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.

5.1 Passive systems

The up-grade costs can strongly vary with local conditions and with the type and size of the transformer

Economical Goals: Table E3

Method	Advantages	Draw-backs	Up-gradeCosts € / MVA
Sealed-Tank Type	Classical, available method Low operational costs	Only for small transformers No or difficult check of proper function	?
Gas-Oil Sealed Type	Classical, available method Low operational costs	High up-grade price * No or difficult check of proper function	250 - 350
Automatic Inert-Gas Pressure Type	Classical, available method Low operational costs	Medium up-grade price ** Buchholz relay alarm possible by pressure drop	150
Bag-In Tank or Membrane-In Tank Type	Classical, available method No operational costs	High up-grade price *** No or difficult check of proper function	300 - 500
TRAFOSEAL	Low up-grade price No operational costs		< 100

^* The old conservator must be removed and a new- or modified one must be installed

^** The automatic inert-gas pressure apparatus must be installed

^*** The old conservator must be removed and a new one equipped by the bag or membrane must be installed

5.2 Active systems

In principle every small vacuum drier can be used as a active degassing device which is simply connected e.g. to the Filter Press Connectors of the main tank. The goal is to reduce the oxygen content permanently under 4000 ppm - it slows down the aging ca 4 - 5x compared to the full saturation ( O2 ~ 22 000 ppm)

Economical Goals: Table E4

Method	Advantages	Draw-backs	Costs € / unit
Small vacuum drier	Low operational costs	No or difficult check of the degassing process Dangerous high vacuum stripping off light fractions of oil	10 - 15 000.-
Vacuum separator VS-04	Low operational costs Remote control of degassing process possible No high vacuum stripping off of light fractions of oil	Higher price	25 000.-

5.3 Combined systems

Only four combinations of passive-active systems are possible - the following cost estimation is valid for ca 100 MVA Transformer.

The goal is to reduce the oxygen content permanently under 1 - 2000 ppm - it slows down the aging by ca min. 10x compared to the full saturation ( O2 ~ 22 000 ppm) .

Economical Goals: Table E5

Method	Advantages	Draw-backs	Costs € / unit
Small vacuum drier & Bag-In Tank or Membrane-In Tank Type	Low operational costs oxygen pemanently under 1000 ppm	No or difficult check of degassing process Dangerous high vacuum stripping of light fractions of oil	Min. 45 000.- max. 60 000
Vacuum separator VS-04 & Bag-In Tank or Membrane-In Tank Type	Low operational costs Remote control of degassing process No high vacuum stripping of light fractions of oil oxygen permanently under 1000 ppm	Higher price	ca 65 000.-
Small vacuum drier & TRAFOSEAL	Low operational costs oxygen permanently under 1000 ppm	No or difficult check of degassing process Dangerous high vacuum stripping of light fractions of oil	Ca 25 - 30 000.-
Vacuum separator VS-04 & TRAFOSEAL	Low operational costs Remote control of degassing process No high vacuum stripping of light fractions of oil oxygen permanently under 1000 ppm -		ca 35 000.-

Read next: 4. Costs of the transformer dehydration

7.1 Introduction

The replacement of an old transformer mostly represents only troubles. The direct (purchase price) undirect costs are relatively very high and the motivation to act is low, because the planned replacement is virtually fruitless. A new unit makes mostly the same job and its efficiency is usually only a bit higher than the old one (the iron and copper loss is usually only a bit lower) - therefore operational costs of a transformer cannot be markedly reduced.

Generally there exist three basic strategies for the replacement of an old transformer:

• A NamePlate Strategy (NPS)- the transformer is replaced immediately after the expiration or its nameplate age
• a Don't Care Strategy (DCS)- a careless manager accepts growing hazard conditions, makes nothing and hopes the best
• a Flexible Response Strategy (FRS) - the manager analyses all existing conditions and risks and flexibly optimizes the approach by the comparising the existing and potential hazard levels versus savings / costs relation

The example of the economical effect of the different strategies is shown as a RTS (Relative Total Saving) in the following picture:

Fig. 1 general comparison of economic effects of different strategies

7.2 NPS - NamePlate Strategy

The origin of the X-scale (Nameplate Point) is equivalent to the nameplate age of the transformer. Nameplate Point (t = 0) means the replacement of the old transformer and the installation of the new unit according to the nameplate strategy.

Let us suppose that the condition of our transformer is in a relatively good shape, no extremely deterioration symptoms are evident and therefore no special treatment is necessary.

The blue curve then shows a negative Relative Total Saving (1) in a time-period Nameplate Point - Decision Point

(1) RTS = -1 * [((1+i)n-1)-((ΔNLL + ΔFLL)*PP)/TRC)*((1 + i)n-1)/i]

• RTS ... Relative Total Saving = Total Saving / TRC
• TRC ... Total Replacement Cost = Purchace price (PP)
  • + removal cost of old transformer
  • + local conversion cost
  • + general logistic cost
• Δ NLL ... cost decrement is equivalent to reduction of No-Load (iron) Losses as a % of Purchace price (PP) per year ^*
• Δ FLL ... cost decrement is equivalent to reduction of Full Load (copper)Losses as a % of Purchace price (PP) per year ^*
• I ... Interest rate
• n ... years

Fig. 1 shows a RTS = RTS (t) relation for: TRS = PP, i = 0.15 , n = 3 ( time- period Nameplate Point - Decision Point), ΔNLL = 0.006 PP, Δ FLL = 0.02 PP

^* See Transformer Life-Cycle Cost
(http://64.90.169.191/applications/electrical/energy/trans_life_cycle.html)

In our example NPS means minimal loss 0.42 TRS for a time-period Nameplate Point - Decision Point

The new transformer is installed even though the old one has been working well. Strictly from the point of the cash flow, the responsible manager is not-our-company-man but a transformer-producer-man. A transformer producer benefits from the company potential saving achieved by the postponed purchase of a new unit.

7.3 DCS - Don't Care Strategy

Let us suppose that after exceeding the Nameplate age, aging symptoms of the transformer gradually increase and during the following 3 years the monitoring has shown an increased deterioration of the transformer - we should take measures. See Decision Point (Fig. 1).

The magenta curve then shows a positive Relative Total Saving (2) in a time-period Decision Point - Failure Point.

(2) RTS = ((1+i)n-1)-((ΔNLL + ΔFLL)*PP)/TRC)*((1 + i)n-1)/i

the equation is the same as a equation (1) only the sign is opposite.

DCS strategy is simple but stupid - the DCS manager does nothing and lets the transformer operate without any adequate maintenance until its failure - See Failure Point.

The owner then faces substantial replacing or refurbishing costs:

• minimum cost of a transformer failure can be as five times the cost of the new unit
• cost of a non-catastrophic failure can be as high as twenty times the price of the unit
• cost of all collateral effects of a catastrophic failure can exceed hundred times the cost of the unit

The DCS manager has ponentialy saved 0.42 TRC from the Nameplate Point to the Decision point and another ca 0.4 TRC from the Decision Point to the Failure Point, but

the potential loss induced by the failure is always much higher than any potential savings

The DCS manager not-our-company but an insurance-man. In the event of a non- or catastrophic failure the insurance company will succesfully argue that the basic security criteria had been systematically neglected and refuse the indemnification.

It is evident that the both mentioned strategies (NPS) and (DCS) are principially wrong and expensive.

The responsible manager is always a FRS manager as clearly shows the Fig.1

7.4 FRS - Flexible Response Strategy

The Relative Total Saving (RTS) corresponding to FRS can be expressed as follows:

(3) RTS = (1-LEC/TRC) * ((1+i)n-1)-((OLEC+(ΔNLL + ΔFLL)*PP)/TRC)*((1 + i)n-1)/i - LEC/TRC

where:

• LEC ... Purchase price of all used Life-Extending Technologies
• OLEC ... Operational cost of Life-Extending Technology

Fig. 1 shows that a well selected and properly operated life-extending technology ( yellow curve) can safely prolong the transformer life-span for another 7 years or more.

FRS potentialy saves 0.4 TRC from the Nameplate Point to Decision point, then the life-extending technology is bought and another ca. 0.9 TRC from the Decision Point to an expected Replacement Point can be saved but
in contrast to DCS

under determined, secured and fully controlled operational conditions.

(FRS) strategy shows potentially the highest gain, but is more sophisticated.

At first the manager must collect all neccessary information, not only about the given transformer, but also about all available life-extending methods.

For him it is most difficult to decide what life-extending technology should be used and what potential advatages could be achieved. The problem is that the manager is usually not a real professional in the life-extending area. The replacement of the old transformers is usually an occasional job for him, but

the optimized FRS conception always means a tailor-made solution for a specific transformer.

Therefore SINDRET (Savings INduced by Deferred REplacement of a Transformer) provides the first help for a FRS manager.