Acid Number and Acid Formation in Transformer Fluids
Acid number is the measurement of the level of acidic compounds that are in transformer fluids. A sample of the transformer fluids is neutralized with potassium hydroxide solution (KOH), thus measuring the amount of acidic compounds in the fluid.
New fluids contain almost no acidic compounds but over time and external and internal conditions, these compounds are formed in the fluid. Accumulation of acids in the fluid lead to further degradation of the fluid, attack the metallic components of the unit, promote acceleration of cellulose degradation, and eventually the formation of sludges.
The acids are formed by oxidation of the fluids into organic acid structures. Oxygen itself is a di-radical and through complex processes the free radicals react with the fluids. The amounts of oxygen and moisture within the oil are facilitated by the heat provided by the load operation on the units.
The rate of acid formation is a direct response to the oxygen present in the fluid and the thermal stress during operation. The oxygen may come from several sources. Improper degassing during manufacture or commissioning can lead to a higher-than-normal content on the onset. Leaks from faulty seals will lead to an external intrusion of oxygen into the oil. This can be present in nitrogen blanket as well as conservator systems. While oxygen is the fuel, thermal operation profile is the fire that drives the process speed. High loading, improper internal cooling, and transformer design are sources of accelerated thermal constituent. Moisture content is another contributing factor to acid content as well as accelerating the degradation of the insulation paper by the acidic compounds.
Reducing the sources of oxygen, regulation of proper cooling, and moisture mitigation are major factors in reducing acid production. Oxidation inhibitors can be added to the fluids to further assist in deterring the formation of acids.
