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Keeping lubricating oils cleaner and dryer can lead to significant improvements in equipment reliability. From gearboxes to hydraulics, engines to steam turbines, cleaner oil results in longer equipment life. But what about electrical transmission and distribution (T&D) equipment?
Many high-voltage transformers, load-tap changers, voltage regulators and circuit breakers use oil as an insulating medium between contacts. Although electrical T&D equipment tends to be more reliable than typical industrial rotating equipment, certain factors necessitate developing predictive and proactive strategies.
Safety concerns, the high cost of repairing transformers and other electrical transmission and distribution equipment, and the loss of revenue associated with failure, particularly during peak demand times, have caused electrical industry organizations such as the Institute of Electrical and Electronics Engineers (IEEE) and Electric Power Research Institute (EPRI) to place considerable emphasis on strategies for extending equipment life and improving operational reliability of electrical transmission and distribution system components.
Fluid cleanliness and condition affect electrical insulating oils’ performance, and directly impact electrical transmission and distribution equipment performance and longevity. For this reason, just like industrial rotating equipment, oil analysis is a key metric for evaluating the condition of insulating fluids and is a vital part of any predictive or proactive maintenance strategy for oil-insulated electrical transmission and distribution equipment.
Insulating oil analysis falls into two distinct categories; tests that monitor key chemical and physical properties of the oil, and tests that determine the concentration of certain key indicator gases dissolved in the oil.
Dissolved gas analysis (DGA) is perhaps the most significant test performed on in-service electrical insulating oils. This test uses gas chromatography to determine the concentration of certain gases dissolved in the oil, including oxygen, nitrogen, carbon dioxide, carbon monoxide, hydrogen, methane, ethane, ethylene and acetylene. Based on the relative concentrations of each gas, specific transformer problems such as electrical arcing, coronas and cellulose media breakdown (used as an insulating medium in the windings of many transformers) along with many other problem conditions can be diagnosed. The analysis of DGA data will be the subject of an upcoming article in Practicing Oil Analysis magazine.
In addition to dissolved gas analysis, some key oil physical and chemical data can help determine the onset of certain problem conditions that can to lead to failures if left unchecked. Some of the common physical and chemical test methods, along with typical industry-accepted limits, are shown in Table 1.
Based on the information in Table 1, it is clear that most tests are geared toward identifying the presence of polar contaminants, which can adversely affect the insulating properties of the oil, resulting in arcing and other associated problems. This is not surprising, given that approximately 75 percent of failures in high-voltage transformers can be directly attributed to arcing caused by dielectric breakdown.
Monitoring the oil’s physical and chemical properties along with DGA data is a vital part of an effective, ongoing predictive maintenance strategy.
Because many problems in transformers and other electrical transmission and distribution equipment are directly related to changes in the dielectric properties of the oil, an effective proactive approach to maintaining oil-insulated equipment is to control polar contaminants such as particles, moisture and by-products of oil oxidation - the forcing factors associated with a change in dielectric constant.
Just like rotating equipment, a proactive approach to insulating oil fluid cleanliness requires a three-step process:
Steps No. 1 and No. 2 can easily be accomplished by routine oil analysis, including both dissolved gas analysis and oil physical and chemical tests. The only effective way to ensure oil condition and desired cleanliness targets are met, however, is to take steps to control contaminants and other forcing factors that affect the oil’s ability to provide effective electrical insulation.
So how can you control oil contaminants in electrical transmission and distribution equipment? Just like industrial rotating equipment, every effort should be made to restrict the ingress of contaminants such as moisture and particles by ensuring free-breathing components are equipped with high-quality desiccant and particulate removing air filters, that seals are well-maintained and inspected regularly, and that any new or make-up oil that is added is prefiltered to remove as much moisture and particulate matter as possible.
However, even with the most stringent contamination control practices, contamination ingression can still occur - particularly oil degradation by-products such as coking particles and moisture - because these contaminants are generated internally, due to the routine operation of the transformer’s load tap changer.
The impact of internally generated contaminants on the performance of electrical insulating oils is well-known. Because of this, it is standard operating procedure, particularly for high-voltage transformers and other oil-insulated electrical transmission and distribution equipment, to use an aggressive preventive maintenance program. This includes annual processing of the oil using a filter press, deploying materials such as fuller’s earth and/or vacuum dehydration, to remove contaminants.
While there is no doubt that such routine maintenance can be effective in ensuring reliable operation of oil-insulated electrical transmission and distribution equipment, there are two major drawbacks with this approach. First, while the oil may be suitable for use immediately after processing, as soon as it is returned to service, contaminants once again start to form in the oil as the transformer, load-tap changer or voltage regulator is energized.
Of course, one approach to controlling this problem is to monitor the oil closely using electrical transmission and distribution equipment oil analysis. The saying “there’s no such thing as oil that is too clean” certainly applies here, with the life of contacts and other ancillary components closely tied to fluid condition and cleanliness.
The second reason why annual oil processing is becoming less desirable is the manpower and associated costs of performing routine preventive maintenance tasks. With modern utilities and other large industries running leaner and meaner than ever before, manpower costs, particularly during scheduled outages where labor costs are at a premium, are becoming the driving factor for changing the approach to ensuring equipment reliability.
The solution to controlling contaminants in insulating oils, without taking equipment out of service, is to install a dedicated online filtration system, running either continuously or semicontinuously.
The filtration system should be installed so that the oil is drawn from the bottom of the reservoir - where most contaminants collect - and passed through a high-quality depth media filter, returning close to the reservoir’s top. Typically, the filter element is comprised of a chemically impregnated cellulose media, with a beta rating of B3=150 and the ability to remove water to levels below 10 ppm.
Because most electrical equipment operates in remote sites and because the effect of draining the oil would be catastrophic, it is important to ensure that certain safeguards are put in place when installing an online filtration system on electrical transmission and distribution equipment.
These include installing the filtration system in such a way that any break in the suction line to the pump causes only minimal fluid to be spilled; ensuring the return line is below the surface level of the oil to prevent aeration; containing spills within the filtration system in the event of leakage from the filter housing or pump; and equipping the system with high and low-pressure alarms, automatic shut-off valves and supervisory control and data acquisition (SCADA) equipment to warn of system failure.
High-quality commercial filtration systems are designed with all of these safeguards and more in place and have been used to safely control contaminants without any serious safety-related problems.
Numerous U.S. and Canadian utility companies have used online oil filtration to successfully control contaminants and hence the insulating properties of electrical insulating oils. Table 2 shows the results of a trial of an online filtration system by Tucson Electric Power. In this trial, new oil was added to the load-tap changer in July 1995.
After only two months of service, water levels rose from 14 ppm to 51 ppm and concurrently, dielectric breakdown voltage dropped from 53 kV to 34 kV. In addition, significant arcing was occurring, giving rise to elevated levels of combustible gases, most notably hydrogen and acetylene. After the installation of an online filtration system, water levels dropped to 10 ppm, resulting in an increased dielectric.
However, perhaps what is most surprising is the drop in combustible gas levels. While there is no suggestion that the filtration system is actually filtering out gases, the removal of polar materials from the oil has increased the insulating properties of the oil, reducing the amount of combustible gases generated inside the load-tap changer.
Clearly, the proactive strategy of continuous, online filtration, in conjunction with careful monitoring of oil physical parameters and dissolved gas analysis was doing an excellent job in this instance of preventing problems by removing the root cause agent; in this case the water.
Figures 1 through 4 show a similar trial conducted by an electrical utility in the Western United States.
Figure 1. No Evidence of Coking
Figure 2. Clear Evidence of Coking
Figure 3. Visual inspection of the tap-changer from Bank #2 after 12,434 operations showing no signs of coking or oil degradation by-products
Figure 4. Visual inspection of the tap-changer from Bank #3 after 12,434 operations. Clear signs of coking can be seen on the tank bottom, along with varnish build-up on the contacts
The utility’s approach was to evaluate two identical load tap changers running under heavy load. Both systems had a history of oil-related problems, with operating temperatures often exceeding 60°C. One tap changer (Bank #2) was equipped with an online filtration system; the second (Bank #3) was left untouched. At the beginning of the trial, both load-tap changer oil compartments were opened, inspected and pressure-washed with solvent to remove all forms of contamination.
Both were then filled with new, prefiltered oil and put into service. After six months and an estimated 12,400 operations, oil analysis data showed little difference in both systems, with only a slight increase in acid number, perhaps indicating the onset of oil degradation. However, a visual inspection revealed a different story.
Photographs of the inside of the two oil compartments at the six-month inspection are shown in Figures 1 through 4. Despite the oil analysis data, it was clear to the utility that the tap changer on Bank #3, the tap changer without an online filtration system, was heavily contaminated with coking particles and filming due to a build-up of varnish on contacts resulting from oil degradation.
By contrast, the tap changer on Bank #2, protected by a dedicated online filtration system, showed no signs of filming or carbonaceous build-up. While the build-up in Bank #3 had not progressed enough to cause significant changes in the oil’s dielectric or other physical properties, coking and varnish build-up is a known precursor to insulating oil degradation and poor tap-changer reliability. Based on this study, the utility concluded that the use of dedicated online filtration offers significant life extension opportunities for both insulating oils and contacts.
With companies striving to run leaner and meaner, significant focus has been placed on extending both equipment and oil life, while reducing maintenance man-hours. To do this successfully, strong emphasis must be placed on recognizing and eliminating the causative effects of poor reliability.
For oil-insulated electrical T&D equipment, there is no doubt that overall fluid cleanliness plays a significant role in both component and oil longevity, making proactive fluid management an important part of an effective maintenance strategy for transformers, load-tap changers, voltage regulators and circuit breakers.