In last week’s article, we introduced hot oil cleaning. In this week’s article, we will carry this discussion further by relating how this maintenance activity extends the life of the equipment that has been hot oil cleaned. How oil ages and what effect this has on the solid insulation are our first concerns in this discussion.
There are three key facts that we need to consider. First, the most critical factor in establishing the cost effective life of a transformer is the mechanical strength of the solid insulation. Mechanical strength of the solid insulation decreases as the transformer continues to operate in service. The solid insulation eventually becomes so weak that it cannot continue to withstand the normal stresses of operation. At that point, the transformer and the insulation are at the end of effective life.
Second, clean and dry transformer oil protects the solid insulation from those conditions that cause it to age prematurely. Excess oxygen, moisture, and high operating temperatures are all conditions that will cause the solid insulation to break down. The dielectric fluid, properly installed and maintained, reduces the exposure of the solid insulation to oxygen and moisture and is also an effective heat transfer fluid allowing the solid insulation to be cooled appropriately during transformer operation.
Third, in addition to oxygen, moisture, and excess heat, there is one more critically important condition that will greatly accelerate the breakdown of the solid insulation – oil aging. As transformer oil ages and oxidizes, whole families of oxidation decay products form, and these polar compounds are very aggressive to the solid insulation. As these compounds start to build up in the solid insulation, they actually start to tear the fibers of the solid insulation apart.
Frank M. Clark of General Electric, one of the pioneers in the study of the aging of insulating materials, found a distinct relationship between the acid number of oil and the mechanical strength of paper when the oil/paper system was aged in the laboratory. The following graph shows the relationship that F. M. Clark established during these experiments:
In the 1980’s, S. D. Myers performed a similar series of experiments, aging the oil/paper system and examining the paper samples at various stages during the process. Comparing new paper in new oil to paper in the experiment where the oil and paper had been aged together to an oil acid number of 0.15 mg KOH/g shows clearly the damage being done to the paper. The following two pictures taken using a scanning electron microscope were taken at a magnification of 750X.
Insulating Kraft Paper in New Oil
Insulating Kraft Paper and Oil Aged to 0.15 mg KOH/g Acid Number
In the F. M Clark study, new paper in new oil had an initial tensile strength of about 17,000 psi. Aged to an acid number of 0.15 mg KOH/g, the tensile strength decreased to about 10,500 psi, a decrease of nearly 40%.
Oxidation decay products will continue to break down the solid insulation as long as they remain in contact with the paper. Hot oil cleaning, by recirculating clean hot oil through the windings, removes the oxidation decay products and arrests the aging process. Nothing can be done to “fix” the damage that has already been done, but you can stop the further decrease in the mechanical strength of the solid insulation.
So, done properly, hot oil cleaning is an oil maintenance procedure that can actually extend the life of the equipment by extending the insulation life. The critical factor is addressing the need for maintenance early enough so that insulation is not needlessly decreased before the maintenance procedure is performed.