Degradation of Cellulosic Insulation in OilFilled Electrical Equipment
In oil-filled electrical equipment like power transformers, the solid insulation is
typically made of cellulose-based materials—for example, kraft paper or pressboard.
These materials provide electrical insulation and mechanical strength to the
windings.
However, over time and under operating conditions, especially when exposed to
heat, oxygen, and moisture, cellulose begins to degrade, and this degradation leads
to the formation of certain chemical by-products.
Cellulose is a long-chain polymer of glucose (a sugar). When it breaks down due to
thermal stress (heating), hydrolysis (moisture), and oxidation (oxygen), it undergoes
several chemical reactions.
The degradation process releases:
Sugars
These are intermediate products from the breakdown of the cellulose chain (glucose
units).
They are not stable and further decompose into smaller compounds.
Furan Derivatives
Formed when the sugars undergo dehydration and cyclization reactions (especially
under heat).
The most important furanic compound is 2-Furfural (2-FAL).
Others include:
5-Hydroxymethylfurfural (5-HMF)
2-Acetylfuran (2-ACF)
5-Methylfurfural (5-MF)
Furfuryl alcohol (FOL)
These furanic compounds are:
Stable in oil
Do not evaporate or degrade easily
Can be measured precisely, even at low concentrations (ppb range)
This makes them excellent indicators (or "markers") for the condition of the paper
insulation.
Why This Matters
Monitoring furan derivatives in the transformer oil gives insights into:
The extent of paper insulation aging
The thermal history of the transformer
When maintenance or replacement might be needed
Summary of the Degradation Chain:
Cellulose (solid insulation) → Glucose (sugars) → Furan Derivatives (2-FAL, 5-HMF,
etc.)
These furanic compounds dissolve into the insulating oil, where they can be sampled
and analyzed—usually using HPLC (High-Performance Liquid Chromatography).