Introduction
Acidity in power transformers is the measure of the amount of acid by-products in electrical insulating fluids like transformer mineral oil. It is represented as an Acid Number which is measured in mg KOH/g. A high value usually indicates that the oil condition has deteriorated and is in the advanced stages of sludge formation.
How is Acid formed in a Power Transformer?
It is important to understand how acids form in the transformer oil. The internal environment of the power transformer is very complex with the different materials operating under chemical, mechanical, electrical, magnetic, and thermal stress. These may be caused by the following processes:
Oxidation: Oxidation is a chemical reaction between oil molecules with oxygen and usually results when transformer oil is exposed to air (oxygen) and high temperatures, especially in the presence of electrical discharges. This oxidation process generates by-products, including organic acids, which contribute to the acidity of the oil.
Hydrolysis: Hydrolysis is the process involving the breakdown of oil molecules in the presence of water (moisture) which usually enters the transformer through various means, such as leaks, faulty gaskets, inadequate sealing, or being formed internally from chemical reactions. When water interacts with the oil, hydrolysis reactions can produce acidic by-products.
Corrosive Sulfur Compounds: The presence of sulfur-containing compounds in the oil, often originating from the degradation of insulating materials, can lead to the formation of corrosive sulfuric acid which is a strong acid that can accelerate the deterioration of transformer materials.
Microbial Action: It is also found that in some cases, microbial contamination of transformer oil can contribute to the production of organic acids through biological processes. Microorganisms thrive in the presence of water and nutrients leading to the formation of acids as metabolic by-products.
Contaminants: Contaminants, such as dust, dirt, and other impurities, can introduce elements that promote chemical reactions leading to acidity in the oil.
It is noted that the formation of acids is part of the natural process of aging and degradation of transformer oil over time.
The Effect of Acids on Transformer Health
The presence of acids in transformers can significantly compromise their health, particularly when it results in the formation of sludge. This sludge tends to build up in critical areas, including components of the cooling system, windings, the bottom of the tank, and cooling ducts within the core and windings.
These effects can impact both the insulating oil and the solid insulation materials within the transformer and high levels may cause damage to the insulation paper with a direct correlation between the level of acidity in the oil and the reduction in the tensile strength of the paper insulation. Some of the key effects of acids on power transformer health are:
Deterioration of Insulation: Acidic compounds can expedite the deterioration of solid insulation materials within a transformer, including materials like paper and pressboard. This degradation diminishes the dielectric strength of the insulation, jeopardizing the transformer’s capability to endure electrical stresses. Ultimately, this process may result in breakdowns and failures of the insulation over time.
Reduced Dielectric Strength: The presence of acids in transformer oil has the potential to diminish the dielectric strength of the oil. The dielectric strength plays a vital role in upholding the insulation properties of the oil. A decline in dielectric strength heightens the susceptibility to electrical breakdowns and arcing within the transformer.
Corrosion: Acids such as corrosive sulfuric acid can cause corrosion of metallic components such as the windings and core thereby weakening the structural integrity of these components, leading to mechanical failures, and reducing the overall lifespan of the transformer.
Increased Heat Generation: Acidic by-products may result in the formation of sludge in the transformer oil which can accumulate in various parts of the transformer clogging of cooling channels and hindering proper oil circulation causing inefficient heat dissipation. This causes increased heat generation within the transformer thereby further accelerating the degradation of both the oil and solid insulation materials.
The detrimental impact of acids leading to sludge formation underscores the importance of proactive monitoring, timely maintenance, and appropriate corrective actions to preserve the health and longevity of power transformers.
How is Acidity Tested?
ASTM D974 – Standard Test Method for Acid and Base Number by Color-Indicator Titration – outlines the procedures for determining the acid number (AN) and base number (BN) of petroleum products and lubricants. The acid number is a measure of the quantity of acidic substances in a sample whereas the base number is the measure of the alkalinity of a sample.
ASTM D664 – Standard Test Method for Acid Number of Petroleum Products by Potentiometric Titration – outlines the procedures for determining the acid number (AN) of petroleum products using potentiometric titration.
The presence of acid in insulating oil is identified using potassium hydroxide (KOH), an agent known for neutralizing acids, hence the term “Neutralization value.” The neutralization number may be used as a general guide for determining when oil should be processed or replaced.
How to Interpret Acidity in Power Transformers?
The acidity of insulating oil is typically assessed by the amount of KOH needed in milligrams to completely neutralize the acidity present in a specific weight of the oil. For instance, an acidity of 0.14 mg KOH/g indicates that 0.14 milligrams of KOH are necessary to neutralize each gram of the insulating oil.
Acidity levels should not exceed 0.3 mg KOH/g, with action being necessary at 0.14 mg KOH/g. For larger units, it is advisable to initiate action when the acidity levels reach 0.07 mg KOH/g.
The Acidity levels can be used in a health index as applied in the Transformer Age Index Model to assess the health of transformer insulating oil.
How to Improve Acidity Levels of Transformer Oil?
Routine monitoring, preventative maintenance practices, and testing of transformer oil, which includes acidity assessments, are essential for identifying acid presence and evaluating the transformer’s condition.
If elevated acidity levels are identified, corrective measures such as oil purification or replacement may be required to mitigate adverse effects and uphold the transformer’s reliability. Regeneration can be conducted continuously while the transformer is in operation or intermittently using mobile regeneration units.
Conclusion
In summary, the presence of elevated acidity levels in transformer insulating oil can compromise its dielectric properties, leading to decreased resistivity and an increased dissipation factor, ultimately impairing the insulation performance of the oil in electrical systems.
Ensuring appropriate transformer design, effective sealing to prevent moisture and oxygen penetration, and promptly addressing factors that could cause excessive heat or electrical stress can reduce acid formation in transformer mineral oil. Regular maintenance, including oil purification and replacement as needed, is vital for prolonging transformer lifespan and preserving reliability.
References
- IEEE Std C57.106, “IEEE Guide for Acceptance and Maintenance of Insulating Mineral Oil in Electrical Equipment,” DOI: 10.1109/IEEESTD.2016.7442048
- ASTM D974, “Standard test method for acid and base number by color-indicator titration”, ASTM, DOI: 10.1520/D0974-22
- ASTM D664, Standard Test Method for Acid Number of Petroleum Products by Potentiometric Titration,” ASTM, DOI: 10.1520/D0664-18E01