EN
Acidity Test
Acidity (neutralisation number) determination in insulation oil: oxidation rate measurement and oil life assessment, reported via accredited methods.
The acidity test determines the amount of acidic compounds formed by oxidation in insulation oil as the neutralisation number (mg KOH/g). Transformer oil slowly oxidises during service under oxygen, temperature and metal catalyst effects, producing carboxylic and lactone type acids. As acidity rises, the oil's corrosivity, conductivity and sludge-forming potential increase — low breakdown strength and accelerated paper degradation are the next step. Acidity is thus the primary indicator of the oil's chemical age.
Method
- Titrimetric basis: The sample is dissolved in solvent (isopropanol-toluene mixture) and titrated with standard potassium hydroxide (KOH); the end point is found by colour indicator (bromothymol blue) or potentiometric reading, reported as the neutralisation number in mg KOH/g.
- Standard: IEC 62021-1 (potentiometric determination of acidity in oils) and equivalent national standards; sample quantity and solvent ratio fixed by the standard.
- Acceptance values (practical guide): Unused mineral oil ≤0.01-0.02 mg KOH/g; in service ≤0.1 trackable, 0.1-0.2 caution, >0.2 candidate for conditioning/replacement — manufacturer and user specifications set the firm limits.
- Monitoring frequency: With the annual oil panel; on inhibited oils, inhibitor depletion becomes visible as an acidity jump, so trend interpretation is critical.
Why It Matters
High acidity damages in three ways: attack on copper and iron surfaces (metallate soaps), acceleration of acid-catalysed depolymerisation of cellulosic paper, and polymerisation of oxidation products into sludge. Sludge blocks cooling channels producing hotspots; soaps deposit on insulation creating leakage paths. In other words acidity is the common variable of the transformer's thermal and dielectric life. High acidity combined with moisture is the most destructive combination for paper ageing.
Where It Is Manageable
Acidity growth is reversible: regeneration (Fuller's earth, molecular sieves, vacuum drying) removes acids and polar degradation products; inhibitor topping-up (e.g. DBPC) lowers oxidation rate again. Which intervention is economic is decided by evaluating acidity together with breakdown, moisture and furan results. In practice three tables are seen: fast rise in a young unit points to air ingress or wrong top-up oil; slow creep in an aged unit reflects normal oxidation and inhibitor depletion; a sudden jump raises arc or overheating suspicion and triggers immediate DGA. The test is part of the periodic plan under Law No. 6331 and the High Current Installations Regulation duties. Results connect to the accredited laboratory report; for the rest of the panel see our Insulation Oil Test, Dielectric Strength and Density pages, or contact us for a programme.
FAQ on Acidity Test
The total acid number (TAN, mgKOH/g) gives the amount of acidic degradation products in the oil; it rises with oxidation. Rising acidity shows the oil ageing and accumulating sludge and corrosive products. This weakens insulation, coats cooling surfaces and opens metals to corrosion; therefore, with moisture-breakdown-furan it is the chemical age indicator of the oil.
TAN is determined by potentiometric-titrimetric method per the relevant ASTM/IEC standard; the sample is prepared in clean-moderate-temperature conditions. The value is interpreted not alone but as a trend against past years and together with dielectric-water-particle-DGA. A sudden rise is linked to high working temperature, air contact (breathing-gasket) or depletion of the oxidation inhibitor.
The threshold is given per oil type and operating condition; a rise beyond the manufacturer-standard limit asks for oil regeneration (filtration-drying) or change. Where acidity is very high and sludge has accumulated, flushing-change may be needed. The aim is to intervene at the point where the acidity curve steepens (before catalytic degradation); if delayed, cellulose and metal damage become permanent.
High TAN with simultaneously rising moisture, falling breakdown strength, increasing furan and disturbed DGA gases shows an active fault-ageing combination and raises priority. Then oil treatment-cooling cleaning-recommissioning verification where needed is planned. In controls the gasket-breather-inhibitor state is also reviewed; preventive maintenance reduces air-chemical contact. The report presents the TAN trend with a removal suggestion.