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Rising energy and transmission economy requirements have produced a demand for increasingly larger

EHV transformer units capable of meeting the very high operational security standards associated with

modern interconnected EHV networks in which the entire energy output is combined.

In addition to appropriate design and material processing methods, efficient drying, degassing and

impregnation process technology is a decisive factor to consider. since it is the one on which optimal

utilisation of the ecellent properties of paper!oil dielectric system depends. Efficient drying ofinsulation, therefore, is a vital requirement for transformers.

Vapour-phase drying

Special Features

"rom the thermodynamic aspect, this process can be considered to be the most suitable of all known

drying methods.

#he "ollowing are its characteristics features$ a% #he entire process takes place in a practically oygen!

free atmosphere. &s a result, drying can be effected at higher temperatures than by conventionalmethods. b% Heat is transmitted by a hydrocarbon vapour. "ull use is being are of its heat of

condensation. c% Heat is released to the entire free surface of the material to be dried, especially to the

coldest point, where condensation is most active. #he result is a larger heat !transfer surface than inconventional processes. &s the net result of these various features shorter heating times are achieved

with smaller temperature differences.

Basic plant features

#he main difference between conventional and vapour!phase drying is that in the latter process, theheat!carrier is a light oil vapour instead of air. #he vapour is condensed out on the ob'ect to be dried

and then re!evaporated in the plant. "or this reason, vapour!phase installations include an evaporator

and condenser system in addition to the vacuum equipment and autoclaves associated withconventional drying equipment.

Cleaning effect

& further advantage of the vapour!phase process is that dirt and dust deposits formed on livecomponents during factory assembly are carried away by condensate during the heating stage.

Drying of oil-impregnanted insulation

&ll drying plants have to deal with oil!impregnated windings at various times, e.g. windingsdeliberately impregnated for stabilising purposes, or in the case of transformers to be dried after

repairs.

&t the vapour!phase heat!carrier washes the insulating oil out of the insulating material, this process is

particularly suitable for such drying applications( the high diffusion coefficient of non!impregnatedinsulation is restored, processing of oil!impregnated material has no effect on final drying quality.

Determination of finaldry condition

"inal dry state is determined from the following parameters$v !insulation temperature

!final vacuum

!residual moisture etraction rate

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Design and installation of vapour-phase drying equipment

#he following are the essential installation requirements for a vapour!phase drying plant$ #he autoclave

should be located at the optimal point for material throughput requirements at the considered

production stage. #wo basic requirements can be catered for, i.e.)% top!loading

*% front loading

"or technical reasons the main vacuum plant should be set up in the immediate vicinity of +or on% theautoclaves

Drying of transformers after repairs

Vapour!phase drying is particularly attractive for this application, if only because of the appreciably

better solid insulation dielectric properties associated with the washing out of +sometimes high aged%insulating oil from the insulating material.

#ransformers under repair showing signs of substantial sludge, graphite or other dirt deposits should

preferable be washed down with flushing oil before processing.

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