Most mineral oil dielectric fluids contain an added oxidation inhibitor which is a chemical additive that acts as a preservative. The purpose of the inhibitor is to prevent oxygen from reacting with the oil, thus slowing the aging rate of the oil (and also of the solid insulation). The two most common oxidation inhibitors used in transformer oils are 2,6-ditertiarybutyl para-cresol (DBPC) and 2,6-ditertiary-butyl phenol (DBP). DBPC is also known as butylated hydroxytoluene (BHT).
SDMyers determines the total concentration of oxidation inhibitor (of DBPC and/or DBP) in transformer oil using a Fourier Transform Infrared (FTIR) spectrometer and following the ASTM method D 2668. We consider an inhibitor concentration of less than 0.1% (by weight) to be unacceptable, because in this range, the concentration of inhibitor is too low to adequately protect the oil from oxidation. When the inhibitor level drops to below 0.1%, we recommend that the oil be re-inhibited and inhibitor added to bring the concentration up to 0.3%.
In some cases, the transformer may be equipped with an oil preservation system designed to keep dissolved oxygen at a low level (below 1000 ppm). Such a system could be in the form of an active nitrogen system (a nitrogen tank or generator), or a conservator tank containing a rubber diaphragm (bladder). Using inhibited oil is not required in these situations; however, having inhibited oil will provide additional protection against oil oxidation if the oil preservation system were to ever fail.
Reinhibiting is done by hiring a contractor who is experienced in reinhibiting transformers. Once the contractor connects the oil processing equipment to the transformer, they will typically heat the oil and circulate it (also referred to as passes) through an adsorbent material such as Fuller’s Earth or activated alumina. These passes will remove old inhibitor, acid precursors (such as peroxides), low levels of acids, and other polar contaminants that may have formed. Removing these contaminants is necessary because the presence of contaminants in the oil and solid insulation will cause them to age and degrade much more rapidly.
Next, the contractor will typically add a concentrated solution of DBPC inhibitor into the oil stream that is returning to the transformer. This process must be monitored to assure that the moisture content of the oil containing the inhibitor mixture is below 10 ppm (parts per million). The final concentration of inhibitor needs to be 0.3% by weight. It should also be noted that if the acid number of the oil has risen to 0.06 mg KOH/g or higher (considered a questionable range) or if the IFT of the oil has dropped to 31.9 mN/m or less (also a questionable range), then the oil may need additional oil processing followed by reinhibiting. In this case, hot oil cleaning (oil processing through an adsorbent material) will first need to be performed to clean both the oil and the solid insulation in the transformer.
Typically, a transformer is de-energized when reinhibiting and hot oil cleaning are performed. These processes may be performed on an energized transformer, but it is extremely important to select a contractor who is experienced in energized reinhibiting or energized hot oil cleaning. This type of contractor must have the knowledge to properly evaluate the circumstances and determine whether energized processing can be done safely and with the proper equipment and expertise.