One of the most reliable indicators today to assess the paper insulation system of power transformers
in service, is the measurement of the concentration of furan compounds in the insulation oil. The
determination of the content of 2-furfuraldehyde, 2FAL, is done by chemical laboratory analysis on oil
samples taken from the transformers. Based on these results, the Belgian Transmission System
Operator categorises the assessed transformer into one out of four different “furan classes”. These
classes, solely based on the furan content of the oil sample, define the different actions and provisions
to be taken in order to improve overall grid reliability.
The 2FAL content measurement is a standard analysis performed in the laboratory, but gives only an
indication of the life consumption of the transformer. Many factors influence the formation of 2FAL
and its correlation with the average degree of polymerisation (DP) depends on several transformer
characteristics. It is therefore unusual to take direct replacement actions only based on these
measurements when the 2FAL concentration is too high. Instead, the furan classification divides the
power transformer assets into different levels of risk of failure related to the insulation system
For each class, a specific set of curative actions and measures are defined. The classes are numbered 1
to 4, with class 1 the class with the lowest concentrations. As most of the transformers reside in this
class, the only action to be taken is the follow-up of the 2FAL content on a regular basis. When the
transformer is categorised in one of the higher classes, the normal exploitation of the substation will be
altered to minimise the impact of the concerned power transformer on the grid. Other measures that
are to be taken can be the obligatory replacement and scrapping, the prohibition of transportation to
another substation, or the extension of the spare transformer assets.
The 2FAL content in the insulation oil of all transformers are assessed in order to discover the units at
risk in the grid. Based on this classification, the Belgian Transmission System Operator changed with
success its transformer asset management: five years ago a strategic forecast was established and a
replacement program was initiated for the power transformers in the highest class. In addition, the
classification served also as one of the steering factors to define a new spare transformer purchase
policy. Over the years, the number of critical transformers (class 3 and 4) has drastically decreased.
Today’s class definitions are based on a unique relationship between the 2FAL concentration and the
degree of polymerisation. Due to recent research in this field, the models used to make up the
classification will be enhanced with the influences of different transformer characteristics on the paper
degradation and furan formation process.