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Choosing between a dry type transformer and an oil immersed transformer is one of the first — and most expensive — decisions in any medium voltage project. The right answer depends on where the unit will be installed, what load it will serve, and how much maintenance your team can realistically perform. This transformer selection guide compares both technologies across fire safety, cost, efficiency and maintenance, then applies the comparison to three common scenarios: indoor commercial buildings, industrial plants, and utility-scale solar projects.
A dry type transformer uses epoxy resin or vacuum pressure impregnated (VPI) insulation, with air as the cooling medium — no liquid at all. In cast resin transformer designs, the windings are fully encapsulated in flame-retardant, self-extinguishing epoxy. Known in French-speaking markets as transformateur sec and in Spanish-speaking markets as transformador seco, these units are the default choice wherever fire safety and low maintenance come first.
An oil immersed transformer (also specified as an oil cooled transformer or liquid-filled transformer) surrounds its windings with mineral oil, which simultaneously insulates the conductors and transfers heat to the tank wall and radiator fins. Chuanli Electric manufactures both technologies — you can browse the full power transformer range for rated capacities, voltage classes and datasheets.
Chuanli Electric’s SC(B)13/14/18 series epoxy cast resin transformer line covers 10 kV, 20 kV and 24 kV grades with rated capacities from 30 kVA up to 2,500 kVA. The high-voltage winding uses copper wire and the low-voltage winding uses copper foil, encapsulated in glass-fibre-reinforced resin. Because the coil does not absorb moisture and the core clamps carry a special corrosion-protection layer, these units operate reliably even at 100% relative humidity.
The trade-offs: cast resin units cost more per kVA than liquid-filled equivalents, and outdoor installation requires a protective IP-rated enclosure because the resin surface is not designed for direct weather exposure.
Explore the SC(B)13/14/18 resin-insulated dry type transformer specifications, including the full no-load and load loss tables for each efficiency level.
An oil immersed transformer remains the most economical way to move serious power outdoors. Chuanli Electric’s fully sealed 6–24 kV and 35 kV oil-immersed units are built to GB 1094 and GB/T 6451, fully compatible with IEC 60076, with CE certification available for export projects. The corrugated fin tank provides both cooling and a “breathing” function that keeps the oil sealed against moisture across a −25°C to +40°C ambient range.
The trade-offs: mineral oil is combustible, so indoor installation demands a dedicated transformer room with oil containment and fire protection; and the units require periodic oil sampling and dissolved gas analysis (DGA) as part of a preventive maintenance program.
See the 6–24 kV oil immersed transformer datasheet for cooling classes, protection devices and operating conditions.
| Selection Factor | Dry-Type (Cast Resin) | Oil-Immersed |
|---|---|---|
| Insulation & cooling | Epoxy resin / air (AN, optional AF) | Mineral oil (ONAN, optional ONAF) |
| Fire risk | Very low — self-extinguishing resin | Combustible oil; containment required |
| Indoor installation | Direct, next to the load | Dedicated transformer room / vault |
| Maintenance | Minimal — visual checks, cleaning | Oil sampling, DGA, seal inspection |
| Typical ratings | 10/20/24 kV, 30–2,500 kVA | 6–35 kV, up to utility substation sizes |
| Efficiency benchmark | SCB18: Level 1, no-load loss −20–25% vs SCB13 | Low-loss designs per GB/T 6451 |
| Upfront cost per kVA | Higher | Lower |
| Best suited for | Indoor, commercial, tunnels, clean processes | Outdoor, industrial, solar, utilities |
For shopping centres, hospitals, data centres, offices and metro systems, the dry type transformer is almost always the correct answer. Fire codes in most jurisdictions either prohibit or severely restrict bulk mineral oil inside occupied buildings, while a cast resin unit can sit in a ventilated electrical room directly beside the load — shorter low-voltage cable runs, lower I²R losses, and no oil containment pit to build. Specify an IP3X or IP4X enclosure and confirm ambient temperature and altitude against the nameplate ratings before ordering.
For petrochemical, metallurgy, mining and utility distribution, the oil immersed transformer wins on cost per kVA, overload tolerance and environmental robustness. Outdoor placement eliminates the fire-protection burden, and corrugated-fin sealed tanks shrug off dust, pollution degree Class III atmospheres and coastal corrosion. For heavier industrial loads at 35 kV, specify an on-load tap changer to hold voltage stable across shifting process loads. The exception is clean production — food, pharmaceutical and electronics plants often still prefer dry-type units inside the process building and oil-filled units only at the incoming substation.
Utility-scale solar combines both technologies. At the inverter stations, indoor cast resin transformer units sit inside the skid or container, close to the inverters where their fire safety and maintenance-free operation matter most. For the medium-voltage collection system, outdoor oil immersed transformer units in pad-mounted or compact substation format step voltage up from roughly 0.8 kV to 35 kV for the collector grid. A hermetically sealed, low-loss pad-mounted design minimises the losses that compound across 25 years of daily generation cycles.
Chuanli Electric supplies dedicated outdoor units for these applications — review the three-phase pad-mounted transformer designed for solar collector systems and underground network distribution.
Not Sure Which Transformer Fits Your Project?
Send us your load profile, primary/secondary voltage and installation environment. Our engineers — suppliers to China’s State Grid on projects including the Three Gorges and West–East Power Transmission programs — will recommend the optimal dry-type or oil-immersed solution with a complete loss table and quotation within 24 hours.
Purchase price is only the entry point of the dry type vs oil immersed decision. Three cost layers sit behind it:
Dry-type (cast resin) transformers. They contain no flammable liquid and release no toxic gases at high temperature, so they can be installed in an electrical room next to the load. An oil immersed transformer indoors requires a dedicated transformer vault with oil containment and fire protection under most fire codes.
ONAN (Oil Natural Air Natural) means oil circulates by natural convection and the tank dissipates heat naturally — the standard rating of an oil immersed transformer. ONAF (Oil Natural Air Forced) adds fans to increase the continuous rating by roughly 25–50% on the same tank.
Both technologies offer low-loss designs; the efficiency class matters more than the cooling medium. Chuanli Electric’s SCB18 cast resin series reaches Level 1 energy efficiency with 20–25% lower no-load losses than SCB13, and the oil-immersed range is built to low-loss GB/T 6451 designs. Always compare guaranteed loss tables (no-load W and load W at 75°C) rather than technology labels.
Usually both, at different points in the system: dry-type cast resin units inside the inverter stations for fire safety and low maintenance, and sealed oil-immersed pad-mounted units along the medium-voltage collector circuit for outdoor economy and environmental tolerance.
“Transformateur sec” (French) and “transformador seco” (Spanish) both mean dry type transformer — the same epoxy-resin or VPI-insulated, air-cooled unit discussed in this guide. If your project specification uses these terms, request the cast resin product line.
With correct loading and maintenance, an oil immersed transformer typically serves 25–30 years, and a cast resin dry type transformer 20–25 years. Insulation temperature is the main life determinant: every 6–8 K of sustained overload above rated temperature roughly halves winding insulation life.
There is no universal winner in the dry type vs oil immersed comparison. Indoors and wherever fire safety, proximity to load and low maintenance dominate, choose cast resin. Outdoors, and wherever cost per kVA, overload capability and environmental robustness dominate, choose oil-immersed. On complex sites — solar farms, industrial campuses, mixed-use developments — the most economical design often uses both technologies in the roles they each do best.
Request a Transformer Quotation from Chuanli Electric
IEC 60076-compliant cast resin and oil-immersed transformers, 6–35 kV, 30–2,500 kVA and beyond. Type test reports, efficiency level documentation and export CE certification available on request.
