Maintaining transformers is not a new idea, but funding for transformer maintenance services continues to decline. The accounting structure incentivizes the purchase of new transformers. Globally, demand for new transformers has lead times extending up to three years. Unless there are enough spare transformers in inventory, it only makes sense to have a robust maintenance program.
The financial pressure of the industry has also impacted transformer manufacturers to reduce costs. Unlike transformers manufactured fifty years ago, new transformers are computer-designed with minimal engineering margins. As a result, new transformers are designed with tight operating clearances. Any prolonged overloading or overheating conditions can dramatically reduce the lifespan of the transformer. Thus, it is important for transformer owners to monitor and test transformer oil on a regular basis. The testing frequency should be based on the oil condition, voltage rating & type of transformer. The question is, are you testing your transformer oil, and more importantly, what are you doing to prolong the life of your existing transformers?
Mineral Oil
Mineral oil is the lifeblood of a transformer and is by far the dominant fluid used in transformers. There are many reasons why mineral oil is used as an insulating fluid in transformers. One, it is an excellent dielectric medium for insulating the components within the transformer and two; it is a good heat transferring agent to dissipate the heat away from the windings to the tank walls and radiators. The radiators are designed to maximize the surface area to effectively cool the oil. The circulation of cool oil back through the windings is a continuous process. And third, it is still the cheapest fluid available for transformer applications.
But transformers will self-destruct over time. Heat and moisture combined with Oxygen will cause a transformer to fail. If the transformer is properly maintained, the destruction process can take many years. But if the transformer is neglected, the transformer aging process can accelerate, and the lifespan can be quickly shortened. This is because of the insulation system within the transformer. The insulation system of a transformer consists of two components: oil and cellulose paper. Although the combination of oil and paper offers an excellent dielectric medium for the transformer, it is the interaction of the oil and the paper that will ultimately destroy the transformer. The hydrocarbons in the oil will form oxidation byproducts that decompose the cellulose insulation system. These byproducts will increase the moisture in the transformer as well as create an environment that will overheat the transformer. In time, the formation of these byproducts can cause the failure of the transformer.
Insulation Analysis
In order to treat the “problem” within a transformer, it is important to understand how the “problem” comes to existence. Moisture can either enter the oil externally through condensation or internally through a chemical reaction.
Given a choice between air or transformer oil, oxygen will leave the air and dissolve into the transformer oil. Thus, a free-breathing transformer will begin to transfer oxygen from the air to the oil. Oxygen is also a problem for sealed transformer tanks that underwent a vacuum-filling procedure.
A transformer that is vacuum-filled will still contain small quantities of oxygen within the oil. This is a problem because oxygen, in the presence of a catalyst, such as copper or water, will chemically react with the hydrocarbons within the transformer oil. This chemical reaction forms byproducts that attack the paper insulation system of the transformer. The decomposition of the paper will release water and peroxide into the oil. Peroxide is the first oil decaying byproduct formed. Just as oxygen has a preference for oil, water and peroxide will leave the oil and adsorb into the paper. Thus, the cellulose paper has an affinity for the very byproducts that contribute towards its destruction. In addition, the adsorption of water and peroxide into the paper further contributes to the release of even more oxygen into the oil.
The problem becomes compounded and in time, creates other decaying byproducts such as acids, alcohols, and sludges. Initially, the paper will adsorb these decaying byproducts and in effect, act as a filter. Although the oil testing results will indicate no breakdown is occurring, in reality, the deterioration process has already begun—it’s just not detectable. Once the paper is saturated with acids, sludges and other harmful byproducts, these byproducts will precipitate out and contaminate the oil. In addition, the higher the operating temperature, the faster the oxidative breakdown occurs. So it is also important to limit or avoid overloading transformers for long periods of time—especially transformers with known poor oil qualities.
Unfortunately, these byproducts will destroy the insulation system and lead to the failure of the transformer. Thus, it is a cyclical “problem” that can never be stopped; it can only be slowed down. By treating the symptoms of the “problem”, you can prolong the lifespan of the transformer.
So, knowing that we can never eliminate the “problem” attacking the insulation system within the transformer, the only thing to do is to monitor the symptoms. This can be accomplished by analyzing the oil. By testing the oil, the results can give you a strong indication of what is occurring inside the substation transformer.
Oil Testing
Before you begin transformer oil testing, it is important to identify which transformers need to be tested, how often, and which tests are necessary for analyzing the oil. It is recommended that all substation class transformers have the following tests performed at least annually. Listed below is a summary of recommended oil tests that should be performed as well as a description of the test1. This list is by no means a complete list of all the tests that can be performed. Rather, it is a recommended list.
The decision is up to each customer to determine which tests should be conducted and analyzed. It is important that when sampling, the individual verifies that there is positive pressure on the pressure gauge so that no air bubbles enter the transformer. Otherwise, serious injury and failure of equipment may result.
The intention of this post is not to provide an in-depth chemical analysis of how each test can measure the chemical changes taking place within the oil and the insulation. For the purposes of this paper, the information provided about each test is to serve as a guide for the decision-maker.
Summary of Oil Tests and Their Objectives
| ASTM Test Method | Test Type | Test Significance |
| D-791 | Interfacial Tension | The IFT test measures the presence of soluble contaminants and oxidation products. A decreasing value indicates an increase in contaminants and or oxidation products within the oil. |
| D-974 | Acid Number | The acid neutralization number is a measure of the amount of acid materials present in the oil. As the transformer ages, the oil will oxidize and increase in acidity. The acid value can also increase from contamination of other foreign material such as paint, varnish, etc… |
| D-924 | Power Factor | The power factor test measures the dielectric losses of the oil, or energy that is dissipated as heat. A low value indicates low losses. It is a useful test for measuring changes within the oil resulting from contamination or deterioration. |
| D-1816 | Dielectric Breakdown | The dielectric test measures the ability of the oil to withstand electric stress without failure. The higher the value, the lower the presence of contaminants such as water, dirt, or other conductive particles. |
| D 3612 | Dissolved Gas Analysis | The DGA test measures various gas ppm levels that are present. Different gases will dissolve in the oil which indicate various types of thermal and electrical stress occurring within the transformer. |
| D-1500 | Oil Color | The color test is a simple test that indicates oil quality. The higher the color number is, the higher the probability of contamination or deterioration of the oil. |
| EPA 8082 | PCB | Test the PCB ppm level of oil. |
Oil Analysis
Although it is important to test the oil in transformers, it is more important to know how to interpret the data from the results. No one test can be used independently to determine the oil condition.
Rather, all of the results should be reviewed simultaneously to give a full understanding about what is occurring in both the oil and the transformer. This will allow the customer to review the options and make a decision as to how to treat the oil. The IEEE Guide for the Reclamation of Insulating Oil and Criteria for Its Use (IEEE Std 637-2015)2 has the following group classifications for oil evaluation.
Group I: Oils that required only reconditioning for further service.
Group II: Oil in poor condition. Such oil should be reclaimed or disposed of depending upon economic considerations.
The difference between reconditioned and reclaimed oil will be discussed later. Listed below is a table with the recommended threshold levels as defined in C57.106-2015 for the oil to remain satisfactory.
Suggested Limits for In-Service Oils Group I by Voltage Class
| Test | Voltage <69kV | Voltage 69kV – 230kV | Voltage >230kV | ASTM Test Method |
| Dielectric Breakdown Voltage, 60Hz, 2 mm gap (min) | 40 | 47 | 50 | D-1816 |
| Neutralization Number, mg KOH/g (max) | 0.20 | 0.15 | 0.10 | D-974 |
| Interfacial Tension, mN/m (min) | 25 | 40 | 32 | D-971 |
| Water ppm (max) | 35 | 25 | 20 | D-1533 |
For oil that does not meet the recommended thresholds above, there are two options. One, the oil can be utilized in a lower voltage application, assuming it was utilized above a 69 kV application. Two, the oil can be reconditioned or reclaimed to meet the Group I classification requirements.
Although the acid test determines conditions under which sludge may form, it does not necessarily indicate that sludging conditions exist. The IFT test is a good indicator of the sludging characteristics of transformer oil because it correlates to the concentration of polar molecules in suspension and in the solution in the oil. Thus, the IFT test serves as an early warning to the beginning stages of deterioration.
It is important to not just consider these two tests as indicators as to whether or not oil treatment is necessary. Other oil quality results should be reviewed as well as dissolved gas analysis (DGA) results should also be considered.
Oil Maintenance
Now that we have developed an understanding of what is occurring inside the transformer, along with which tests to perform and how to interpret the results, now we need to understand the available options available to protect the transformer. Listed below are different options depending on the specific data for each transformer that needs to be processed. Obviously one of the most important considerations is cost. Because each scenario depends on the specific circumstances, this area can not be properly addressed in this paper; however, surprisingly enough, performing an energized hot oil reclamation process on substation transformers is typically the most cost-efficient option.
- Hot Oil Reclamation
- Alternative Options
Hot Oil Reclamation
The IEEE Guide for Acceptance and Maintenance of Insulating Oil in Equipment, (C57.637-2015) has the following definitions5:
Reclamation of Oil: The restoration of usefulness by the removal of contaminants and products of degradation such as polar, acidic, or colloidal materials from used electrical insulating liquids by chemical or adsorption means.
Reconditioning of Oil: The removal of insoluble contaminants, moisture, and dissolved gases from used, electrical insulating liquids by mechanical means.
From a technical position, these are different processes to treat different problems. The first is chemical and the second is mechanical. However, from a practical position, it is necessary to remove all types of contaminants that destroy the insulation characteristics of a transformer. Since the purpose of a hot oil reclamation process is to prolong the life of the transformer, a hot oil reclamation process must have the ability to perform both chemical and mechanical oil treatment. Specifically, the reclamation process must have the ability to remove:
- Water from the oil and the windings
- Contaminants from the oil
- Acids, alcohols, oxides and other oil-decaying byproducts
- Dissolved Gases resulting from overheating or electrical stress
- Sludge “layered” within the coil/core assembly as well as in the radiators
For on-site energized oil treatment, the Hot Oil Reclamation process begins mobilizing an oil processing rig to the site and connecting two sets of high-pressured hoses to the transformer. This will allow the oil to circulate the hot oil in a closed-loop path. The oil needs to be pulled from the bottom of the transformer and returned to the top.
There are five components to the removal of harmful contaminants within the oil. The five components are as follows:
- Heat
- Fuller’s Earth
- Vacuum Degassing/Dehydration Column
- Inhibitor
- Filter stages
Heat
Heat is accomplished by literally heating the oil when it enters the rig. Heating the oil has two purposes: one, to help remove water adsorbed in the paper insulation of the transformer; two, to dissolve the sludge layered in the transformer into a soluble form where it can be removed from the oil after entering the rig. This is why it is advantageous to perform the reclamation process on an energized transformer. The heat generated from the load will aid in transferring the moisture from the paper to the oil as well as dissolving the sludge buildup internal to the core and coil assembly. In addition, the mechanical vibration at 60 Hertz also aids in the circulation of the oil through the windings. This circulation is important to remove the sludge within the windings.
As the temperature increases in the transformer, the water will transfer from the paper to the oil. This will continue until the moisture reaches a new equilibrium between the oil and the insulation. As the oil circulates through the processing rig, the water is removed before re-entering the transformer. Below you will find a graph that compares water absorption between paper and oil. Note the higher the temperature, the higher the water ppm is in the oil, and the lower the moisture content is in the paper. Remember, water has an affinity for paper, but when you introduce heat, it transfers the moisture into the oil. This is important because water in the paper will decompose the insulation integrity4. It is important to understand that if the insulation is wet, the circulation of hot oil will not dry it out.
The second benefit of heat is at approximately 180° F, the oil reaches the aniline point and becomes a solvent for it’s own decaying products within the oil. As the hot oil circulates through the transformer, it begins to dissolve the sludge that has built up within the core and coil as well as the radiators. The insoluble sludge that is carried out by the oil can be removed via filtering stages within the rig, while the soluble sludge can be removed via Fuller’s Earth, which will be explained later. The amount of sludge accumulated in the transformer will affect how many circulation passes are required. Transformers in poor condition may require as many as 20 passes to de-sludge the core and coil assembly. A pass is defined as the total gallons processed through the rig divided by the nameplate gallon rating. Thus, if a transformer has 2,000 gallons, and the rig circulates 40,000 gallons, then 20 passes are performed on the oil.
Fuller’s Earth
Fuller’s Earth is a naturally mined adsorbent clay. Adsorption is the attraction of one substance to another substance. Fuller’s Earth will adsorb water, acids, sludge, alcohols and other oil-decaying products. Thus, as the oil passes through the Fuller’s Earth, the adsorption process will remove the dissolved acid, sludge and water from the oil. In addition, Fuller’s Earth will also adsorb microscopic particles and suspended carbon created from internal arcing. As a result, Fuller’s Earth improves both the power factor and the interfacial tension of the transformer oil while subsequently reducing the acid content. The continuous circulation of hot, dry clean oil through the energized transformer will also clean the transformer as it continues to dissolve sludge, oil-decaying products and moisture from the insulation. Upon returning the “dirty” oil through the rig, the adsorption process will again remove the contaminants from the oil before returning to the transformer. This process is repeated until all incoming oil properties meet the minimum standards as outlined by the customer and the contractor. Attached is a graph that outlines the effectiveness of Fuller’s Earth in relationship to the acid content of the oil5.
Vacuum Degassing-Dehydration
Before the oil returns to the transformer, it is important to have the oil pass through a vacuum degassing/dehydration column. Although there are several ways to achieve a high vacuum on the oil, the general idea is to maximize the surface area of the oil in order to remove dissolved moisture, oxygen and other gases. The vacuum degassing column will also remove volatile forms of acids, but Fuller’s Earth will be the primary method to extract acid from the oil. Since the circulation process will continue to remove moisture from the insulation, it is important to continuously operate the dehydration column so that wet oil does not return to the top of the transformer.
Inhibitor
Most transformers have an inhibitor additive to the oil. An inhibitor is important because it aids in the resistance of oxidation. On a sealed transformer, the benefits of an inhibitor last for many years; however, as the inhibitor depletes, the rate of oxidation increases while the oil quality deteriorates. Most inhibitor utilized in transformer oil is 2,6 di-tertiary-butyl para-cresol (DBPC). Because of the reclamation process, an inhibitor is removed from the oil through the Fuller’s Earth bed and the vacuum degassing column. Thus, during the last pass, it is important to add DBPC to the oil as it is returning to the transformer for protection against oil oxidation.
Filter Stages
Because of the dangers of metallic particles and other foreign particles found inside the transformer, it is critical to have multiple filter houses throughout the rig to guard against particles from re-entering the transformer. Multiple filters throughout the rig ensure the protection of the transformer. Contaminants and particles in the oil not only degrade oil qualities such as dielectric and power factor, but contaminants such as iron particles also introduces the risk of failure. Thus, the very last step before the oil exits the rig is to pass through a 0.5 micron particle filter to trap any remaining particles that have not been previously captured.
Safety
The advantage of the hot oil reclamation process is that, in most cases, it can be performed on energized transformers. Thus, the customer never has to take an outage and incur additional expenses of setting up a mobile transformer. In addition, the energized process helps to aid in the cleaning of the transformer. Typically, 230kV is considered the ceiling for performing hot oil reclamation on energized transformers. But not all transformers qualify. Performing a hot oil reclamation process on energized transformers is safe as long as:
- Properly designed oil processing equipment is utilized on energized transformers.
- The crew is well-trained and experienced with processing on energized transformers.
- The transformer size meets the acceptance criteria for processing energized.
- The oil condition is safe to process while energized.
The oil processing system must have the ability to eliminate bubble formation. Bubbles are stress points and have low dielectric strength. As a result, a bubble or a conglomerate of bubbles can cause arcing and or failure. Static charges can also develop when mineral oil flows through pipes and hoses so it is important to have the system grounded. In addition to the equipment, the crew must be very well trained to perform this service on energized transformers– both for the protection of the personnel and the equipment.
If a transformer is too small, or the voltage rating is above 230 kV, then it is recommended that the transformer be processed de-energized. Likewise, if the condition of the transformer oil is too poor because of high acids, high moisture, high dissolved gases, low dielectric or low interfacial tension, then the transformer cannot undergo a hot oil reclamation energized. Instead, the poor conditions force the transformer to be processed de-energized. Every time a transformer is de-energized and re-energized, there is a significant amount of electrical and mechanical stress on the transformer.
There is always the added risk that this stress could lead to the failure of the transformer. This is especially true in older transformers. In addition, the added benefits of performing the reclamation process energized are lost when the transformer is reclaimed de-energized. Thus, it is advantageous to monitor and prevent the transformer from reaching a point where it cannot be reclaimed energized.
Alternative Options
Retrofilling
In some cases, retrofilling is the preferred option. A straightforward process, retrofilling is where the oil is drained and transported for disposal; the transformer is flushed, and then refilled with replacement oil. Small transformers or transformers without valves are good candidates for retrofilling. Another good retrofill candidate is an oil with really poor oil qualities. However, it is recommended that the transformer undergo a hot oil reclamation process following the retrofill.
This is because only 10% of the interior surface of the windings will be reached during the retrofill process. Retrofilling cannot remove or clean the sludge deposits located within the transformer. In
addition, the remaining oil that was not drained will still be oxidized. Like PCBs, a small quantity of oxidized oil can contaminate a large batch of oil. Within a year, the transformer oil can deteriorate to the previous poor condition if the transformer does not receive a hot oil reclamation process.
It is critical to make sure that there is enough settling time after a retrofill is performed. Every time a transformer is drained and refilled, there is an increased probability of transformer failure upon re- energizing because of the risk of an air bubble entrapped within the windings.
Vacuum-Filling
In some cases, the primary voltage is too high to perform a retrofill. Instead, the transformer must be vacuum-filled. The vacuum-filling process will remove air and water, the biggest contaminants of an insulating system. In addition, it is also important to remove the air so that no voids are left within the insulation. These voids can create corona and electrically stress the windings. How long a transformer needs vacuum-filled depends on the size of the transformer and the recommendation of the manufacturer. After a sufficient amount of vacuum time has passed, then the transformer is ready to be filled with oil while still under vacuum. This allows the oil to fill all of the voids. There are cases when it is important to circulate the oil through the transformer. This will allow any remaining moisture or dissolved gases to be removed through the oil circulation process.
Conclusion
It is important to test and monitor transformers. Testing is inexpensive and very informative as to what is occurring inside the transformer. Oil testing allows the owner to prevent minor “problems” from turning into major “problems” by taking corrective action to save the transformer before it fails. When the need arises to treat the oil, it is important to understand the options available and how the processes work to treat the symptoms of the problem. By maintaining the transformer oil, it can prolong the life of the transformer and delay the inevitable need to replace the unit.
References
1 Facilities Instructions, Standards and Techniques, “Maintenance of Liquid Insulation Mineral Oils and Askarels”, United States Bureau of Reclamation
2 IEEE Guide for Reclamation of Insulating Oil and Criteria for Its Use (IEEE Std 637-2015)
3 IEEE Guide for Acceptance and Maintenance of Insulating Oil in Equipment, (C57.106-2015)
4 C. Manger, “High Voltage Equipment Maintenance 101, Mineral Insulating Oil”, TechCon 2002
5 C. Bandt, “Filtering Insulating Oil”, Presentation for American Society for Testing and Materials, 2002
