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Technical Goals of Transformer Maintenance

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,