Transformers
Oil immersed transformers
An oil immersed transformer is filled with mineral oil or an ester liquid. It is the usual choice outdoors and in dedicated transformer rooms.
- 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
The liquid both insulates and cools. The unit suits outdoor installation, and the choice against a dry type unit usually turns on site conditions, fire strategy and cost at the rating in question.
The liquid is the next decision. Mineral oil is the conventional choice. Natural and synthetic esters have a higher fire point and better biodegradability, and a fire strategy, a water protection zone or an environmental policy can call for them. The containment that local rules require for the liquid volume belongs to the civil design, so settle it before the order.
In the UK this is the construction the network operators adopt. ENA TS 35-1, now Issue 8 of March 2026, specifies two winding, liquid immersed, naturally cooled units from 25 to 2000 kVA, in separate parts for ground and pole mounting. In Dubai, DEWA's standards set 50 °C as the outdoor temperature the equipment must work in.
The tank decides how the unit breathes. A hermetically sealed tank is closed to the air and takes up the liquid's expansion in flexible walls or a gas cushion, so the liquid never meets the atmosphere. A conservator tank breathes through a dehydrating breather and can carry a Buchholz relay, which detects the gas a developing fault produces. The protection scheme you already run often settles the choice.
What gets specified
| Parameter | Typical options | What decides it |
|---|---|---|
| Insulating liquid | Mineral oil, natural ester, synthetic ester | Fire strategy, water protection rules and the owner's policy |
| Tank design | Hermetically sealed, or free breathing with a conservator | Rating, maintenance approach and the owner's standard |
| Cooling | ONAN, with ONAF on larger units | Whether fans may add rating for peaks; IEC 60076-7 guides loading above nameplate |
| Protective fittings | Pressure relief, liquid level and temperature indication; a gas and pressure relay, or a Buchholz relay on conservator types | 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 | Tier 2 ceilings of Table I.1, the liquid immersed table | Regulation at the destination, and the running cost of a unit that will often stay in service for decades |
Temperature rise limits, and what a hot site does to them
| Limit, IEC 60076-2 Table 1 | Normal site | At a 50 °C site |
|---|---|---|
| Top liquid | 60 K | 50 K |
| Average winding, ON and OF cooling | 65 K | 55 K |
| Winding hot spot | 78 K | 68 K |
A normal site reaches at most 40 °C, 30 °C in the hottest month and 20 °C over the year. Where it exceeds any of these, every limit falls by the excess (clause 6.3.2), so a 50 °C site takes 10 K off each. The right hand column is that arithmetic.
Applicable standards
| Standard | What it covers |
|---|---|
| IEC 60076-1 | General requirements, read with Part 7 for loading above nameplate |
| IEC 60076-7 | Loading guide for mineral oil immersed power transformers |
| IEC 60296 | Mineral insulating oils for electrical equipment, the specification for the unused oil |
| IEC 62770 and IEC 61099 | Unused natural esters, and unused synthetic organic esters, where an ester replaces mineral oil |
| 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 |
Information particular to a filled unit
| Item | Why it matters |
|---|---|
| Liquid type, standard and volume | The liquid to IEC 60296, IEC 62770 or IEC 61099; the volume sizes the containment local rules require |
| Total mass filled | Decides the crane, because the unit normally arrives filled |
| Fittings schedule | Which relay, pressure relief and indicators are fitted, and their contacts for your protection |
What an oil immersed transformer request needs
- Mineral oil, natural ester or synthetic ester, and the reason when it is not mineral oil
- Hermetically sealed or conservator type, where the owner's standard fixes it
- The protective devices to fit and the contacts they report to
- Bushings or cable boxes on each side
- Whether containment is already designed, and the liquid volume it allows
Delivery and the transformer room
A liquid immersed unit normally travels filled and ready to connect. The transport rules for its liquid apply on the road, the lifting plan uses the filled mass, and the plinth and containment have to be ready before the vehicle arrives.
Indoors, the room is part of the specification. Kahramaa in Qatar asks for naturally ventilated oil transformer rooms with openings of at least 3.00 m² per 1,000 kVA and walls and doors rated for three hours of fire. In the UK, regulation 11 of the ESQCR requires generators and distributors to take all reasonable precautions to minimise the risk of fire from their substation equipment.
Specification template
Fill it in on screen or by hand, then upload it with your request.
Questions buyers ask
What is the difference between hermetically sealed and conservator types?
In a hermetically sealed tank the liquid never meets the outside air, and the tank takes up the expansion itself. A conservator type uses a separate expansion vessel and breathes through a dehydrating breather. The rating, the design and the owner's maintenance approach decide between them.
When is an ester worth its price?
When the fire strategy, a water protection zone or the owner's environmental policy asks for a higher fire point or better biodegradability than mineral oil. Without such a requirement, mineral oil remains the conventional choice.