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Technical Goals of Transformer Maintenance

1. Introduction - IEEE Study

A study by the IEEE, of 3.787 transformers, with an accumulated 37.692 years of operation, concluded that:

"The insulation is the origin of the majority of the transformer defaults"

Extract from a conclusive study and report, titled Transformer Diagnosis: Part 1.
A Statistical Justification of Preventative Maintenance:
http://www.electricityforum.com/et/june99/trans.html
Presented by Mr. Michael Belanger, of Seidel Inc. in 1999.

The first and most important step of the identification of internal problems of the power transformer is a precise and relevant diagnostic of the actual state of its oil-cellulose system.

Unfortunately it is not commonly recognized that only consequent and complex monitoring only of the content of the aging products in the oil - as water, acids and gases (and of course particles) help avoid big troubles and unnecessary costs.

Therefore universal advice for any user is:

Keep your transformer clean.

2. Moisture Problem

The presence of moisture in the transformer, no matter how small, potentially decreases the dielectric strength of the oil and degrades and permanently damages the solid insulation materials.

A transformer when manufactured, has a cellulose water content of about 0.5% or lower. After the installation and commissioning, this level has usually increased about 1.0%. When the transformer is energised, the production of water in the insulation materials is a natural and inevitable occurrence.

The ageing of oil cellulose insulating systems is always accompanied by the formation of water as a product of decomposition from the hydrocarbons contained in these systems. High temperatures and oxygen content in the oil promote and accelerate this process. Depending on the conservator system used, atmospheric air may act as a source of oxygen and as a second source of moisture.

Once water has formed, it acts not only as an agent reducing the dielectric (decreasing an operational reliability of the transformer), but as a catalyst which accelerates the ageing process and subsequently and inevitably reduces the life-span (long-term reliability) of the given unit.

2.1 Moisture Diagnostic & Evaluation

Motto: In the solid insulants is always deponed more than 95% of the water contained in the transformer

The accurate recording and managing of the water content in the transformer's solid insulation Qp (weight %), the tracking and limiting of the impact on the aging rate of the paper, and the maintaining the desired dielectric strength of oil Ud (kV/2.5mm) at maximum process temperatures, have never been cost effective and easy to achieve. However, it is one of the most pro-active, life extending, and cost reducing preventative strategies available to a transformer manager.

One oil sample a year collected in a glass bottle or syringe, processed in a lab, with a high degree of variability due to the process and lack of controls, does not provide the degree of data and accuracy necessary for the competent failure risk management and for managing the appropriate insulation treatment program of the transformer.

The ARS solution and answer to the problem is Tx-Multiscan on-line measuring method..

2.2 Tx-MULTISCAN

Tx-Multiscan is a small portable diagnostic system, which can be easily and quickly connected to the oil sampling points of any transformer. Then, Tx-Multiscan automatically tracks the desired system temperatures and water content in the oil, and can provide the first data snapshot already within 30-40 minutes.

However, for an accurate diagnosis of the moisture contamination of the insulation system it is necessary to carry out the precise evaluation of the transformer equilibrium condition - the water movement between the oil and the paper during the measurement must be minimized or stopped.

Therefore, the first and basic question to be asked after the measurement is: are the adequate equilibrium conditions (relatively) constant temperature TTS and water content in oil Qw in the transformer reached or not.

This basic evaluation and subsequent advanced diagnostic evaluation of water/dielectric problem of given transformer is then made by a lap-top connected by cable to the Service Unit of Tx-Multiscan.

2.3 The comparison of off/ on-line dehydration methods.

TThe common, simple and plausible determination what method is technically and economically better is in practice very difficult - See Economical Goals, Table E1 .

The off-line aproach is ideal for new or slightly aged transformers and the emergency cases. For the aged and water heavy contaminated units these methods are mostly rated as risky, because the aged and therefore relatively brittle cellulose insulants can be easy damaged by the combined effect of high vacuum, lateral pressure gradients in the high density insulants, hydrodynamical forces etc.

The overdrying of the solid insulants cannot be completely controlled and the lost of the clamping forces is inevitably very high. Subsequently, the restoration of the clamping forces is neccesary and it means mostly in-situ dismantling of the transformer and relatively high additional costs. On the other hand the on-line drying is commonly rated as a very soft method and therefore ideal for aged transformers. The solid insulants can not be damaged and the dangerous overdrying can be easily controlled. However, these methods are rather time-consuming.

For more detailed analysis of advantages / drawbacks of existing dehydration (degasing) methods See Maintenance of wet power transformers

To conclude - for the transformers at the end of its life-span are on-line methods generally better and remains to select which of them is technically and economically the best for given transformer(s).

2.4 On-line (on-power) dewatering of power transformers

Two separate technologies are employed by ARS Altmann Systems, in their On-line Transformer Dehydration and Oil Conditioning Machines:

2.4.1 The Vacuum Separation

The principle of the VS technology as developed by ARS avoids high vacuum and temperature separation processes, because the long-term high vacuum and temperature treatment permanently deteriorates the transformer oil and changes its chemical composition.

The fundamental principle is as follows; oil is drawn from the transformer into the VS - 06 machine, the water and gasses are removed through the use of vacuum level only 3 - 10 kPa and temperature under 60 - 70 C.

The procedure removes only the water and partly attached acids and gases. Due internal cyclical process the natural inhibitors remain within the oil. Only the water and water-near components are frozen in the freezing chamber and then deposited in the water trap. At the outlet a fine filter bank is connected in series, in order to collect any possible particles of dirt.

The VS System therefore provides a permanent solution to ensure a defined sink for water, gas and particles and other harmful substances in the Oil Cellulose system. This is an active process, which ensures that only harmful substances get extracted, they cannot be fed back into the transformer.

The VS System can be put into permanent use with absolute confidence, as it cannot have any harmful effect on the oil in use, as ensured by the operation principle. Particular attention has been paid to the permanent use of the VS - 06 System, in that through inherent comprehensive safety features, any risk or damage to the transformer's operation will be totally avoided under any circumstances.

There is no risk involved in using the VS System in continuous duty, because its mode of operation is insufficient to distillate lighter fractions from the oil, or cause any damage.

2.4.2 The ADT Technology

The ADT System Series Technology is based on the absorption principle. The wet oil from the transformer is forced into ADT columns filled by molecular sieve 3A. Diluted water is bonded to molecular sieve and dry, filtered oil is forced back into the transformer. The drying on-line process continues till the whole absorption capacity of columns is exhausted (columns has then to be replaced) or the amount of water in the transformer is reduced to a requested level.

The ADT System acts as permanent sink for water and other water-near contaminants in the oil cellulose system. This is an active (one-way) method, which ensures that any contaminants which are extracted cannot find their way back into the transformer.

For the costs of the on- and off-line dehydration methods and their comparison see Economic Goals,

3. Acid Problem

Motto: In most cases the delicate transformer insulation is allowed to stew in a chemical cocktail of its own waste.

To extend the life of a transformer, the content of the aging products in its oil inventory must be very often strongly reduced, because the organic acids, alcohols, metallic soaps, aldehydes and ketones not only atack the insulants, but work as a catalysts and promote (with the diluted water and the oxygen) so-called avalanche aging effect resulting in the exponential increase of the failure probability density in the time.

For the decision what treatment process should be used, serves most often NN- value (Neutralization Number) of the oil.

NN (mgKOH/g oil) Treatment
<< 0.1 no treatment necessary
>= 0.1 the change or reclamation of oil inventory
> 0.3 in situ long-term reclamation of oil inventory combined with desludging
> 0.5 long-term reclamation of oil invetory combined with the high- powered desludging

Continually working at, or allowing the transformer to reach the upper limits of contamination, rather than maintaining absolute limits of all, considerably increases the cost of maintenance, the risk of failure, and dramatically shortens the life span.

For the cost of the "transformer detoxication" See Economical Goals, Table E6

4. Particles problem

There is a lot of a theoretical and experimental work about the sources, sizes and amount of particles and its efects on e.g. dielectric strength of oil.

From the practice point of view exists only two succesfull ways to keep the problem under control:

  • to minimize the sources of particles e.g. by the reducing of the intensity of the cellulose aging
  • continuous or semicontinuous filtration of the transformer oil inventory

5. Gas Problem

In the last time is there a new and growing industry awareness, that by maintaining consistently low oxygen level in the transformer oil inventory, the transformer will be considerably less likely to fail partially or fully, and the life-expectancy can be considerably extended.

The theoretical and experimental works explicitly show - See e.g. Lampe, Spicar: Oxygen-free transformer, reduced Ageing by continuous Degassing, CIGRE 1976, paper 12-05, that the aging process in the cellulose insulants can be successfuly suppressed by the permanent:

  • reduction of operational temperature of the transformer
  • reduction of oxygen content in its oil invetory

The permanent reduction of the average temperature of a transformer can be relatively difficult ( auxiliarry radiators, ONAN » OFAF change, …).

The present methods for the strong reduction of the oxygen ingress into transformer / reducing O2 content in the oil are:

  • passive - the oxygen ingress in the transformer oil inventory is only strongly retarded
  • active - the oxygen is forced out from the transformer oil inventory
  • the combined - the oxygen ingress in the oil inventory is retarded by the suitable passive system and simuntaneously is oxygen removed by active system from the oil inventory

As a comparison level is used - 100% aging intensity of given transformer = oil inventory fully saturated with oxygen (~ 22 000 ppm) & temperature over 50 C

Oxygen content in oil (ppm) Intensity of aging process (%) Treatment
4 - 7000 ca 30 active systems
ca 4000 10 - 20 passive systems
< 1000 < 5 combined systems

The costs, advantages and draw-backs of a various methods are described in Economical Goals , Table E3, Table E4 and Table E5.

Because of the installation cost of classical "hermetization of transformer e.g. Bag-In Tank or Membrane-In Tank " is relatively very high and the transformer must be long-term shut-down for up-grade - in fact the whole conservator must be changed, the ARS-Altmann Group invent a completely new method without above mentioned draw-backs.

5.1 TRAFOSEAL

The free breathing conservator provides a continuous supply of fresh oxygen and other atmospheric gases to the transformers main tank.

ARS Altmann Systems have completely and permanently removed the problem of oil and air contact, by redesigning the plumbing or physical connection between the conservator and the main tank.

As a "seal" the stratification layer is used, a natural and therefore long-lasting (and) indestructible membrane-like physical phenomenon, which is always spontaneously created in:

between the hot oil from the main tank and the cold oil from the conservator.

The thermal stratification layer, between the cold (gas contaminated) oil from the conservator and hot (protected) oil in the main tank, acts therefore as a very thin "membrane" which divides both liquids and prohibits its mixing.