Oil-Immersed vs Cast-Resin Transformers: Fire, Environment, Losses and Selection

Distribution transformers at medium voltage are built in two main ways. In one, the core and windings sit in a tank filled with an insulating liquid. In the other, they are cooled by air, with the windings encapsulated in epoxy resin. Both are mature technologies covered by the IEC 60076 series. The right choice depends less on which is “better” and more on where the transformer will stand, what fire and environmental rules apply there, how it will be loaded, and how it will be maintained.

This article answers the question: When should a project choose an oil-immersed transformer, and when a cast-resin dry-type transformer, and what should the specification state for each?

How the two technologies are built

Oil-immersed transformers

The core and windings are placed in a steel tank filled with an insulating liquid that both insulates and carries heat to the tank walls and radiators. The liquid may be mineral oil (IEC 60296), a natural ester (IEC 62770) or a synthetic ester (IEC 61099). Distribution transformers are often hermetically sealed, with the tank fully filled and expansion taken up by flexible corrugated walls. Larger units more often use a conservator. Cooling is usually ONAN (oil natural, air natural), with ONAF (forced air) used to increase output. With ester liquids the first letter of the cooling code becomes K.

General requirements are in IEC 60076-1, and temperature-rise limits for liquid-immersed transformers are in IEC 60076-2.

Cast-resin dry-type transformers

A dry-type transformer has no insulating liquid. In a cast-resin design, the HV windings are cast in epoxy resin under vacuum, and the LV windings are typically foil or strip with resin impregnation or encapsulation. Cooling is by natural air (AN), and fans may be added for forced-air cooling (AF) to increase output temporarily or continuously. Cast-resin is one form of dry-type construction; others, such as vacuum-pressure-impregnated windings, are also covered by IEC 60076-11, which applies to dry-type transformers with highest voltage for equipment up to 72.5 kV.

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Side-by-side comparison

Fire behaviour and location

Fire safety is often the deciding factor, but local regulations, not the transformer standard, decide what is acceptable in a given building.

For liquid-immersed transformers, IEC 61039 classifies insulating liquids by fire point. Class O covers liquids such as mineral oil with a fire point below 300 °C. Class K covers higher-fire-point liquids, such as many ester liquids, and class L covers liquids with no measurable fire point. Higher-fire-point liquids reduce, but do not remove, the fire risk. The quantity of liquid, the need for spill containment, fire separation and distance from buildings remain project and code questions.

For dry-type transformers, IEC 60076-11 defines fire behaviour classes F0 (no special fire risk to consider) and F1 (transformers subject to a fire hazard, with restricted flammability and limited emission of toxic substances and opaque smoke). In the 2018 edition, the fire behaviour test is carried out on a test transformer of not more than 1000 kVA, as part of a test sequence with the climatic and environmental tests; it is not performed on every rating. For other ratings, the F1 declaration relies on that certified test sequence under the conditions specified in IEC 60076-11, so ask which test report supports it.

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If the transformer will be inside an occupied building, near escape routes, or in a basement, a cast-resin transformer with F1 classification is often the starting point. If it will stand outdoors or in a separate substation, an oil-immersed transformer is often simpler and more robust.

Climate, environment and enclosure

Cast-resin windings are exposed to the surrounding air. IEC 60076-11 therefore classifies dry-type transformers by:

• Climatic class (C1 to C5), which defines the lowest ambient temperature for operation and for transport and storage. C1 corresponds to operation down to −5 °C and transport and storage down to −25 °C; C2 to operation, transport and storage down to −25 °C. The 2018 edition added classes C3 to C5 for colder conditions, as specified in IEC 60076-11.
• Environmental class (E0 to E4 in the 2018 edition, which redefined the earlier E0 to E2 and added E3 and E4), which reflects exposure to condensation and pollution, from clean, dry installations to frequent condensation with heavy pollution. The 2018 edition also has separate classes for unenclosed outdoor units.

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Choose the environmental class from real site conditions: coastal air, dust, unheated rooms and periods de-energised in humid weather all increase the risk of surface tracking. Where a dry-type transformer is placed outdoors or in a kiosk, the enclosure’s degree of protection, ventilation and solar gain must be considered together, because an enclosure reduces cooling.

Oil-immersed transformers protect their active parts inside a sealed tank, so they are less sensitive to humidity and pollution. Their external bushings, corrosion protection and paint system still need to suit the site. Defining Site and Service Conditions explains how to document these factors.

Temperature rise, loading and overload

Both technologies are rated at a continuous load within temperature-rise limits based on ambient temperature assumptions in IEC 60076-1. If the site ambient is higher, or the altitude above 1000 m, the rating or the limits must be adjusted. Declare these conditions rather than leaving the manufacturer to assume standard values.

• Oil-immersed transformers have a large thermal mass, so they tolerate short overloads well. IEC 60076-7 gives a method for estimating hot-spot temperature and insulation ageing under cyclic loading and emergency overloads.
• Cast-resin transformers are designed to insulation thermal classes such as 155 (F) or 180 (H), with average winding temperature-rise limits that follow from the class in IEC 60076-11. They respond faster to overloads. Forced-air fans can increase output, but the fans and their supply then become part of the reliability chain. IEC 60076-12 is the loading guide for dry-type transformers.

Non-linear loads, such as drives, UPS systems, rectifiers and LED lighting, add harmonic losses to both types. IEC 60076-11 expects the purchaser to declare harmonic content. If the transformer will be energised frequently, state the expected number of energisations per year.

Losses and efficiency

Both technologies can be designed to a range of loss levels. Compare offers on guaranteed no-load and load losses at stated reference conditions, with their tolerances, and use a loss capitalisation method where the client has one. Some regions set minimum efficiency or maximum loss levels for transformers placed on the market. Where these apply, state the applicable rule in the specification, and avoid general assumptions about which technology is more efficient.

Size, weight, noise and installation interfaces

For the same rating and voltage, footprint, weight and sound level differ between technologies and between manufacturers. Compare declared values rather than assuming. Sound levels are determined to IEC 60076-10; state a limit if the installation is near occupied spaces.

Practical interfaces to agree early:

• Enclosure for dry-type units: degree of protection, ventilation openings and access doors interlocked with the MV supply if required.
• Clearances from cast-resin windings to walls and enclosure. The resin surface is not earthed and must be treated as live when energised.
• Cable or busbar connections on HV and LV sides, including busduct flanges for close-coupled LV switchboards.
• Liquid containment for oil-immersed units: bund or pit, and drainage.
• Lifting, transport and handling, including access routes into buildings for dry-type units.

Monitoring and maintenance

Oil-immersed transformers are commonly fitted with liquid level and temperature indicators. Hermetically sealed units often use an integrated protection device that detects gas, pressure and temperature; conservator types use a gas-actuated relay. Periodic analysis of the liquid, including dissolved gases, is a useful condition-assessment tool for larger units.

Cast-resin transformers are fitted with temperature sensors in the windings, connected to a temperature monitoring relay that controls fans and gives alarm and trip signals. Maintenance focuses on cleanliness, connection tightness and checks of the monitoring system. Dust and condensation are the main enemies.

Frequency and IEC vs ANSI/IEEE practice

A transformer is designed for its rated frequency. A 50 Hz design used at 60 Hz, or the reverse, changes flux density, losses and impedance, so use at the other frequency must be confirmed by the manufacturer. In North American practice, liquid-immersed and dry-type distribution transformers are specified under the IEEE C57.12 series, with different standard ratings, temperature-rise conventions and nameplate practice. Use one system consistently in a specification and do not combine values taken from both.

How to decide

For guidance on choosing the rating itself, see Sizing a Distribution Transformer: A Practical Checklist. For the broader difference between distribution and power transformers, see Distribution vs Power Transformers: Ratings, Construction and Uses.

Specification and procurement considerations

Whichever technology is chosen, state:

• rated power, voltage ratio, tappings, vector group, impedance and frequency;
• technology: oil-immersed (with liquid type) or dry-type (cast-resin or other), and cooling method;
• installation: indoor or outdoor, enclosure requirements and degree of protection;
• site conditions: ambient temperature range, altitude, humidity and pollution;
• for dry-type: climatic, environmental and fire behaviour classes, insulation thermal class and temperature monitoring;
• for oil-immersed: sealed or conservator design, protection devices, containment needs;
• guaranteed losses and the loss evaluation method, or the efficiency rule that applies;
• sound level limit, if relevant;
• harmonic content and expected energisation frequency;
• HV and LV termination arrangements and interfaces with switchgear;
• type tests and special tests required, and the routine tests to witness.

Preparing an RFQ for Electrical Power Equipment gives a general structure for presenting this information.

Common specification mistakes

• Specifying “dry-type” without classes. Without C, E and F classes, offers can differ widely in suitability for the site.
• Putting an indoor dry-type unit outdoors without considering the enclosure. The enclosure affects cooling and therefore rating.
• Assuming any liquid is acceptable indoors. Fire point class, liquid quantity and local rules determine what is allowed.
• Comparing prices without comparing losses. Lower purchase price can mean higher losses for the life of the transformer.
• Ignoring harmonic loads. Harmonics increase losses and temperature in both technologies.
• Forgetting touch safety for cast-resin units. Resin surfaces are not safe to touch when energised; enclosures or barriers are needed.
• Assuming 50 Hz and 60 Hz designs are interchangeable. Confirm the rated frequency explicitly.

Conclusion

Oil-immersed and cast-resin dry-type transformers each have a natural home. Cast-resin suits indoor, occupied and fire-sensitive locations where its classes match the environment. Oil-immersed suits outdoor and compact-substation installations where its thermal robustness and sealed construction are valuable. Decide on location, fire and environment first, then specify the classes, losses and interfaces that make offers comparable.

Related AdepaTrade solution

AdepaTrade sources oil-immersed and cast-resin dry-type distribution transformers and power transformers up to 36 kV, matched to the project’s ratings, losses and site conditions. Transformers

Relevant standards

IEC standards are the primary reference. Confirm the applicable edition for each project; local fire and environmental rules may add requirements.