Online moisture management for transformers

The transformer's life expectancy is directly linked to the condition of solid insulation (cellulose-based paper).

byRajeev Shevgaonkar

The transformer’s life expectancy is directly linked to the condition of solid insulation (cellulose-based paper). Compared to normal kraft paper, thermally upgraded kraft (TUK) paper degrades slower.

The insulation life is defined by the degree of polymerization (DP), which represents the average length of polymer chains. In new insulation, the chains are long, and with aging these chains break down into smaller and smaller lengths. The longer the chain is, the higher the tensile / mechanical strength of the insulation. Breaking up of the polymer chains weakens the insulation, thereby making the transformer prone to failures. The DP value of the new insulation is 1,200, and it drops to around 200 at the end of its life.

The rate of aging of the insulation depends on the following factors:

    1. Load on the transformers – the higher the load, the insulation ages faster
    2. Operating temperature – the higher the temperature, the insulation ages faster
    3. Moisture in the transformer – the higher the moisture content in the solid insulation, the insulation ages faster. Every 1 % increase in moisture reduces life expectancy by a factor of 2 [3].

The moisture in the paper also creates bubbles at higher loads, causing gassing and tripping of the Buchholz relay. A dry transformer can be loaded to the desired levels.

The dielectric properties improve with low moisture levels, and the transformer can better withstand electrical stresses.

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