Cellulose degradation mainly involves the oxidation, hydrolysis or fracture of polymer chains under the influence of heat.
The overall life expectancy of power transformer depends largely on the ability of insulating fiber to withstand the mechanical stress generated by fault conditions. The resistance of this kind of solid insulation will decrease with the increase of operation time.
The aging of power transformer insulation board is usually attributed to thermal degradation. In thermal degradation, the average length of cellulose polymer chain is shortened due to fracture, and the fracture frequency is higher with the increase of temperature. The shortening of the cellulose polymer chain results in the decrease of the physical properties of the pressboard, which leads to the decrease of its tensile strength, elongation before tensile failure, tearing strength, breaking or bursting strength.
In the daily patrol inspection of distribution transformer in operation, it is often found that the oil level of transformer in some transformers is not within the specified range.
When the oil in the transformer drops below the transformer shell, the contact surface between the transformer oil and air increases, the oil is easy to absorb moisture, oxidize and deteriorate, reduce the withstand voltage strength of the transformer oil, and damage the insulation performance of the coil.
When the oil shortage is serious, the insulation of the conductive part of the transformer to the ground or between phases will be reduced, which will cause the breakdown and discharge between phases or to the ground. The continuous use will make the oil temperature rise sharply, shorten the service life of the transformer and even burn down the transformer.
Therefore, in order to ensure the normal and safe operation of the transformer, it is necessary to find out the cause in time and refuel the transformer