Long term improvements of all power transformers insulating system characteristics

Continual maintenance of transformers insulation

There are two basic approaches in any oil system treatment:

» Off-Load which requires machinery shutdown and lasts for a certain period of time

» On-Load which imply to a certain continual treatment on the machinery that is in operation

In the world of hydraulic, lubricating, gear and other industrial oils on-load preventive methods of filtration and dehydration are well known and accepted as the best way to protect machinery, prolong oil lifetime, reduce maintenance and shutdown costs, increase MTBF and avoid unnecessary waste oil disposal procedures and costs related.

The goal in the maintenance of transformers is absolutely the same- to keep oil and cellulose permanently water and degradation products free. This way significantly longer transformer lifetime and safe operation are secured. At the same time costs of new oil purchasing and waste oil disposal are reduced.

Our approach is implementation of VIMAP continual preventive on-load oil and paper insulation revitalisation as the most efficient and most comprehensive treatment.

VIMAP technology enables this without oil heating and vacuuming. Complete process takes place at the ambient temperature and atmospheric pressure. This means that this is absolutely non-invasive treatment without any harmful consequences to the oil and cellulose.

There are some other very important reasons why continual online power transformer maintenance should be implemented.

It is well known that 95-99% of water is contained in cellulose while only 1-5% of water is in the oil.

The main goal is to remove water from cellulose.

Obviously it takes much more time for cellulose to dehydrate which means that technology applied must be such to be active for a longer period of time.

Many oil dehydration technologies, among which best known and most applied is vacuum-dehydration, reduce water content in the oil to 3-4 ppm within very short period of time and increase dielectric strength of the oil to above 250 kV/cm, even above 300 kV/cm.

But according to researches 50-70 days after off-line dehydration treatment is finished both water content in the oil (ppm) and dielectric strength of the oil (kV) reach the same values as before treatment.

Results are more or less the same after any short term oil dehydration.

Following charts are the best illustration of water in oil and corresponding dielectric strength development after short time oil dehydration.

Documentation \ Continual maintenance of transformers insulation

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According to law of equilibrium soon after the process is finished water and oxidation products will start to migrate from cellulose into the oil, thus annulling results of such treatment and making it worthless.

Most of technologies require oil heating to temperatures above 70°C in combination with oil vacuuming. This cause oil degradation and if implemented frequently or for a long time would significantly reduce oil lifetime.

Oil analysis and results evaluation often take place immediately after certain treatment is finished resulting in highly wrong and misleading conclusions. In such case both water content and dielectric strength values readings are very possible to be wrong.

It is known that oil dissolves more water at high temperatures then at the low ones. When oil temperature decreases, water will move to the cellulose indicating lower water content in oil. But it doesn’t mean that water content in transformer is lower, it only means that it is differently distributed.

As oil-cellulose insulation system tend to create state of equilibrium at any given temperature it is important to make all measurements at the same temperatures.

Only appropriate approach is to compare results of measurements at the same temperatures before treatment, after treatment is finished and in following months.

The very same mechanism applies to power transformers oil-cellulose system revitalozation.

As we explained due to Oil-Cellulose Insulatin System Aging water, acids, peroxides and sludge are created. They all consists of polar molecules and are distributed in transformer according to the equilibrium law. In the case they are removed from oil only, the moment treatment is finished they will migrate back to the oil, pollute it and trigger the oil oxidation process immediately.

VIMAP continual on-load oil-cellulose insulating system revitalization secure removal of all polar molecules from both oil and cellulose thus permanently keeping following parameters within recommended limits for new oil in new transformer (acc. to. IEC 60422, Table 3):

› Water in solid insulation

› Insulation resistance

› DDF- tgδ of solid insulation

› Dielectric strength

› Water content in oil

› Neutralization number

› Interfacial tension

› DDF- tgδ of oil

› Particles Content (ISO 4406)