{"id":1825,"date":"2026-08-22T02:34:02","date_gmt":"2026-08-21T18:34:02","guid":{"rendered":"https:\/\/q8bbzjnpm5.wpdns.site\/?p=1825"},"modified":"2026-08-29T23:43:46","modified_gmt":"2026-08-29T15:43:46","slug":"dry-type-and-oil-immersed-transformers-in-depth-analysis-of-differences","status":"publish","type":"post","link":"https:\/\/subian-electric.com\/es\/news\/dry-type-and-oil-immersed-transformers-in-depth-analysis-of-differences\/","title":{"rendered":"Dry-Type and Oil-Immersed Transformers: In-Depth Analysis of Differences"},"content":{"rendered":"

The engineer’s consultant has done this comparison chart many times in the past, but the question from the client has not changed: “We are making a 6-story hospital, with an MRI wing, a solar rooftop panel, and a switch room in the basement above the parking space. What transformer do we buy?” The answer is not a name or a trade mark, but rather a variety of choices between cost, fire safety, maintenance, efficacy and lifespan. Hospitals, data centers, skyscrapers, and tunnels opt for dry-type transformers, and factories use the oil-immersed ones. Why is this so, is what this article covers.<\/p>\n

In this article, we compare oil-immersed transformers and dry-type transformers based on nine characteristics, providing the minimum required information and prices based on IEC 60076 standard. The article includes a detailed side-by-side table, with costs of ownership and comparisons, that\u2019s why you end up as a real expert who can make your project colleagues understand a choice of a transformer.<\/p>\n

Quick answer: The main distinction between dry-type transformers and oil-immersed transformers lies in their cooling and insulation process: whereas dry-type transformers rely on either air, resin or vacuum pressure impregnation, oil-immersed transformers use mineral oil for insulation. Although clients are able to save 25-40% in cost with oil-immersed transformers, dry-type transformers are free from fire and oil containment risks that are the leading factors causing popularity of dry-type transformers in various places.<\/p><\/blockquote>\n

\"Dry<\/p>\n

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The Basics: What Each Technology Is<\/h2>\n

Though both types of transformers perform the same function of electromagnetic induction-based voltage transformation, they employ different methods of insulation and cooling. The cooling of a dry-type transformer is achieved in air (AN\/AF classes), and the insulation is either in the form of solid cast-resin windings, where the coil is cast in epoxy and silica-rich filler, or through the VPI windings, in which the coil is treated with varnish under vacuum pressure.<\/p>\n

In contrast to that, oil-immersed transformers use oil in both the insulation process and for heat conduction to the tank. The difference is said to not be only superficial. The dry-type transformer needs to be much bigger and made of more copper because air is not as good of a thermal conductor as oil. Moreover, cast-resin types yield temperature rises of 100\u00b0C (F class) or 125\u00b0C (H class) according to IEC 60076-11, compared to 65\u00b0C of oil-immersed types (IEC 60076-2).<\/p>\n

Cooling and Insulation: Air vs Oil<\/h2>\n\n\n\n\n\n\n\n\n\n
Aspecto<\/th>\nTipo Seco<\/th>\nSumergido en Aceite<\/th>\n<\/tr>\n<\/thead>\n
Medio de refrigeraci\u00f3n<\/td>\nAir (natural or forced)<\/td>\nMineral oil, ester, or silicone<\/td>\n<\/tr>\n
Cooling classes<\/td>\nAN, AF, ANAF<\/td>\nONAN, ONAF, OFAF<\/td>\n<\/tr>\n
Winding insulation<\/td>\nCast epoxy resin or VPI varnish<\/td>\nOil-impregnated kraft paper<\/td>\n<\/tr>\n
Aumento de temperatura<\/td>\n100 K (F), 125 K (H) typical<\/td>\n65 K oil \/ 78 K winding (IEC)<\/td>\n<\/tr>\n
Flammability<\/td>\nSelf-extinguishing (resin)<\/td>\nMineral oil is combustible; esters are less so<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

\"Cooling<\/p>\n

The fire aspect is more important than any other aspects. Cast-resin transformers are self-extinguishing and pass IEEE 1414\/BS 476 flame tests, which is the main reason why NFPA 70 and most national codes allow them in buildings without a fire-rated vault. Mineral-oil units need either a vault, fire barrier, or 3\u20135 m distance from flammable material. For undergroth and skyscraper projects, this has often been the only consideration.<\/p>\n

Nine-Dimension Comparison Table<\/h2>\n\n\n\n\n\n\n\n\n\n\n\n\n\n
Dimension<\/th>\nTipo Seco<\/th>\nSumergido en Aceite<\/th>\nWinner<\/th>\n<\/tr>\n<\/thead>\n
First cost per kVA<\/td>\nHigher<\/td>\nLower (25\u201340% less)<\/td>\nOil<\/td>\n<\/tr>\n
Fire \/ explosion risk<\/td>\nLow, self-extinguishing<\/td>\nCombustible liquid<\/td>\nDry<\/td>\n<\/tr>\n
Indoor approval<\/td>\nNo vault needed in most codes<\/td>\nVault\/barrier often required<\/td>\nDry<\/td>\n<\/tr>\n
Overload capability<\/td>\nLimited beyond nameplate<\/td>\nGood thermal mass<\/td>\nOil<\/td>\n<\/tr>\n
Maintenance burden<\/td>\nVisual checks, fan servicing<\/td>\nOil sampling, DGA, leak checks<\/td>\nDry<\/td>\n<\/tr>\n
Humidity sensitivity<\/td>\nHigher; needs dry room<\/td>\nSealed against moisture<\/td>\nOil<\/td>\n<\/tr>\n
Vida \u00fatil<\/td>\n20\u201330 years<\/td>\n25\u201340 years<\/td>\nOil<\/td>\n<\/tr>\n
Ruido<\/td>\nQuieter (no oil pumps)<\/td>\nCan be noisier with fans<\/td>\nDry (marginal)<\/td>\n<\/tr>\n
Efficiency at high ratings<\/td>\nLarger, more copper needed<\/td>\nCompact for same rating<\/td>\nOil<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Total Cost of Ownership: Real Math<\/h2>\n

The cost of buying first transformer is only a starting point for the whole analysis. Now we will analyze a 1,000 kVA transformer operating at an industrial facility with a load factor of 60% producing losses of $0.12\/kWh for the period of twenty years. The transformer losses will be calculated based on IEC 60076 International Standard.<\/p>\n\n\n\n\n\n\n\n\n\n\n\n
Cost Component<\/th>\nTipo Seco (Resina Fundida)<\/th>\nSumergido en Aceite<\/th>\n<\/tr>\n<\/thead>\n
First cost (FOB, 10\u201335 kV)<\/td>\n$20,000\u2013$45,000<\/td>\n$18,000\u2013$38,000<\/td>\n<\/tr>\n
P\u00e9rdida en vac\u00edo<\/td>\n\u2248 2,000\u20132,600 W<\/td>\n\u2248 1,500\u20132,000 W<\/td>\n<\/tr>\n
Load loss @ 60% load<\/td>\n\u2248 6,000\u20138,500 W<\/td>\n\u2248 5,500\u20137,500 W<\/td>\n<\/tr>\n
Annual loss cost (@$0.12\/kWh)<\/td>\n\u2248 $8,400\u2013$11,600<\/td>\n\u2248 $7,400\u2013$10,000<\/td>\n<\/tr>\n
20-year loss cost<\/td>\n\u2248 $168,000\u2013$232,000<\/td>\n\u2248 $148,000\u2013$200,000<\/td>\n<\/tr>\n
Maintenance over 20 years<\/td>\n\u2248 $4,000\u2013$9,000<\/td>\n\u2248 $12,000\u2013$22,000 (oil tests, reconditioning)<\/td>\n<\/tr>\n
Approx. 20-year total<\/td>\n$192,000\u2013$286,000<\/td>\n$178,000\u2013$260,000<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Two things immediately stand out. First of all, losses exceeds expenditures, which is why choosing based on price alone is a wrong move. Secondly, the 20-year figures show that the total costs are very similar to one another, due to the lower maintenance costs of the dry-type motor that compensate for the higher price. Although oil-immersed motors still get better lifetime costs in comparison to the dry type, the difference is not as big as the first-cost difference suggests.<\/p>\n

Ratings and Efficiency: What the Standards Say<\/h2>\n