Oil-type transformers – also known as oil-cooled transformers – use mineral oil or ester fluids for the electrical insulation and cooling of the windings. For over a century, they have been the most widely used design for distribution and power transformers worldwide. Oil-type transformers have been continuously developed for over a century – modern units now have almost nothing in common with their predecessors from the 1960s.
How does the cooling of an oil-type transformer work?
The insulating oil flows around the hot windings, absorbs their heat, and releases it to the surroundings via radiators or coolers. Depending on the application, a distinction is made between passive convection (ONAN), forced air cooling (ONAF), and forced oil circulation cooling (OFAF, ODAF). As the cooling stage increases, a transformer can deliver higher continuous power.
Key Characteristics at a Glance
- Cooling by mineral oil or biodegradable ester fluids
- Ratings from a few kVA to over 1,000 MVA
- Multi-stage cooling (ONAN/ONAF/OFAF/ODAF)
- Proven technology with service lives of 40 years or more
- Oil containment basin and fire protection concepts for outdoor or indoor installation
- Hermetically sealed designs without a conservator
- Nitrogen blanket to prevent oxygen ingress
Frequently Asked Questions
What advantages does an oil-type transformer offer over a dry-type transformer?
Oil-type transformers are generally more compact, quieter, and better suited to very high power ratings. Heat dissipation under heavy load is also significantly more efficient. Dry-type designs, on the other hand, have their strengths in fire protection and indoor use.
What is dissolved gas analysis (DGA)?
In DGA, gases dissolved in the oil are analyzed. Certain gas patterns indicate internal faults such as partial discharges, overheating, or arcing. Regular DGA is a proven tool for preventive maintenance.
What advantages do hermetically sealed oil-type transformers offer?
They have no direct contact with the surrounding air, so oxygen and moisture cannot enter the oil. This significantly extends the oil’s service life and drastically reduces maintenance effort. Ideal for locations with unfavorable environmental conditions.
How is an oil-type transformer properly taken out of service?
First, the unit is de-energized and earthed, then the oil is drained into certified containers for reprocessing or disposal. The components are then dismantled, with copper, steel, and aluminum recycled separately. Environmentally sound disposal is legally required and thoroughly documented.
Do you need an oil-type transformer – whether distribution or power? Talk to our specialists about design, oil selection, and connection conditions.