The acid value (or acid number) of transformer oil is a critical parameter indicating the level of acidic contaminants formed due to oil degradation (oxidation). Here's a detailed breakdown:
What it Measures:
The concentration of acidic components (primarily organic acids, but also inorganic acids or oxidation by-products) in the oil.
Expressed in milligrams of Potassium Hydroxide (KOH) required to neutralize the acids in one gram of oil (mg KOH/g).
Why it Matters:
Corrosion: Acids attack cellulose insulation (paper, pressboard) and metal components (windings, tank), accelerating aging and potentially causing failures.
Sludge Formation: Acids promote the formation of insoluble sludge, which reduces heat transfer efficiency (leading to overheating) and blocks oil ducts.
Dielectric Strength: While not a direct 1-to-1 relationship, severe acid formation is linked to overall oil degradation, which can eventually compromise insulating properties.
Indicator of Oxidation: It's a primary indicator of the oil's oxidation state and overall aging.
Acceptable Levels:
New Oil: Typically very low, often < 0.01 mg KOH/g (per standards like IEC 60296 or ASTM D3487).
In-Service Oil: Maximum acceptable limits are defined by standards and utility practices:
Common Action Limit: 0.1 to 0.3 mg KOH/g is widely used as the threshold for corrective action (like reclamation or replacement). Consulting the transformer manufacturer's recommendations and relevant standards (like IEEE C57.106, IEC 60422) is crucial.
General Guidance:
Below 0.1 mg KOH/g: Generally acceptable.
0.1 - 0.3 mg KOH/g: Indicates active degradation. Investigation and planning for corrective action (oil processing/replacement) are needed.
Above 0.3 mg KOH/g: Strongly indicates significant degradation. Oil reclamation or replacement is usually required urgently to prevent damage and failure. Sludge formation becomes very likely.
Testing Standards:
ASTM D974: Standard Test Method for Acid and Base Number by Color-Indicator Titration (common, but less precise for darker oils).
ASTM D664: Standard Test Method for Acid Number of Petroleum Products by Potentiometric Titration (more precise, especially for darker/used oils, detects weaker acids).
IEC 62021-1: Insulating liquids - Determination of acidity - Part 1: Automatic potentiometric titration.
Relationship to Other Tests:
Acid value often correlates with Interfacial Tension (IFT). As acids increase (high acid number), IFT decreases. Both are strong indicators of oxidation and sludge potential.
Dielectric Dissipation Factor (DDF or Tan Delta) can also increase with acid formation.
In summary: The acid value is a vital indicator of transformer oil health. A rising acid number signals active oxidation and the formation of corrosive, sludge-forming acids. Maintaining the acid value well below 0.1 mg KOH/g is essential for reliable transformer operation and longevity. Regular testing and taking action when it approaches or exceeds 0.1-0.3 mg KOH/g are critical maintenance practices. Always refer to specific manufacturer guidelines and relevant industry standards for precise limits and actions.