Page 53 - Energize August 2022

TECHNICAL

Furan measurements have gained popularity in the past 20 Acids (oxidation by-products)
years because they offer measurements of specific chemical Various organic acidic compounds are also released as by-products
compounds such as 2-FAL, which can be directly correlated of oxidation. These acids in turn also attack the cellulose. In
with the ageing of the transformer’s paper insulation. Several particular, the degradation of insulation due to acidic reactions has
equations describing a degree of polymerisation (DP) as a the consequence of producing sludge. As this sludge is produced, it
function of log 10 (2-FAL) for kraft paper have been developed, could be deposited in areas critical for the cooling processes of the
and it is shown that accurate DP estimation is not always coils, such as cooling ducts, that would block the free circulation
possible. It is advisable not to apply any Furan-DP correlation of oil, which in turn increases the temperature and accelerates the
without proper analysis directly. Additionally, certain actions may whole ageing cycle once again. As we can see, there are a variety of
affect the Furan concentration in the oil, such as oil reclamation mechanisms that cause degradation of the insulation system which
and oil treatment, among others. effectively “ages” the transformer.

Comparing various models in predicting DP from
2-FAL

Figure 3: Electron microscope picture of paper aged in oil (Acid value 0,15
mg/kg) Courtesy of SD Myers

Figure 2: Comparing Furan-DP models [Courtesy of SD Myers] Water (hydrolysis)
The presence of water will increase the rate of cellulose
degradation. At the beginning of a transformer’s life, the Kraft
Ageing rates of insulation paper insulation contains less than 0,5% water, and the mineral oil is
There are three main processes of degradation: Hydrolysis, also dried. The water content levels within the transformer (in
Oxidation, Pyrolysis the paper insulation) may increase up to 5% during its lifetime.
In a real transformer all these processes – hydrolysis, The rate of degradation of the paper, starting from an initial water
oxidation and pyrolysis act simultaneously, resulting in a non- content value of 4%, was 20 times greater than that at 0,5%. So, in
linear Arrhenius plot. principle as the transformer ages, the rate at which the insulation
deteriorates is expected to increase.
Temperature (pyrolysis)
Ageing rates increase with operating temperature. The ageing of
cellulose can be described by the equation:

This equation describes how the DP-value decreases from the start
value (DP 0) to a value DP t after a time t by a so-called Arrhenius
relation.

Oxidation
In a similar way to hydrolysis, oxygen is a highly reactive element
which causes the breakage of cellulose bonds to form by-products
such as water, carbon monoxide and carbon dioxide. These three
mechanisms, oxidation, hydrolysis and pyrolysis normally do not
act in isolation but rather as a group of reactions that reinforce
each other. Figure 4 - Influence of (Hydrolysis) water in oil on ageing of Kraft paper

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