Maintenance and Reconditioning of Insulating Oil and Fluids
This section covers the maintenance and reconditioning of the oils and less flammable fluids such as silicone, RTemp, and Wecosol. As discussed earlier, moisture and oxygen are the most prevalent contaminants present in transformer oil and fluids. As a result of these contaminants and other catalysts and accelerators, oxidation of these liquids takes place. Overtime oxidation results in deterioration of the transformer insulating system. If this degradation is not corrected in time, it eventually leads to terminal stage of deterioration called sludge. Sludge is a resinous, partially conductive substance that eventually causes the transformer to fail. Before this condi- tion is reached, it is imperative that the oil should be maintained so that this condition does not occur. However, all is not lost; even badly deteriorated oils can be reconditioned and reclaimed by removing the oxidation prod- ucts and other contaminants.
Reconditioning Used Insulating Oils
All known oil purification methods are shown in Table 4.10. Modern vacuum oil purification systems with integral Fuller’s earth or activated alumina can correct all contamination conditions of deteriorated oils. First the contami- nated oil condition has to be identified and then an appropriate method or combinations of the methods are used for a complete purification of the oil.
Oil that has low dielectric strength or contains deposits of sludge or other contamination should receive maintenance attention. Low dielectric strength indicates the need for drying by mechanical filter or vacuum dehydrator. High acidity, high power factor, or low IFT values indicate the need for reclaim- ing treatment. When used, insulating oils are to be subjected to reconditioning and/or reclaiming processes, every advantage possible should be taken of natu- ral precipitation. Considerable savings can frequently be realized in processing used oil if it is allowed to remain in its container undisturbed for at least 24 h so that water and suspended solids can settle out. The oil can then be removed without disturbing the residue in the bottom of the container, thus obviating the necessity of removing the residue from the processing machinery.
Filter presses (Figure 4.3) vary somewhat in form, but are based upon the principle of forcing oil under pressure through a series of absorbing materials, such as paper, Fuller’s earth, etc. Filters of this type are capable of removing carbon, water, sludge, and the like, when they are in suspension, but except for certain special arrangements, they cannot remove them effectively when they are dissolved or in colloidal form. These devices (particularly those with centri- fuges) will not remove air, but, in fact, tend to aerate the oil. Experience has shown that the most efficient temperature at which to filter insulating oil is between 20°C and 40°C. Below 20°C, the viscosity increases rapidly, while at temperatures above 40°C the moisture is more difficult to separate from the oil.
Filter Press Operation
When the oil is to be purified by the use of a filter press using blotting paper, the paper should be well dried to obtain the most efficient operation; other- wise, the paper may actually add moisture to the oil. An oven should be used for drying the paper, and the sheets should be separated as they are hung on rods in the oven to permit free circulation of air and to ensure the most rapid drying. The filter paper should be dried from 6 to 12 h at a temperature of 101°C to 105°C. After drying, the paper should be taken from the oven directly to the filter, or it may be stored in dry transformer oil for future use. When transferring the paper, care should be taken to handle it as little as pos- sible to avoid the absorption of moisture from the hands and to minimize the time of exposure to the air.
When purifying very wet oil with a filter press, the back pressure will not increase appreciably as the filter paper absorbs moisture. Therefore, the operator should make frequent dielectric tests of the oil discharged from the filter press to determine when the paper should be replaced. When purifying oil containing materials such as sludge or small carbon particles, consider- able back pressure will develop as the filtering progresses because of the materials clogging up the filter paper. When the back pressure reaches about 75 psi, the paper should be replaced.
In recent years, mobile cartridge-type filters (Figure 4.4) for reconditioning transformer oil are being used. These units are available in various sizes with oil-processing capacities ranging from 10 to 75 gal/min and utilize disposable cartridges with filter densities ranging from 1 to 25 micrometers (μm). (Note: 0.5 μm filters are recommended for transformer oil.) These mobile filter units are smaller, lighter, and more portable than large filter presses, have greater oil-flow capacities, and in most cases provide better water and particle removal. In addition, a drying oven is not required since the filter cartridges are hermeti- cally sealed in plastic for shipment and storage. Once used, the filter cartridges are properly disposed of. Each cartridge typically can hold up to 3 qt of water.
Another means of separating free and suspended contaminants, such as carbon, water, and sludge, from oil is the continuous centrifuge. In general, the centrifuge
can handle much greater concentrations than can the conventional filter press, but it cannot remove some of the contaminants as completely as a filter press. Consequently, the centrifuge is generally used for rough bulk cleaning where large amounts of contaminated oil are to be handled.
Frequently, the output of the centrifuge is put through a filter press for the final cleanup. The centrifuge cannot remove dissolved water from oil; since the final centrifuge is sealed with water, the oil leaving the centrifuge may be saturated at the temperature of operation and conceivably could contain more dissolved water than when it entered. Neither the centrifuge nor the filter press is designed to treat oil chemically.
Throughout the power industry, coalescers are replacing centrifuges for use in removing free water from both lubricating and insulating oils. Coalescing is a technique that has been borrowed from the aviation fueling field. Fiberglass cartridges trap small water particles; increasing differential pres- sure across the filter media forces the particles of water together, and the large water drops are extruded at the outer surface of the fiberglass element. Large water drops are retained within a water-repellent separator screen and collect, by gravity, at the bottom of the filter while dry oil passes through the
separator screen. This method is quite similar to centrifuging with respect to performance and limitations; however, coalescing filters have no moving parts and, therefore, are simpler in operation and maintenance and suitable for unattended and automatic operation.
The vacuum dehydrator (Figure 4.5) is efficient in reducing the water content of insulating oil to a very low value. In this apparatus, the oil is exposed to a vacuum and heat for a short interval of time. Vacuum dehydrators can be used to treat oil without removing associated equipment from service. In addition to removing water, vacuum dehydrators will degas the oil and remove the more volatile acids. Vacuum dehydrators are frequently used by the manufacturer during initial filling of new transformers.
Maintenance of Less Flammable Fluids
Maintenance of RTemp
As a general rule, RTemp transformers may be handled in the same manner as conventional oil-filled transformers. However, some of the characteristics of RTemp fluid require special attention. The maintenance of RTemp fluid can be carried out similarly to that for oil and askarel. The sampling procedures for RTemp are similar to askarel. Special maintenance instructions for RTemp are the following:
Filtering: If it is necessary to filter RTemp fluid to remove excess moisture, sludge, and the like, it can be filtered through conventional filtration systems. The filtering system should be flushed before connection.
Care should be taken to assure that the pump has sufficient capacity to handle the relatively high viscosity of RTemp fluid at lower temperatures. The heating of the fluid and piping system will increase the speed and ease of filtration and is completely acceptable.
Cold start: RTemp fluid has a pour point of −30°C. At this point, the dielec- tric strength is still sufficient to allow safe energization of the transformer. Because the possibility exists of energizing a transformer into a fault, RTemp transformers should not be energized if the fluid temperature (top oil) is below −15°C. At −15°C top oil temperature or above, full load may be applied to the transformer.
If the top oil temperature is below −15°C, immersion heaters placed near the bottom of the tank, external heating blankets, or some other means should be used to raise the top oil temperature to −15°C, thus assuring ade- quate cold spot temperature.
Precautions: RTemp transformers are high-fire point liquid-insulated trans- formers. Relatively small quantities of conventional transformer oil or other low-fire point materials can substantially reduce the fire point of RTemp fluid. Care must be taken in processing and handling RTemp transformers not to introduce such contaminants. External systems, such as filtration sys- tems, should be thoroughly flushed with RTemp fluid before connection to a RTemp transformer.
Maintenance of Silicone
Silicone insulating fluid is used in transformers to provide heat transfer. Transformers containing silicone should be installed, operated, and serviced by competent and trained maintenance personnel who are familiar with good safety practices. The sampling procedures for silicone are similar to askarel. The following are special maintenance instructions relating to silicone-filled transformers.
Receiving and handling: Immediately upon receipt of shipping drums or a transformer filled with silicone fluid, an examination should be made for leaks. If leakage is evident either at this time or at any time thereafter, the cause should be corrected and the spillage soaked up with absorbent materials such as saw- dust or fuller’s earth, followed by a cleanup of the affected area with rags soaked with kerosene or other approved solvent, such as 1,1,1-trichloroethane. Adequate ventilation must be provided when using such solvents.
On those infrequent occasions when silicone fluid is removed for shipment, the transformer may be shipped gas filled and is to be liquid filled at instal- lation. If the transformer is located outdoors, adequate precautions must be taken to ensure that no dirt or moisture enters the liquid during the filling operation. Before opening a container of silicone fluid, allow it to stand until the liquid is at least as warm as the surrounding air.
Before placing the liquid in the transformer, take a sample from each con- tainer and make dielectric tests as outlined under Section 184.108.40.206. If the tests are unsatisfactory, restore the dielectric strength by filtering before placing the liquid in the transformer. When transferring from containers to the trans- former, it is recommended that the liquid be passed through a filter press to remove any undetected moisture or sediment that may be present. A vacuum purifier is also commonly used.
Silicone fluid must be handled in containers, pipes, oil-resistant hoses, and the like, that are free from oil, grease, pitch, or other foreign materials, since these contaminate the liquid and decrease its nonflammable properties. All apparatus used in sampling, filtering, storing, or transporting silicone fluid must be maintained for exclusive use with silicone fluid, since it is extremely difficult to remove all traces of oil or other silicone fluid contami- nants from equipment of this type. Also, mineral oil is completely miscible in silicone fluid, and it is practically impossible to separate the two liquids after they have been mixed.
Use kerosene or other approved solvent to remove all traces of silicone fluid on the outside of the transformer tank. This precaution should be taken since silicone fluid has a tendency to affect adhesion of additional coats of paint.
Storage: Shipping drums should be stored indoors in an area specially selected for this purpose. If it is necessary to store drums or cans containing silicone fluid outdoors, protect the containers from the weather and direct contact with water. Regardless of location, all drums should be stored in a position that results in the bungs being under a positive pressure. Do not open a drum or can until the liquid is actually needed. Any change in tem- perature while the containers are open will cause an exchange of air, with the possibility of moisture entering the liquid. Partially emptied drums must be tightly resealed and stored in the same manner outlined previously.
Periodic inspection: The insulating liquid must be maintained at the proper level, and for the longest possible service life of the transformer, the dielec- tric strength of the silicone fluid should be maintained at a high value. It is recommended, therefore, that the liquid be sampled and tested after the first few days of operation, again after 6 months, and yearly thereafter. Keep accurate records of the tests, and filter or replace the liquid as indicated. The entire transformer should also be thoroughly checked for leaks at these same intervals. If the pressure-vacuum gauge consistently reads zero, a leak in the gas space is indicated. If there is any reason to believe that water may have entered the transformer, check a top sample immediately for water.
Filtering: If test results indicate that moisture or other contaminants are present, they can usually be removed by passing the insulating liquid through a filter press. This device may be used either as a paper filter press for drying or with fuller’s earth and paper for purifying. All apparatus used in sam- pling, filtering, storing, or transporting silicone fluid must be maintained for exclusive use with silicone fluid, since it is extremely difficult to remove all traces of oil or other silicone fluid contaminants from equipment of this type.
Filtration can be accomplished in the transformer or other container by circu- lating the silicone fluid from the bottom to the top through a filter press. Filtering can be done faster and more efficiently by passing the liquid from the transformer through the filter and into a separate, clean, dry container and then back through the filter again to refill the transformer. In this manner all the liquid will be given two complete passes through the filter press. If additional filtering is still required, the entire procedure can be repeated. As moisture is extracted from the liquid during the filtering process, the filter- ing medium will become wet. Frequent samples of the outgoing liquid should be tested to determine when the filtering medium should be replaced.
The filter press will not remove large quantities of free water from the silicone fluid. When a large quantity of free water is introduced into the filter, it will be passed on through, emerging as finely divided droplets dispersed throughout the liquid. Therefore, if free water is present, it should be removed before filtering is started. A transformer contaminated with moisture may not only have moisture suspended in the insulating liquid, but also in the windings and insulation. The most efficient temperature for filtering mois- ture from the liquid is between 20°C and 40°C, but at this temperature the transfer of moisture from the windings and insulation to the insulating liquid is quite slow. If free water is present in the transformer or if the dielectric strength of the silicone fluid is still below 30 kV after filtering, consult the nearest office of the transformer manufacturer for additional information.
Safety precautions: As a class, silicone liquids are nontoxic. Silicone fluid in contact with the eyes may cause local irritation, but this irritation is only temporary. If desired, eyes may be irrigated with water, and if irritation per- sists, consult a physician.
Precautions: Static charges can be developed when silicone fluid flows in pipes, hoses, and tanks. Fluid leaving a filter press may be charged to over 50,000 V. To accelerate dissipation of the charge in the liquid, ground the filter press, the piping, the transformer tank, and all bushings or the winding leads during flow into any tank. Conduction through silicone fluid is slow; therefore, it is desirable to maintain these grounds for at least 1 h after the flow has been stopped.
Arcs can occur from the free surface of the charged liquid even though the previous grounding precautions have been taken. Therefore, explosive gas mixture should be removed from all containers into which liquid is flowing.
Maintenance of Wecosol
Wecosol fluid is a transformer grade of tetrachloroethylene (sometimes called perchloroethylene). Wecosol fluid will slowly evaporate to produce Wecosol vapors. It is necessary to use the proper safety procedures to prevent adverse effects resulting from vapor inhalation and skin contact with fluid. Overexposure to Wecosol vapors will result in symptoms such as headaches, confusion, nausea, and lack of coordination. Extreme overexposure to Wecosol vapors could result in fatal personal injury. The fluid is considered as
less than 50 ppm polychlorinated biphenyl (PCB) dielectric fluid in accordance with Federal PCB regulations 40 CFR 761, dated July 01, 2002. It is considered as a nonflammable fluid with boiling point of 121°C at atmospheric pressure. The safe limits of vapor exposure are shown in Table 4.11.
Low lying areas such as pits can slowly accumulate vapors which are nearly six times heavier than air. The tank should not be opened in those areas which may accumulate the vapors to prevent excessive vapor concentrations. The odor of Wecosol vapors is noticeable at concentrations of 50 ppm and often as low as 10 ppm. Do not use odor to determine vapor concentrations since odor threshold varies between individuals. Also, the ability to recognize Wecosol vapors diminishes after exposure, due to temporary desensitization. Wecosol fluid is a solvent. Like any solvent, contact will dry out the skin by removing its natural oils. The natural oils can be replaced by the use of common hand lotions. Gloves resistant to Wecosol fluid should be worn to avoid skin contact and transformer fluid contamination.
Fluid splashed into the eyes may cause pain and irritation. Safety goggles should be worn if tasks to be performed risk splashing of fluid into the eyes.
If the fluid is splashed into the eyes, flush the eyes with water for approximately 15 min and consult a physician. The following are special maintenance instructions for Wecosol.
Receiving and handling: Immediately upon receipt of drums or a transformer filled with Wecosol fluid, examine for leaks. Take the necessary action and precautions so that PCB contamination is not introduced from the leaks or any filling or maintenance of the transformer. If the fluid is received in drums, they must be stored in ventilated dry area in an upright position. Before a drum is used to fill the transformer, it must be sampled and tested for dielectric strength. The dielectric strength must be at least 30 kV for drummed fluid to be used.
Sampling: Samples should be taken to prevent air from entering the tank. To prevent air from being drawn into the tank, the tank pressure must be greater than zero psi. If necessary, increase the tank pressure by injecting dry nitrogen until a positive pressure of about one half psi is reached. The sample should be taken when the unit is warmer than the air to avoid condensation. Care should be taken to procure a sample which fairly represents the liquid in the tank. All samples must be taken from the top liquid sampler near the top fluid level. A sufficient amount of liquid should therefore be drawn off before the sample is taken to insure that the sample will not be that which is stored in the liquid sampler. If the sample taken contains free water, it is not suitable for dielectric tests and the sample must be discarded. A second sample should then be taken after at least 2 qt of liquid have been withdrawn.
If free water still exists, all transformer fluid must be dried as explained in the following section.
Drying: The transformer fluid can be dried by either chemical drying or insulation drying methods. Take the necessary precautions so that PCB con- tamination is not introduced during field filling or maintenance of the trans- former. In the chemical drying method use hoses, gaskets, and threaded fitting seals made of viton rubber and Teflon-lined copper or steel. The liquid temperature must be less than 60°C for chemical drying procedure to be used. Circulate the tank fluid through glass or paper filters and calcium sul- fate. The filters should be a 50 μm filter on the calcium sulfate inlet with a 1 μm absolute on the exit (filter will pass no particle larger than 1 μm). Continue the drying process until the water content is less than 35 ppm, and the dielectric breakdown is greater than 26 kV.
In the insulation drying method, the windings, insulation, and fluid can be dried by circulating current through the windings. The tank coolers must be blanketed off to reduce heat loss. The low-voltage winding is short circuited and sufficient voltage is impressed across the high-voltage winding to circu- late current through the windings to maintain a liquid temperature between 90°C and 100°C. The voltage necessary to accomplish this task is approxi- mately one-third the rated impedance divided by 100 and multiplied by the rated voltage. Current requirements are approximately one-third the rated current. During the heating, monitor the liquid level to make certain the liquid level is at least to the 25°C level. Stop the heating if the liquid level falls below the 25°C level. When the liquid has reached the required tempera- tures, purge the gas space with dry nitrogen. Minimize the Wecosol vapor exhaust. Continue the purging and heating until the liquid water content is less than 35 ppm, and dielectric breakdown is greater than 26 kV.
Reprocessing: If necessary the Wecosol fluid can be reprocessed by filtering. When reprocessing, use hoses and tubing lined with Teflon or made of copper or steel. All gaskets and threaded fitting seals used during this pro- cess must be made of viton rubber. Circulate the fluid through paper or glass filters and fuller’s earth. The inlet filter should be a 50 μm filter. The exhaust filter must be a 5 μm absolute filter.
Inspection and maintenance: Periodic inspection and maintenance test should be conducted to determine whether a transformer fluid should continue to be used, dried, or reprocessed.
Maintenance of Environtemp (FR3)
This section discusses natural ester-based transformer insulating fluid known as Envirotemp® (FR3) and offers a guide on testing and evaluation, as well as criteria and methods of maintenance for it. These base fluids are also known as vegetable seed oils. These fluids are currently being used in the range of small distribution class transformers to medium power transformers. They are being applied in new equipment and for retrofilling existing equipment. The dielectric fluid FR3 has been available for transformers in the last several years and today this fluid is being used in transformers produced by many manufactures. This fluid is soy-based product and has an exceptionally high fire point of 360°C and flash point of 330°C. The chemical composition of FR3 fluid is a mixture of triglycerides (long-chain fatty acid ester molecules) that are relatively polar, are less prone to saturate, and readily form hydrogen bonds. It has the highest ignition resistance of less-flammable fluids currently available. It is referred to as a high fire point or “less-flammable” fluid, and is listed as a less-flammable dielectric liquid by Underwriters Laboratories (UL) for use in complying with the NEC and insurance requirements. Because FR3 fluid is derived from 100% edible seed oils and uses food-grade additives, its environmental and health profile is unmatched by other dielectric coolants. Its biodegradation rate and completeness meets the U.S. EPA criteria for “ulti- mate biodegradability” classification.” The manufacturer (Cooper Power System, USA) also claims FR3 fluid extends insulation life by a factor of as much as 5–8 times because it has the unique ability to draw out retained mois- ture and absorb water driven off by aging paper. It also helps prevent paper molecules from severing when exposed to heat. These properties can result in increase ability to overload or longer transformer insulation life, resulting in both lower life cycle costs and delayed asset replacement. FR3 fluid is fully miscible with conventional mineral oil or R-Temp®, and may be used to retro- fill or top off units filled with these fluid types. It appears the only negative that can be attributed to this fluid is the fact that it has a relatively high first cost relative to mineral oil and could easily add 15%–30% to the transformer first cost. FR3 fluid is a fire-resistant natural ester dielectric coolant specifi- cally formulated for use in distribution and power transformers where its unique environmental, fire safety, chemical, and electrical properties are advantageous. Because of its excellent environmental, fire safety, and perfor- mance characteristics, applications for FR3 fluid have expanded into a variety of other applications, including power transformers, voltage regulators, sectionalizing switches, transformer rectifiers, electromagnets, and voltage supply circuits for luminaries. The fluid is also used in retrofill applications for transformers and other fluid-filled distribution and power equipment.
Storage and handling: The same basic procedures for storing and handling conventional transformer oil should be followed with FR3 fluid. To maintain the extremely low percent moisture saturation at time of fluid manufacture, it is recommended that exposure time to air be as minimal as practical. Drum and tote storage should be indoors or outdoors protected from the elements. To maintain the optimal fluid properties for its intended use as an electrical insulating fluid, exposure to oxygen, moisture, and other contami- nants must be minimized. Except for short storage periods, material that has been immersed in FR3 fluid should not be exposed to air. Thin films of natural esters tend to polymerize much faster than conventional transformer oil. For equipment drained of FR3 fluid, it is recommended that the equipment be placed in an inert gas environment or be reimmersed as soon as is practical. Hot air drying is an unacceptable process for assemblies already impreg- nated with a natural ester fluid. For impregnated assemblies that require additional drying, a method of drying that does not expose the impregnated insulation to air is required to avoid polymerization of the dielectric fluid. Avoid extremes of temperature of storage. FR3 fluid should be stored in labeled, tightly closed containers at 10°C–40°C in dry and well-ventilated areas away from sources of ignition or heat.
Fluid maintenance tests: Physical, chemical, and electrical properties are used to evaluate new and in-service electrical insulating fluids. Periodic mainte- nance tests for FR3 fluid-filled equipment should follow the same schedule used for transformers filled with conventional transformer oil. However, some traditionally acceptable indicators of mineral oil performance may not apply or may have different values for Envirotemp FR3 fluid. When compar- ing the standard ASTM tests and mineral oil specifications to those for natu- ral esters indicate that many tests may require special consideration. The battery of tests may be separated into performance, quality, and diagnostic. These tests are discussed below.
Insulating fluids provide both electrical insulation and cooling for the electrical apparatus. The dielectric breakdown voltage and viscosity are two key properties that affect the function and performance of an electrical insu- lating fluid. The dielectric breakdown voltage measures the integrity of the insulation. The viscosity influences the cooling performance.
Dielectric breakdown voltage tests: The dielectric breakdown voltage tests that are conducted for insulating fluids are ASTM D1816 and D877. The only modification to the D816 test method is the stand time before the test. The stand time for the mineral oil is between 3 and 5 min. Because of the viscosity of the FR3 fluid is higher than the mineral oil, a 15 min stand time is recom- mended between pouring the room temperature equilibrated fluid sample and the start time of the test. This added time gives the entrained air suffi- cient time to escape after pouring of the sample. The stand time recom- mended for the D877 is 2–3 min. The D1816 is the preferred test for FR3 even though D877 test works well for this fluid. The reason is that the D877 test is less sensitive to dissolved gas, water, and particulate than the D1816.
Viscosity: The kinematic viscosity of the FR3 fluid is the lowest of the less- flammable fluids, and is higher than that of mineral oil. The viscosity test using ASTM D445 may be performed without any modification.
These tests are conducted to give indicators of changes in the electrical insulating fluid over time due the operation of the equipment. Their usefulness is not so much in the test values (pass or fail) themselves, but in the trends over time. The quality of FR3 fluid is measured using the same battery of tests that are used for oil. However due to the differences in the chemistry of FR3 and mineral oil, the normal base line values are different for certain properties. For example, dissipation factor (power factor), water content, pour point, and acid (neutralization) number are typically higher than those of mineral oil. Interfacial tension, gassing, and resistivity are normally lower than that of oil. These tests provide a good indication of possible fluid contamination or unusual degradation. Acceptable limits for continued use of service-aged FR3 fluid-filled equip- ment are listed Table 4.12.
Dielectric strength per ASTM D1816 and D877: The acceptable limit for continued use of service-aged FR3 fluid is 30 kV minimum for equipment rated 69 kV and below. For applications greater than 69 kV line voltage con- tact manufacturer for recommendations. As was discussed above the stand time for the D1816 should be 15 min to allow entrapped air to escape before the test is conducted.
Water content: The ASTM D1533 method can be used for the FR3 fluid with- out modification. If erratic or unusual results are observed while conducting this test, then use the Karl Fisher reagents for aldehydes and ketones instead of those used for mineral oil. The high capacity for water (1100 versus 60 mg/ kg for mineral oil) is one of the important attributes of FR3 fluid that gives kraft paper insulation longer life in it compared to its life in mineral oil. New FR3 fluid typically contains 20–60 mg/kg of water, and standard specification for natural ester fluids used in electrical apparatus allow up to 200 mg/kg.
Dissipation factor: The ASTM D924 method can be used without modifica- tion. When using the same test cell for both mineral oil and FR3 fluid dissi- pation measurements, it is of utmost importance to clean the test cell meticulously when changing from one type of fluid to another. This is espe- cially true when measuring FR3 fluid after mineral oil, otherwise high values may be seen if the cell is not sufficiently cleaned. Also, clean the test equipment immediately after completion of the test due to the higher tendency of thin films of natural esters to oxidize and eventually polymerize when exposed to air. A 0.05% value at 25°C for new FR3 fluid is typical and values up to 0.2% are acceptable per ASTM D6871.
Acid number: The ASTM D974 can be used without modification for deter- mining the neutralization (acid) number for FR3 fluid. Because new FR3 fluid contain small amounts of free fatty acids that result in acid neutralization number being higher than seen in mineral oil. As the FR3 fluid ages, it reacts with water (hydrolysis), generating additional long chain fatty acids that are considered to be noncorrosive and milder than short chain organic acids found in mineral oil.
IFT: The IFT can be measured in the same manner as for mineral oil using STM D971 method. The FR3 fluid has an inherently lower IFT value com- pared to mineral oil. It is considered that IFT should be as useful for FR3 fluid as it is for mineral oil however more test data is needed to establish safe limits for in-service FR3 fluid.
Color: A low color number of FR3 insulating fluid is desirable to permit inspection of assembled apparatus in a tank. An increase in color number during service is an indicator of oil deterioration or contamination. New FR3 may initially be slightly darker in color, typically a slight amber appear- ance, than highly refined new mineral oil. Other tests (such as dissipation factor and neutralization number) are better measures of fluid deterioration or contamination. Note that natural ester fluid manufacturers may add clear colorants for identification purposes. Such tints should not impact the ASTM color and visual examinations.
Three tests are important for diagnostic and safety purposes. Flash and fire point analyses per ASTM D92 serves both for quality verification of new FR3 fluid and diagnostics and safety evaluation of in-service fluid.
The lower flash and fire point values indicate contamination by more volatile fluids. Flash point values can be used to estimate the amount of mineral oil in transformers that are retrofilled with FR3 fluid.
Flash point and fire point: Relatively small amounts of conventional oil should not significantly reduce the flash point and fire point of Envirotemp FR3 fluid. Contamination above 7.5% may reduce the fire point to below 300°C. If it is suspected that the fluid may be contaminated, flash point and fire point should be measured in accordance with ASTM D92.
Dissolved gas analysis: This test is recommended particularly for high value equipment or equipment servicing critical loads. ANSI/IEEE guide C57.104- 1991 for detection and analysis of generated gases can be applied, except the use of ratio methods. Limited testing and field experience indicates that the same fault gases are produced in FR3 (natural esters) as are produced in mineral oil. Under the same magnitude of electrical overstress, natural esters typically produce somewhat less volume of the gases compared to mineral oil. Under the same thermal overstress, natural esters typically produce sig- nificantly more volume of the gases. There are differences in gas solubility coefficients between natural esters and mineral oils and their respective values should be used for data interpretation. There are differences between mineral oil and natural ester gassing tendency per ASTM D2300. During normal operation, the levels of dissolved hydrogen and ethane gases can increase at a rate greater than the typical rate in mineral oil.
Safety and care procedures
Typically, natural esters covered have been formulated to minimize health and environmental hazards. Although no known hazard is involved in the normal handling and use of natural ester fluids, additives to the base oil may differ. Users should obtain a material safety data sheet (MSDS) for each natural ester fluid in use. Follow the manufacturer’s instructions at all times.
Personnel should avoid eye/fluid contact and inhalation of spray mists, and take appropriate steps if such incidents occur. MSDSs should provide appropriate guidelines with respect to handling these fluids. Although not listed as a hazardous substance or waste by any federal agency, disposal of natural ester fluids may require certain precautions. Currently, the U.S. EPA Spill Prevention, Control, and Countermeasure (SPCC) regulation (40CFR112) makes no practical distinction between mineral oils and vegetable oils, except for possible reduction in spill remediation requirements. Refer to IEEE Std C57.147-2008, IEEE Guide for Acceptance and Maintenance of Natural Ester Fluids in Transformers, for more details.
Maintenance of Askarels
Askarel is a generic name for PCBs which were used extensively in electrical transformers, capacitors, and other equipment since the 1940s. Askarel was used by many electrical equipment manufacturers under their own trade
name such as Pyranol (GE), Inerteen (Westinghouse), and so on. The open use of askarel was banned in electrical equipment in the early 1980s by the Environmental Protection Agency (EPA) in the United States. The use of askarel in closed systems in some industries may still be allowed with man- datedproceduresforhandlinganddisposalofaskarelandaskarel-contaminated materials. The discussion of maintenance of askarel is undertaken here for those facilities that are still allowed to use askarel as a dielectric medium for electrical equipment.
Inspection: Visual inspection and dielectric strength tests should be made on askarel when installing equipment and on a regular schedule at 1-year intervals thereafter. Visual inspection that reveals a clear, faint yellow or light brown color indicates good askarel condition. The presence of a green, red, or blue cast, cloudiness, or turbidity indicates the presence of insulation or moisture contamination, and further tests on the askarel and inspection of the associated equipment should definitely be made. If the askarel sample appears black or contains suspended carbon particles, severe arcing has occurred. The askarel should be discarded and a thorough inspection of the equipment performed.
Sampling: Samples of askarel should be taken in a clean, dry glass quart bottle. If the sample is to be stored indefinitely or sent to a service labora- tory for moisture content tests, the bottle should be filled only to within 1 in. of the top; the sample should be sealed by wrapping the top and the threads of the jar with aluminum foil before tightly securing the cap. When the sample is to be sent to a laboratory, indicate the temperature of the askarel when the sample was taken. Samples of askarel should be taken when the relative humidity of the environment is low and when the temperature of the askarel is as high or higher than the surrounding air. It is best to take the sample when the unit is near operating or maximum temperature. Samples taken during regularly scheduled intervals should be taken at nearly the same temperature as previous samples. Samples should be taken as close to the top of the liquid surface as possible because askarel is heavier than water.
Testing: The testing procedure for askarel is the same as for mineral oil, but care should be taken to see that there is no mineral oil in the test cup. The dielectric strength test for askarel is the most important maintenance test. A higher dielectric strength indicates that the insulating efficiency of the askarel is high and that any cloudiness or turbidity present is not due to dam- aging moisture contamination. If the dielectric strength of the askarel decreases abruptly or a decreasing trend is observed, inspections should be made at more frequent intervals. When the dielectric strength is 26 kV or less, a sample of askarel should be sent to a laboratory for a moisture test (ASTM D 1533, Karl Fischer method). A high power factor alone is an insufficient criterion for replacing or reconditioning askarel; however, where abnormally low values of dielectric strength occur, the power factor of the askarel can be expected to
abnormally high. Power factors of 20% and more are frequently encountered in askarel transformers during normal service. Although this is an indication that some contamination has occurred, experience has shown that askarel is serviceable long after power factor test values increase greatly. Again, the determining criterion that indicates the serviceability of askarel is the dielec- tric strength. Reconditioning askarel to obtain a lower power factor is usually not justifiable.
Contamination: Water contamination is the primary cause of deterioration of askarel dielectric strength. Inspect all askarel-filled equipment for possible areas that would allow the equipment to breathe moisture-laden air. Numerous sealing compounds are available for sealing these areas. Where gaskets are located, use silastic seals at the flanges, viton for an elastomeric seal, and Teflon tape on pipe threads. On older equipment, cork-type gaskets should have the outside edges sealed with epoxy cement.
Reconditioning askarel: The fact that the transformer tanks are generally sealed and that any condensation will float on top of askarel makes filtering by a blot- ter press rarely necessary. If such filtering is necessary, it can be done with an ordinary press from which all mineral oil has been removed. A centrifugal purifier designed for mineral oil will not function on askarel. Special combina- tion activated clay purifiers and blotter presses are manufactured for askarel.
Handling and disposal of askarel: While askarel is generally considered to be noncombustible, under arcing conditions gases are produced that consist predominately of noncombustible hydrogen chloride and varying amounts of combustible gases depending on the askarel composition. Care should therefore be taken to handle askarel-filled apparatus as potentially combus- tible when accumulated gases are released.
Askarels (PCBs) have been used in many applications for over 40 years, but in the 1980s evidence was discovered that PCBs were widely dispersed in the environment. Studies have shown that PCBs are an environmental contami- nant. Simultaneously, significant steps have been taken by the EPA to limit fur- ther releases of PCBs to the environment. The National Electrical Manufacturers Association (NEMA) prepared Publication No. TR-P6-1973 (January 25, 1973), “Proposed American National Standard Guidelines for Handling and Disposal of Capacitor and Transformer-Grade Askarels Containing Polychlorinated Biphenyls.” The guidelines in this official standard should be rigidly followed by all personnel when handling or disposing of askarel-soaked materials.
The following excerpts from the official standard are presented as a handy reference:
Safety precautions: Based on about 40 years of industrial usage, askarels are considered harmful materials to humans. There has been no known instance of human injury when askarels are used under the normally prescribed con- ditions of precaution and handling. Nevertheless, exposure to askarel should be avoided at all times.
Vapors: The odor of askarel is noticeable well below the maximum safe air concentrations. Depending upon the composition of the askarel used, from
0.5 to 1.0 mg/m3 of air has been determined to be the upper safe level of expo- sure during an 8 h workday. (See American Industrial Hygiene Association Hygienic Guide Series January/February, 1965.) Breathing vapor or fumes from heated askarels should be avoided. High concentrations of vapors can cause irritation of the eyes, nose, throat, and upper respiratory tract. Provision should be made for adequate ventilation and regulation of manufacturing operations to avoid open exposure of hot askarels (55°C or higher). The gases produced when askarel is decomposed by very high temperature (such as that of an electric arc) in the presence of air or organic insulating materials contain a high percentage of hydrogen chloride and small percentages of other gases. Minute concentrations of this combination of gases are very unpleasant and irritating, thus giving ample warning of their presence.
If exposure to high concentrations of askarel or its arced products is necessary under emergency conditions, an approved gas mask of the organic canister-type or self-contained breathing apparatus must be worn. Such exposure should be under the surveillance of other personnel capable of rescue in case of accident. If the odor of askarel or its arced products is detected by the person wearing protective equipment, he should immediately go into fresh air. All gas masks, respirators, and replacement parts should be approved for the purpose and be maintained on a regular schedule in accordance with the manufacturer’s recommendation.
Liquid: Unlike mineral insulating oil, there is no fire hazard in handling askarels. A limited solvent action (similar to that for paint thinner) on the fats and oils of the skin with prolonged contact may lead to drying and chap- ping of the skin. As with insulating oil, some people are allergic to askarel, and continued exposure may result in skin irritation. Both the liquid and vapor are moderately irritating to eye tissue.
Operating procedures should require avoidance of contact with any askarels. The use of porous gloves that can absorb and retain askarels is to be avoided. Resistant gloves and aprons of the neoprene, polyethylene, or viton type should be used if contact is unavoidable. In case of spillage, clothing should be removed as soon as practical, the skin washed, and the clothing discarded. Medicinal washes or mild detergents followed by the application of cold cream will reduce the irritation resulting from the contact of an open cut or abrasion with askarel.
Safety glasses with side shields or face shield should be worn when han- dling askarel. Eyes that have been exposed to liquid askarel should be irri- gated immediately with long quantities of running water for 15 min and then examined by a physician if the irritation persists. (A drop of castor oil has been found to reduce irritation.)
Persons developing a skin irritation or respiratory tract irritation while working with askarels should be placed under supervision of a physician. Ingestion or swallowing of askarels is not generally regarded as a problem of the industry. Should accidental ingestion occur, consult a physician immediately.
Hands should be washed with warm water and soap before eating, drinking, smoking, or using toilet facilities.
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