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The majority of power transformers in operation today are filled with mineral oil. The primary function of the oil is to provide a high dielectric insulating material and an efficient coolant. The effectiveness of the oil as an insulating material is reduced as the moisture level increases, while cooling is reduced as the oil oxidizes. Paper insulation will also absorb moisture from the oil, thus increasing power factor readings.

The oxidation of transformer oil begins as soon as the transformer is energized. A chemical reaction occurs when the oil is exposed to a combination of heat, oxygen, and core and coil components. As the process of oxidation progresses, acids and polar compounds are formed and in turn become sludge. This sludge will then coat heat transfer surfaces on the core/coil and the tank/radiators, reducing the heat transfer capacity of the system. The operational temperatures are increased, thus accelerating the degradation of the oil.



· Oil Which Is In The Initial Stages Of Oxidization, Forming Acids And Polar Compounds. Some sludge deposits will be found in a small percentage of oils in this initial stage of oxidization (Acidity levels <.20mg KOH/g oil).

· Oil Which Has Advanced In The Oxidization Process To The Point Where Sludge Deposits Have Been Formed. This precipitating sludge coats all surfaces of the transformers tank and radiator walls, as well as the core and coil oil ways. In so doing, heat transfer is reduced causing the transformer to operate at higher than normal temperatures, which in turn speeds up the oxidation process (Acidity levels of .20mg KOH/g oil or greater).


· Through Absorption From The Atmosphere Above The Oil Level. Many transformer tanks are designed to seal the transformer from the outside atmosphere; however, top side leaks may develop that allow normal temperature changes to cause breathing . With each new inhalation comes more moisture to be potentially dissolved in the oil. Units designed as free breathing also can experience a build-up of dissolved moisture. In extreme cases, top cover leaks may be present which can allow rain to enter into the unit directly.

· Condensation Inside Transformers. The moisture is introduced by exposure to the atmosphere above the oil level. Sudden temperature changes can condense the moisture allowing it to run down the tank walls into the oil. There it will dissolve slowly.

· Oxidation Of Oil And Paper Insulation. Since oil and paper are organic compounds containing hydrogen, gradual oxidation will allow the formation of moisture. This can account for a major portion of the moisture in badly deteriorated oils.


TSI has found that production of moisture can become a problem if oil is allowed to deteriorate beyond an acidity level of .05mg KOH/g oil; therefore, we recommend treatment of oils that have reached this level. In cases where acidity levels do not require treatment, I.F.T.'s of less than 24 dynes/cm, dielectrics of less than 25Kv, and moisture contents above 30ppm signal the need for hot oil treatment. Units with primary voltages above 15Kv should have dielectric readings of 30Kv or above and moisture contents below 25ppm.


Correcting the problems of oil oxidation can be accomplished in several ways with varying degrees of success. Changing the oil will result in clean oil, but will do little to remove sludge adhering to the radiators, tank walls, and core and coil. Within a year of changing the oil, oxidation products not removed will be redissolved into the new oil resulting in acidity and polar compound levels appreciably above those of new oil. Subsequent oil changes may be required to remove these redissolved products of oxidation. Each time this is done, the transformer must be de-energized.

Another method is to filter press the oil. The only thing accomplished by filter pressing is the removal of solid particles that have been in suspension and free water. This process does not significantly change the acid or polar compound levels, or remove dissolved water. Oxidation and sludge formation will continue as soon as filtering is stopped. Very little is gained from this method.

A third method is to un-tank a unit, flush the tank, radiators, and core and coil with solvents, then refill the unit with new oil. This method can result in a successful stabilization of the oil, but there are several major drawbacks. The units must be de-energized and sent to a service shop. This means days or weeks without the use of the unit, plus expensive handling, transportation, and service charges.


TSI has developed, and introduced in 1952, an alternative procedure for restoring transformer insulating mineral oil, often while the units remain energized. It has come to be generally recognized as the most efficient and cost effective method for restoring transformer oil available today. The process starts by heating the oil to a maximum of 200°F. Processing begins when the oil reaches a minimum of 150°F. The heaters have the capability of 100°F temperature rise at the rate of 600 to 1200 GPH. From the heaters the oil enters the Fullers Earth towers, which contain a minimum of 1000 pounds of Fullers Earth. The Fullers Earth removes acid and polar compounds from the oil. Next, the oil enters the vacuum degassing chamber. Vacuum is maintained at a minimum of 28 inches. This part of the process removes dissolved moisture, air, and dissolved gases from the oil enabling a unit to be processed while energized.

TSI has developed procedures using this equipment to process transformer oils that have oxidized. The thrust of this program is that oil be exposed to operational components of the re-refiner at temperatures above 150°F long enough for the purification process to be completed. Pumping speed is of little importance. Under normal conditions TSI recommends flow rates between 400 and 900 GPH. Higher flow rates should be used with caution on energized equipment.


The treatment of oil in the initial stages of oxidization is called HOT OIL TREATMENT. Within this category there are two exposure time periods.

· Oils With Acid Levels Below .10mg KOH/g oil have an exposure time based on six (6) passes at a flow rate of 600 to 900 GPH.

· Oils With Acid Levels From .10 To .19mg KOH/g oil have an exposure time based on ten (10) passes at a flow rate of 600 to 900 GPH.


The treatment of oil in the advanced stages of oxidation is called D-SLUDGING, which is a two step treatment process. Oils with acid levels greater than .20mg KOH/g oil are exposed to 10 passes for Step 1, and six (6) to ten (10) passes for Step 2, at an average flow rate of 600 to 900 GPH. A time interval of at least six (6) months occurs between Steps 1 and 2. This time interval is referred to as the D-SLUDGING period. The clean oil from Step 1 redissolves decay products into the oil which are removed from the oil in Step 2. After treatment the oils will meet or exceed the following specifications:





34.0 dynes/cm MIN

.03mg KOH/g oil MAX

35Kv MIN

<15Kv - 30ppm >15Kv - 20ppm


Hot oil treatment through the vacuum degasser will remove moisture and gasses from the oil. Drying of the solid insulation is a multi pass process. The number of passes is a function of the actual moisture and gas content. Processing from ten to fifty passes is the average range. The higher passes are needed when the goal is to dry the solid (paper) insulation that has absorbed moisture from the oil.

During the final phase of HOT OIL TREATMENT and D-SLUDGING, oxidation inhibitor is added to the oil (.3% by weight) to replace natural inhibitors that have decayed.

Most transformer oil has some PCB contamination even though it may be very low. TSI insists on measuring that PCB level before oil treatment service. This insures that cross-contamination from previously processed units will not elevate the legal PCB "classification" of the treated unit to a more restrictive category, however, the process is likely to change the absolute PCB level. Usually the change is downward as we process from the lowest PCB concentration to the highest if that sequence is at all possible. Some customers may find that PCB levels after treatment are below the threshold (50 or 500 ppm) for the previous classification. A transformer such as this may be a candidate for "reclassification" to a lower classification once all EPA mandated criteria have been satisfied.

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