It is necessary to ensure that the insulation does not exceed a temperature of about 130ºC to prevent ageing damage: when this temperature is reached, the white spirit and water vapor is pumped off.
Finally, a vacuum equivalent of between 0.2 and 0.5 mbar absolute pressure is applied to the tank to complete removal of all air and vapors. During this phase, it is necessary to supply further heat to provide the latent heat of vaporization; this is usually done by heating coils in an autoclave, or by circulating hot air around the tank within the dry-out oven.
The vapor phase dry-out process is similar to systems used previously, the only difference being in the use of the vapor to reduce the heating time. It is not a certain method of achieving a drier transformer and, in fact, it is possible that the drying of large masses of insulation might be less efficient since, being limited by the rate of diffusion of water through the material, it is a process which cannot be speeded up. This is an area where further research might be beneficial. Particular problem areas are laminated pressboard end support structures and laminated wood used in the same location, where moisture will tend to migrate along the laminations rather than cross through the inter-laminar layers of adhesive. Designers need to give special consideration to such structures and can often improve the dry-out process by arranging to have holes drilled in places where these will assist the release of moisture without weakening the structure. Another aspect of this system of drying-out which requires special attention is that of the compatibility of the transformer components with the heat transfer medium. For example, prior to the use of the vapor phase process, some nylon materials were used for transformer internals, notably in a type of self-locking nut. This nylon is attacked by hot white spirit, so it was necessary to find an alternative.
Even in the case of small transformers, where dry out will probably be carried out using a heated oven, there is still a need for careful attention in certain difficult areas. One of these is for multilayer HV windings using round conductors. This type of winding usually has a layer of paper insulation between conductor layers. The moisture trapped within this interlayer insulation will have to travel up to half the length of the layer in order to be released to the atmosphere. This can take many hours, even days, at 130ºC.
FIG. 113 Insulation resistance and power factor curves during dry out
Monitoring insulation dryness during processing usually involves measurement of some parameter which is directly dependent on moisture content.
Insulation resistance or power factor would meet this requirement. Since there are no absolute values for these applicable to all transformers, it is usual to plot readings graphically and dry out is taken to be completed when a level ling out of power factor and a sharp rise in insulation resistance is observed.
FIG. 113 shows typical insulation resistance and power factor curves obtained during a dry out. Vacuum is applied when the initial reduction in the rate-of-change of these parameters is noted: the ability to achieve and maintain the required vacuum, coupled with a reduction and leveling out of the quantity of water removed and supported by the indication given by monitoring of the above parameters, will confirm that the required dryness is being reached. For a vapor phase drying system, since it could be dangerous to monitor electrical parameters, drying termination is identified by monitoring water condensate in the vacuum pumping system. At this point oil filling is begun with dry, filtered degassed oil at a temperature of about 75ºC being slowly admitted to the tank and at such a rate as to allow the vacuum already applied to be maintained.
Drying-out of insulation is accompanied by significant shrinkage, so it is usual practice for a large transformer to be de-tanked immediately following initial oil impregnation to allow for re-tightening of all windings, as well as cleats and clamps on all leads and insulation materials. This operation is carried out as quickly as possible in order to reduce the time for which windings are exposed to the atmosphere. However, once they have been impregnated with oil, their tendency to absorb moisture is considerably reduced so that, provided the transformer is not out of its tank for more than about 24 hours, it is not necessary to repeat the dry-out process. On returning the core and windings to the tank, the manufacturer will probably have a rule which says that vacuum should be re-applied for a time equal to that for which they were uncovered, before refilling with hot, filtered and degassed oil.
Before commencement of final works tests, the transformer is then usually left to stand for several days to allow the oil to permeate the insulation fully and any remaining air bubbles to become absorbed by the oil.
References
1. Montsinger, V.M. (1930) 'Loading transformers by temperature.' Trans. A.I.E.E., 49, 776.
2. On-load Tap-changers for Power Transformers A technical Digest. Maschinenfabric Reinhausen, Falkensteinstrasse 8, 93059 Regensburg, Germany. www.reinhausen.com
3. Shroff, D.H. and Stannett, A.W. (1984) 'A review of paper ageing in power transformers.' Proc. IEE., 132, 312-319.