Types of Oil-Immersed Transformers: Construction, Phases, Cooling and Duty

Overview of Oil-Immersed Transformer Types

Oil-immersed transformers can be categorized by construction, phase configuration, cooling method and application. Understanding these categories helps buyers and engineers specify the right transformer for a substation, a distribution network or an industrial plant. The four main axes are construction (core-type or shell-type), phase (single-phase or three-phase), cooling (ONAN, ONAF, OFWF, OFAF) and duty (power or distribution).

Classification by Construction

Core-type transformers have windings that surround the magnetic core limbs. This layout is common for high-voltage power transformers because it allows efficient cooling of the windings and easy insulation of the high-voltage side. Shell-type transformers enclose the windings within the core, with magnetic flux passing through the outer shell. The shell design gives excellent mechanical support and short magnetic paths, which suits traction transformers and other applications subject to vibration and short-circuit forces.

Classification by Phase Configuration

Single-phase transformers use one pair of windings and are suitable for lower-load applications such as rural distribution and railway auxiliary supplies. Three-phase transformers use three pairs of windings connected in star or delta and supply higher loads in industrial areas, commercial buildings and grid substations. The three-phase design reduces the material per kVA and is the standard for distribution networks.

Classification by Cooling Method

Cooling classes are defined in the IEC 60076 series; the rating of a transformer can differ depending on which cooling stage is in service, for example ONAN/ONAF ratings on the same unit.

Classification by Application

Power transformers step voltage up or down for high-voltage transmission over long distances and are installed in power stations and substations. Distribution transformers step the grid voltage down to the utilization level for homes and businesses; they are smaller, often pole-mounted or pad-mounted, and optimized for low no-load loss because they stay energized continuously. The design and test requirements for both types follow IEC 60076 and, in China, GB/T 6451 for oil-immersed power transformers.

How to Choose the Right Type

Confirm the system voltage, rated power and impedance requirement.

Select the phase configuration according to the network: single-phase for small loads, three-phase for general distribution.

Choose the cooling class from the load profile and ambient temperature; reserve the forced-cooling stage for peak load if the duty varies.

Check the site conditions: indoor or outdoor, fire safety, noise limits and space constraints.

FAQ

1. What does ONAN mean?

ONAN stands for Oil Natural Air Natural: the oil circulates by natural convection and the tank is cooled by natural air flow, without fans or pumps.

2. When is OFWF cooling used?

OFWF uses pumped oil circulation and water heat exchangers. It is applied where very high heat must be removed and a reliable water supply exists, such as in large generator step-up transformers.

3. What is the difference between a power transformer and a distribution transformer?

Power transformers handle high voltages and large power for transmission; distribution transformers step voltage down to the utilization level and are optimized for low no-load loss.

4. Can one transformer have two cooling ratings?

Yes. Many units are designed with dual ratings such as ONAN/ONAF, where the nameplate rating depends on which cooling stage is in service.

5. Which standards apply to oil-immersed transformers?

General requirements and tests follow IEC 60076-1, temperature-rise limits IEC 60076-2, and Chinese technical parameters GB/T 6451.