Читать книгу Alternative Liquid Dielectrics for High Voltage Transformer Insulation Systems - Группа авторов - Страница 49

Total acid number (TAN) or neutralization value gives the measure of acidity in the sample. It indicates the number of milligrams of KOH required to neutralize the H+ ions present in 1 g of oil. The total acid number provides the purity of the dielectric oil. Stability toward oxidation of a liquid insulation can be analyzed by determining the neutralization value, DDF, and specific resistance. Natural esters undergo oxidation and decompose by the process of hydrolysis and generate a variety of by‐products like acids and alcohols. Some reports showed that the acid number of natural esters increased correspondingly with passing time [61]. The acidity for some of the oils ranged from 1 to 3 mg of KOH/g when stored for a period of three months [62]. According to ASTM D4625 (30 °C/50 weeks), the total acidity of Pongamia biodiesel rose up to 6 mg KOH/g. Studies have shown that Low Molecular Weight Acids (LMA) is higher than High Molecular Weight Acids (HMA) in MO, while HMA is higher than LMA in NEOs. Also, the total acid is much greater in NEO and hence the absolute quantity of LMA in a natural ester is higher than MO. This might be because that natural ester is much more polar and hence the interface between solid insulation and natural ester favors the stay of LMA in natural ester.