Transformers
Oil immersed transformers
Liquid immersed transformers filled with mineral oil or an ester liquid, the usual choice outdoors and in dedicated transformer rooms.
At a glance
- Insulating liquid
- mineral oil or esterester where fire point or biodegradability matters
- Tank
- hermetically sealed or with conservatorthe design and the rating decide
- Cooling
- natural oil and air, commonly ONANfans add output on larger units
- Placement
- outdoors or a transformer roomliquid containment is set by local rules
- Standard
- IEC 60076-1IEEE C57.12.00 on North American specifications
An oil immersed transformer uses its insulating liquid for both insulation and cooling. It suits outdoor installation, and the choice against a dry type unit is usually made on site conditions, fire strategy and cost for the rating in question.
The liquid is the decision that follows. Mineral oil is the conventional choice. Natural and synthetic esters have a higher fire point and better biodegradability, and they are specified where a fire strategy, a water protection zone or an environmental policy asks for them. Where the unit stands, the containment that local rules require for the liquid volume is part of the civil design and is worth settling before the unit is ordered.
What gets specified
| Parameter | Typical options | What decides it |
|---|---|---|
| Insulating liquid | Mineral oil, natural ester, synthetic ester | Fire strategy, environmental requirements, owner preference |
| Tank design | Hermetically sealed, or free breathing with a conservator | Rating, maintenance philosophy and specification |
| Cooling | ONAN, with ONAF on larger units | The load profile and any overload requirement |
| Protective fittings | Pressure relief, liquid level and temperature indication, gas and pressure relays where specified | The protection scheme and the owner's standard |
| Bushings and cable boxes | Porcelain bushings, or cable boxes on either side | How the unit is connected on site |
| Losses | At or below the limit of the destination market | Regulation and running cost |
Typical options are the ones buyers most often specify. They are not a list of what is available, which is confirmed per enquiry.
Applicable standards
| Standard | What it covers |
|---|---|
| IEC 60076-1 | Power transformers, general requirements |
| IEC 60076-7 | Loading guide for mineral oil immersed power transformers |
| Regulation (EU) No 548/2014, as amended | Maximum losses for liquid immersed transformers placed on the EU market, Table I.1 |
| IEEE C57.12.00 | General requirements for liquid immersed transformers, used on North American specifications |
The standard that applies is set by the specification and the destination market. Editions change, so the edition cited in a specification is confirmed in the quotation.
Documents normally requested
| Document | What it evidences | When it is normally provided |
|---|---|---|
| Technical datasheet | Rating, voltages, losses, impedance, dimensions and weight of the unit offered | With the quotation |
| Outline drawing | Footprint, lifting points and cable connection positions | With the quotation or at order |
| Routine test report | The individual unit's tests to its standard | With the delivered unit |
| Type test report | The design has passed the type tests of its standard | On request, where a type test exists for the design |
| Rating plate details | The declared values carried on the unit itself | With the delivered unit |
| Declaration of conformity | Conformity with the legislation of the destination market | Where the destination market requires one |
| Operating and maintenance manual | Installation, operation and maintenance instructions | With the delivered unit |
Which documents apply depends on the destination market, the standard and the manufacturer. Documents are confirmed in the quotation for the specific unit offered.
What we need to quote accurately
| 01 | Rated power in kVA, or the connected load and the largest motor if the rating is not fixed yet |
| 02 | Primary and secondary voltage, and the tapping range if the network operator sets one |
| 03 | Phase, frequency and the vector group, or the earthing arrangement it has to suit |
| 04 | Construction: liquid immersed or dry type, and whether oil is acceptable where the unit will stand |
| 05 | Indoor or outdoor, ambient temperature, altitude and any corrosive or dusty exposure |
| 06 | The standard and the loss requirement the destination market applies |
| 07 | Quantity, destination and the date the equipment has to be on site |
| 08 | Whether surplus or refurbished units are acceptable, or new only |
| 09 | The insulating liquid required, and any containment or fire point requirement behind it |
Delivery and logistics
Distribution equipment is heavy, and a delivery date is only real once the site is ready for it. The dimensions and weight of the unit offered decide the vehicle, the lifting equipment and the route to the foundation, and they come from the manufacturer's drawing rather than from the rating.
Liquid immersed transformers are usually shipped filled, which affects transport rules and the handling on arrival. Anything crossing a border also carries customs documentation, and where the destination applies its own conformity or energy efficiency rules the paperwork has to match that market rather than the country of manufacture.
Lead times vary widely with rating, specification and manufacturer loading. We state a delivery basis in each quotation for the specific unit offered and do not publish a general lead time we cannot hold.
Common questions
What is the difference between hermetically sealed and conservator types?
A hermetically sealed tank has no contact between the liquid and the outside air, and the tank accommodates expansion itself. A conservator type uses a separate expansion vessel and breathes through a dehydrating breather. The choice depends on the rating, the design and the maintenance approach the owner prefers.
Related
Request a quote for this equipment
Send the rating, the voltage and the destination, or submit the specification, drawing or nameplate. Anything missing is raised before it reaches a supplier.