This testing is used to provide one of the most important early warning diagnostics for LTCs.<\/p>\n\n\n\n <\/p>\n\n\n\n An LTC failure can easily cause collateral damage to the transformer and decrease its lifespan. Thus, there is much emphasis placed on making sure the LTC operates correctly over time<\/p>\n<\/blockquote>\n\n\n\n <\/p>\n\n\n\n Although many of the common oil tests such as dielectric breakdown voltage, interfacial tension, acid content, particle count, metals analysis, etc. provide valuable information used in many LTC diagnostic programs offered by commercial and utility laboratories, this article will focus specially on DGA and water content.<\/p>\n\n\n\n Dissolved Gases in Oil:<\/strong> This is the most important oil test for LTC diagnostics. As insulating materials deteriorate byproduct gases are generated. Normal deterioration produces gassing patterns which are typical for a specific family which includes the model, type of breathing, and compartment. As problems manifest the gassing behavior changes which includes both the amount and most often the pattern or relative composition of gases in oil. Problems include localized overheating and\/ or excessive arcing and other abnormalities. Localized overheating of conductors and surrounding insulation may lead to carbonization and byproduct polymeric films forming on conductors, which creates a runaway thermal condition.<\/p>\n\n\n\n Water Content:<\/strong> Excessive water reduces the dielectric breakdown strength of the oil and can accelerate the aging of the contacts. Excessive water in the solid insulation can result in tracking and ultimately an insulation failure. The water content will vary depending on if the compartment is sealed to the air, free breathing or has a desiccant breather. Compartments with arcing contacts in oil will often be vented to reduce the combustible gas concentration generated in the operation of the device. In such cases, the relative saturation of the water in oil will be driven by the ambient humidity and altered by the difference in the temperature of the ambient air and the oil. Increasing the temperature of the oil above that of the ambient air lowers the relative saturation of water in oil. Under equilibrium conditions, expected amounts of water in oil can be quantified and therefore makes it possible to estimate what concentrations are excessive.<\/p>\n\n\n\n As the oils from LTCs can contain considerable byproducts from the deterioration process, it is very important to take care in sampling to avoid cross contamination and obtain a representative sample. Experts recommend flushing enough oil to remove condensation in the valve and excessive carbon and debris that has formed as sediment on the bottom of the LTC. If enough oil is not removed, then a \u201cfalse positive\u201d can occur where the analysis indicates a problem with the LTC where one really may not actually exist.<\/p>\n\n\n\n It is critical to use new tubing each time as plastic materials have a \u201cmemory effect\u201d where some of the gases dissolved in the oil will adsorb in the plastic only to desorb in the next sample that contains less of the gas. This can be particularly troublesome as the gases are not visible and different types of LTCs have very different normal gas concentrations. Cross contamination is of concern when taking samples from LTCs and thus the use of clean compatible sample containers is required.<\/p>\n\n\n\n Make sure the LTC is under positive pressure. This can be done by introducing a slug of oil into the tubing attached to the valve. Open the valve slowly and watch the movement of the oil slug. If it moves away from the valve, then there is positive pressure and flushing can continue. If the oil slug moves towards the valve, the valve should be immediately closed and the LTC should be brought back to positive pressure.<\/p>\n\n\n\n <\/p>\n\n\n\n Condition-based maintenance relies on tools which can detect most problems early enough that maintenance can be scheduled and performed. In most cases, condition-based testing is performed while the apparatus is in-service so that any down time is reduced or eliminated.<\/p>\n<\/blockquote>\n\n\n\n <\/p>\n\n\n\n As with any sample from electric apparatus, it is necessary to have sufficient identification that the test data can be linked to the equipment and proper diagnostics can be provided as many of the test results are based on the construction of the electric apparatus. This would include information from the nameplate such as manufacturer and serial number. For diagnostic purposes the minimum information that should be supplied with LTC samples is:<\/p>\n\n\n\n Other information such as if the unit has a filtering unit on it or when the LTC was last maintained or oil handled is useful information for those performing the interpretation of results.<\/p>\n\n\n\n The goal of diagnostics is to provide a ranking that prioritizes maintenance activities. This goes beyond the simple good\/bad distinction, to provide some grading to permit different management options.<\/p>\n\n\n\n The primary test for LTC diagnostics and condition assessment is that for dissolved gas-in-oil as this detects most of the problems. There are three main types of LTCs, reactive with arcing contacts in oil, resistive with arcing contacts in oil and arcing contacts in a vacuum bottle. There should be differences in the gassing behavior between resistive and reactive types as the shorter time of arc extinction of the resistive type (5-6 ms after contact separation) should lower the concentrations of gases generated. However, it has been our experience that the gassing behavior of different models of LTCs are so different that generic rules for reactive and resistive LTCs are not adequate. The primary diagnostic gases used to develop condition codes are methane, ethylene and acetylene. In addition, three ratios are used:<\/p>\n\n\n\n A matrix has been developed for each model that includes concentration limits for each of the three diagnostic gases plus the three ratios. Points are assigned for each limit and then summed, with a total point range between 0 and 15. Condition codes are then determined from the points as shown in Table 1. A Condition Code of 1 indicates an LTC in the worst possible condition that should be the number one priority. A Condition Code of 5 would indicate an apparatus in good condition and the lowest possible on the list for sample frequency.<\/p>\n\n\n\n Condition codes 3-5 have a range for sample frequency. This is based upon the experience that the time from inception to complete failure, particularly for thermal runaway conditions, is shortened considerably if overloading significantly beyond nameplate on a regular basis. The matrix is structured to find a variety of problems which may be detected by excessive gassing rate, change in gas ratios or both. For example, localized overheating of contacts or the reversing switch will generally show increasing combustible gas generation with ratios of gassing going from an arcing pattern to characteristics of high temperature overheating of oil. Excessive arcing between contacts is most likely to develop high gas concentrations until the later stages when heating occurs (causing the combustible gas ratios to change). Examples of causes of overheating include:<\/p>\n\n\n\n Excessive combustible gas buildup can result when the vent becomes plugged. This eventually leads to low oxygen contents as it is consumed in oxidation reactions and is not replenished. Typically, the ratios will remain normal unless another problem is present at the same time.<\/p>\n\n\n\n Various other thermal and electrical problems can also be detected depending upon the model of LTC.<\/p>\n\n\n\n The diagnostic matrix for selector compartments and vacuum type models that break under vacuum is different. The ratios do not apply in the same way for vacuum type LTCs and therefore, absolute concentration limits for the gases are relied upon to determine the condition codes. Condition codes are generated with the same type of ranking as for arcing contacts-in-oil compartments.<\/p>\n\n\n\n This diagnostic scheme has been very successful over the past years in determining which LTCs have problems that need remediation and many \u201csaves\u201d have been reported on an annual basis.<\/p>\n\n\n\n <\/p>\n\n\n\n DGA has gained wide acceptance in the industry for assessing the LTC condition and has become standard practice electrical networks all over the world.<\/p>\n<\/blockquote>\n\n\n\n Oil testing is just the first step in the diagnosis of LTCs with potential problems. There are a number of nonoil tests that have been employed to confirm or identify LTC problems. These include the following:<\/p>\n\n\n\n Infrared thermography and temperature differential:<\/strong> A frequently used method to detect or confirm overheating of contacts or the reversing switch in externally mounted LTCs is to determine the temperature difference between the main tank and the LTC. Normally the main tank should be operating at a higher temperature than the LTC compartment except for the occasional transient such as when the pumps initially come on to cool the main insulation. As thermal problems develop in the LTC compartment the oil temperature will consistently be higher than the main tank. This difference in temperature can be detected using continuous temperature monitors mounted to the tank wall or by periodic inspections using infrared thermography.<\/p>\n\n\n\n Electrical tests:<\/strong> There are a number of electrical tests that can be used to confirm or help identify the source of LTC problems before entering for visual inspection.<\/p>\n\n\n\n Acoustic and vibration analysis<\/strong>: Some investigators have developed a database of LTC signatures using acoustic analysis to complement diagnostic programs for LTCs.<\/p>\n\n\n\n <\/p>\n\n\n\n To illustrate the LTC diagnostic program several case studies are presented. The details of the LTC for this case are listed in Table 2. The DGA results in Table 3 show the combustible gas values were high and the ratios indicative of an abnormal condition. The OLTC was assessed as being in an emergency condition and immediate remedial action was needed. The concentrations of the hydrocarbon gases associated with overheating were extremely elevated and the ethylene\/ethane ratio was far more than what was considered normal.<\/p>\n\n\n\n These markers pointed to a severe thermal condition. Based on the DGA, an internal inspection was performed. Figure 1 shows the issue was on the stationary side of the reversing switch on one of the outside phases. Two of the good phases showing normal wear are given in Figure 2. The damaged part was replaced and the LTC returned to service.<\/p>\n\n\n\n DGA has gained wide acceptance in the industry for assessing the LTC condition and has become standard practice electrical networks all over the world. Doble has found that once oil diagnostic programs are started for LTCs, problems that were not being detected by other methods are revealed and, in many cases, sooner as the time intervals between sampling is shorter than most other maintenance or testing activities. After using oil diagnostics for all the LTCs in a system, the number of problems found the second time around is reduced showing the effectiveness of the program.<\/p>\n\n\n\n <\/p>\n\n\n\n Lance Lewand <\/strong>has been in the utility industry for 39 years, and with Doble Engineering for the past 32 years. He is the R&D and Technical Director for the Insulating Doble Insulating Materials Laboratories. Since joining Doble in 1992 he has published over 85 technical papers pertaining to testing and sampling of electrical insulating materials and laboratory diagnostics. He received his Bachelor of Science degree from St. Mary’s College of Maryland. He is actively involved in professional organizations including the American Chemical Society, a representative of the U.S. National Committee for TC10 of the International Electrotechnical Commission (IEC) and ISO TC28, ASTM D-27 since 1989, is the sub-committee chair 06 on Chemical Tests, former vice-chair of D-27, recipient of the ASTM Award of Merit for Committee D-27, and is current Chair of ASTM D-27 committee on insulating liquids. He is also vice-chair of IEEE C57.146 and 155 and is the chair for the new IEEE committees on cellulose degradation and corrosive sulfur, as well as involved in several CIGRE technical committees.<\/p>\n\n\n\n <\/p>\n\n\n\n Paul J. Griffin,<\/strong> now retired, started working for Doble in 1979 and worked there for 42 years. He held the position of Laboratory Manager before becoming Vice President of Laboratory Services. While at Doble, Paul Griffin has published over 70 technical papers pertaining to testing of electrical insulating materials and laboratory diagnostics.<\/p>\n\n\n\n <\/p>\n\n\n\n Harry Heulings<\/strong> is a Senior Chemist in the Professional Services group at Doble Engineering, focusing on condition assessment of critical electrical assets and large-scale fleet assessments using laboratory diagnostics. Harry was a member of the Doble Windfarm Sub-Committee and was the co-author of the Windfarm Subcommittee Report on Wind Turbine Step-Up Transformer Asset Management (2021). Prior to joining Doble in 2017, Harry held the position of Laboratory Manager at Morgan Schaffer USA and Intertek Testing Services and Research and Development Chemist at Rohm & Haas Company. Harry has a BA and MS in Chemistry from Rutgers University, and an MBA from Rutgers School of Business.\u00a0<\/p>\n","protected":false},"featured_media":12076,"parent":0,"template":"","meta":{"_acf_changed":false},"article-category":[4],"class_list":["post-12075","article-hub","type-article-hub","status-publish","has-post-thumbnail","hentry","article-category-in-focus"],"acf":[],"_links":{"self":[{"href":"https:\/\/transformer-technology.com\/wp-json\/wp\/v2\/article-hub\/12075","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/transformer-technology.com\/wp-json\/wp\/v2\/article-hub"}],"about":[{"href":"https:\/\/transformer-technology.com\/wp-json\/wp\/v2\/types\/article-hub"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/transformer-technology.com\/wp-json\/wp\/v2\/media\/12076"}],"wp:attachment":[{"href":"https:\/\/transformer-technology.com\/wp-json\/wp\/v2\/media?parent=12075"}],"wp:term":[{"taxonomy":"article-category","embeddable":true,"href":"https:\/\/transformer-technology.com\/wp-json\/wp\/v2\/article-category?post=12075"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}![\"Image\"\/](https://transformer-technology.com/"images//2025//06//23//figure-3-coking.jpeg/")
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![\"Image\"\/](https://transformer-technology.com/"images//2025//06//23//figure-4-poor-surface-contact.jpeg/")
<\/figure>\n\n\n\nOil Tests for LTCs<\/h3>\n\n\n\n
Sampling for LTCs<\/h3>\n\n\n\n
![\"Image\"\/](https://transformer-technology.com/"images//2025//06//23//figure-5-evidence-of-coke-formation.jpeg/")
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<\/figure>\n\n\n\nNomenclature for LTCs<\/h3>\n\n\n\n
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Diagnostics and Rankings (Condition Codes)<\/h3>\n\n\n\n
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![\"Image\"\/](https://transformer-technology.com/"images//2025//06//23//table-1.jpg/")
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Confirmation and Complimentary Test<\/h3>\n\n\n\n
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![\"Image\"\/](https://transformer-technology.com/"images//2025//06//23//table-2.jpg/")
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<\/figure>\n\n\n\nCase Study<\/h3>\n\n\n\n
Conclusions<\/h3>\n\n\n\n
![\"Image\"\/](https://transformer-technology.com/"images//2025//06//17//lancelewand-2.jpg/")
<\/figure>\n\n\n\n![\"Image\"\/](https://transformer-technology.com/"images//2025//06//17//paul-griffin.png/")
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