Vapor Phase Drying System

Vapour - Phase drying for EHV transformers

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 excellent properties of paper-oil dielectric system depends. Efficient drying of insulation, therefore, is a vital requirement for transformers.

Vapour-phase drying

Special Features
From the thermodynamic aspect, this process can be considered to be the most suitable of all known drying methods.

The Following are its characteristics features: a) The entire process takes place in a practically oxygen-free atmosphere. As a result, drying can be effected at higher temperatures than by conventional methods. b) Heat is transmitted by a hydrocarbon vapour. Full 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. The result is a larger heat -transfer surface than in conventional processes. As the net result of these various features shorter heating times are achieved with smaller temperature differences.

Basic plant features

The main difference between conventional and vapour-phase drying is that in the latter process, the heat-carrier is a light oil vapour instead of air. The vapour is condensed out on the object to be dried and then re-evaporated in the plant. For this reason, vapour-phase installations include an evaporator and condenser system in addition to the vacuum equipment and autoclaves associated with conventional drying equipment.

Cleaning effect

A further advantage of the vapour-phase process is that dirt and dust deposits formed on live components during factory assembly are carried away by condensate during the heating stage.

Drying of oil-impregnanted insulation

All drying plants have to deal with oil-impregnated windings at various times, e.g. windings deliberately impregnated for stabilising purposes, or in the case of transformers to be dried after repairs.

At 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-impregnated insulation is restored, processing of oil-impregnated material has no effect on final drying quality.

Determination of finaldry condition

Final dry state is determined from the following parameters:v -insulation temperature
-final vacuum
-residual moisture extraction rate

Design and installation of vapour-phase drying equipment

The following are the essential installation requirements for a vapour-phase drying plant: The autoclave should be located at the optimal point for material throughput requirements at the considered production stage. Two basic requirements can be catered for, i.e.
1) top-loading
2) front loading
For technical reasons the main vacuum plant should be set up in the immediate vicinity of (or on) the autoclaves

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.

Transformers under repair showing signs of substantial sludge, graphite or other dirt deposits should preferable be washed down with flushing oil before processing.