4.1 Off-line transformer dehydration

For the treatment is a transformer always shutt. Preliminary work takes usually 1-2 days with regarding the size and operational voltage of transformer and used methods (e.g. dismantling of bushings for Oil Spray …), and can therefore be relatively very expensive (extra manpower, oil tanks, cranes….).

The treatment generally non-reversibly removes water, gas and particles from the oil inventory of the transformer . But the effective drying of the cellulose insulants can be done only when the oil inventory of the transformer is discharged and then the vacuum interacts with the hot cellulose insulants directly.

The costs of all dehydration methods is a very rough estimation of the cost of the removal of 1 kg water which can strongly vary with local conditions.

Economical Goals: Table E1

Method	Advantages	Draw-backs	Costs: € / kg water
Vaccum Oil Drying	Classical, available method	Only oil invetory can be effectively dried *	?
Oil Spray	Treatment is relatively short Soft (paper) and hard cellulose insulants (boards) can be dried very effectively	Main tank and oil coolers must be vacuum proof transformer has to be opened to install internal oil sprinklers aged cellulose can be non-reversibly damaged loss of clamping forces by overdrying possible	min. 370 - 500
LFH Method Low Frequency Heating Method	The treatment is relatively short Paper on wires can be dried very effectively	Boards cannot be effectively dried (only paper on wires is effectively heated by LFH) ** Main tank and oil coolers must be vacuum proof aged cellulose can be non-reversibly damaged losss of clamping forces by overdrying possible	min. 300 - 500

^* By the treatment the oil inventory of a transformer is not discharged. The oil is heated over 80C and the water is removed from the oil by means of external high vacuum drier. Because the diffusion coefficients between the paper (boards) and oil are always very low and the time-period of the treatment is usually too short (days), the paper and especially the boards cannot be effectively dried. Don´t forget - there is always more than 95% water in the solid insulants and only 5% or less is solved in the oil.

^** tBoards and other high density cellulose insulants of a transformer can be heated by repeated discharge of cold and recharge of hot oil inventory but this process prolongs the drying process and increases the costs dramatically

4.2 On-line transformer dehydration

The transformer remains on-power , no special adaptations on the main tank are necessary. The installation of the drier is very simple and can be done by one man without troubles within a hours.

The costs of all dehydration methods is a very rough estimation of the cost for the removal of 1 kg water which can strongly vary with local conditions especially with the operational temperature of the transformer.

Economical Goals: Table E2

Method	Advantages	Draw-backs	Costs: € / kg water
Vaccum Oil Drying	low operational costs water, gas and particles are non-reversibly removed aged cellulose cannot be non-reversibly damaged the overdrying and loss of clamping forces can be controlled * removed water can be measured volumetrically from remote ****	High purchase price Long-term treatment Dangerous high-vacuum stripping of light fraction ***	min. 120 - 250
Adsorption process	Low purchase price water and particles are non-reversibly removed aged cellulose cannot be non-reversibly damaged the overdrying and loss of clamping forces can be controlled *	High operational costs Long-term treatment	min. 250 **

^** BThe costs of column replacement (if molecular sieve is exhausted) can be substantially reduced by the external high-temperature regeneration(dehydration)

^*** By the permanent application of the high vacuum / high temperature are removed (stripped) from the oil not only water and gases but its light fraction as well. Chemical composition of the oil can be changed in this way. This process can be effectively suppressed. See - Liquid Piston Principle - / Product Range / VS-06

^**** See Product Range / VS-06

^**** See Product Range / ADT

4.3 Economical comparison of existing on-line drying methods

The costs of transformer dehydration depend on:

• selected technology
• duration of dehydration treatment or number and size of dehydrated transformers
• the arragement made to acquire the dehydrator

As mentioned there are three basic on-line methods and apparatus:

• small vacuum drier . e.g. Vacuum Separator VS-06
• adsorption drier whose adsorption cartrige(s) must be routinely changed

The purchase of the drier is usually the most economical, especially if there are several smaller units to be maintained at acceptable moisture levels in a preventative maitenance program, or we have a large units which have to undergo a lengthy corrective or even continuous life-extending treatment.

The main cost components of the dehydration are therefore:

• purchase price
• installation cost
• operational cost (energy , routinely exchangeable items, labour costs, transportation etc.)

Installation costs are for both driers approx. the same and can be excluded from the comparison.

A typical cost comparison is showed as an example in folowing figure. The costs are expressed as a function of the amount of water which have to be removed.

Economical Goals: Boundary conditions

Drier	Purchase price €	Operational costs € / kg water
Vacuum drier	60 000.-	120.- *
Adsorption drier	25 000.-	700.- **

^* the operational cost = power consumption

^** the operational cost = power consumption + purchace price of changed cartridges + labor costs + transportation (estimation)

Fig. 2 Comparison of the costs of on-line drying methods

At the first sight it seems that dehydration by the vacuum method is much less advantageous than an adsorption one. The cost break-point No.2 comes not until after removing of ca 66 kg water or ca 3-4 years of the operation.

In this case we would be comparing the incomparable.The vacuum drying is, in contrast to the absorption, an ultimately complex process which can successfuly remove all main transformer enemies - water, gases and particles.

4.4 Conclusion:

When the problem of the aged transformer is strictly limited to a higher content of the water in the cellulose insulants (and increased amount of particles) the proper, sufficient and the most economical solution is the on-line drying by absorption drier.

But mostly the problem is not so simple, because for the extending of the life-span of transformer we must not only "discharge" the undesired aging products (water, acids and particles)from its insulation system, but control the internal conditions that the aging process would be permanently and strongly reduced or stopped in future.

The simplest way to stop the oxidation aging of the transformer is a passive or active longterm reduction of the oxygen content in the oil and decreasing of operation temperature of given transformer.