Optical sensor for determining the degree of polymerization of the insulation paper inside transformers
The transformer represents one of the most important assets in the electrical power network so that damage to the asset can have a significant impact on the power supply; therefore, regular condition monitoring, as well as assessment of the transformer, is of great importance . Condition monitoring and evaluation can cover various areas concerning a transformer, whereby the most important field is the condition of the insulation .
The aging condition of the insulation has a considerable influence on the life expectancy of the unit . The age-related decomposition of the mostly used oil-impregnated paper insulation considerably limits the life expectancy of a transformer since the mechanical strength of the paper is reduced over time and, as a result, can no longer withstand failures such as short circuits .
In power transformers, insulation paper is commonly based on the various forms of cellulose. Mechanical and electrical performance is depending on the degree of polymerization of cellulose, which is the average number of monomeric units (glucose rings) in the cellulose chain. New insulation paper has a high degree of polymerization with typical DP-values of 1000 or more. The DP-value of 200 is regarded empirically as a threshold value for safe operation and the end of life of the transformer .
Therefore, a permanent and accurate online monitoring of the degree of polymerization would be essential for effective lifetime management of a transformer fleet. The fact that in many countries, the transformer population has reached an average age of more than 30 years intensifies the question of the remaining life expectancy determination .
Accurate monitoring of DP-values over time, especially at the hottest temperature spot at the coil windings, can reflect the status of the insulation quality. It has to be considered that the solid insulation system is the only part of a transformer that cannot be repaired in an economical way.