Long term improvements of all power transformers insulating system characteristics

Water in power transformers

There are three sources of water in power transformers:

» water left from production

» moisture absorbed through unmaintained breathers and gaskets

» water  created by cellulose depolymerization, i.e. thermal degradation

When a new transformer leaves the factory, the insulation has a moisture content of up to 0.5% of the dry weight of the insulation.

Cellulose insulation starts to degrade from the very beginning due to the presence of remained water from production, oxygen and temperature.

Oil also starts to degrade due the temperature, presence of metals, water and oxygen.

By time, acids, peroxides, free radicals and other oxidation products are formed in oil-paper insulating system.

Resin is also created as a result of oil oxidation. It is oil-soluble polar substance which in the case of continuous oxidation turns into insoluble deposit- sludge which is ultimate product of oil deterioration.

These compounds cause cellulose depolymerization which further leads to creation of more water and more oxidation products.

Result is that most of the water in power transformers is chemically created in solid insulation by degradation of cellulose, i.e. cellulose depolymerization. According to the analysis 95-99% of water is contained in cellulose while only 1-5% of water is in the oil.

It is known that oil dissolves more water at high temperatures then at the low ones. When oil temperature decrease water will move to the cellulose.

Oil-cellulose insulating system tends to create state of equilibrium and therefore water will be divided between oil and cellulose in a definite ratio at any given temperature.

Following chart explains correlation between water content in oil and cellulose depending on the oil temperature:

Documentation \ Water in power transformers

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0            10          20         30        40           50             60       70         80          90       100

Water in Oil

Oil/Cellulose Humidity Correlation at Different  Oil Temperatures

Water in oil

Water in solid

insulation

350

300

250

200

150

100

50

0

About 99% of water is stored in solid insulation

Example:

Power transformer 300 MVA at 50°C

60 t of oil with 20 ppm (µg/kg) water content: => 1,2 lit

10 t of cellulose with 3% water content:          => 300 lit