What an oil cooled transformer actually is

An oil cooled transformer is a power or distribution transformer whose core and windings are immersed in insulating oil inside a sealed steel tank. The oil does two jobs at the same time: it insulates the live parts and it carries heat away from the windings to the tank walls and radiators, where it is released into the surrounding air. Because oil circulates naturally or with forced pumps, this cooling method handles high loads reliably and keeps internal temperatures within safe limits, which is why oil cooled transformer units dominate medium-voltage distribution networks worldwide.

In most standard designs the cooling mode is ONAN (oil natural, air natural). Larger units may use ONAF with fans, or forced oil circulation for heavy duty applications. The basic structure includes the core, windings, oil tank, radiator or corrugated tank wall, conservator, breather, Buchholz relay on larger units, off-circuit or on-load tap changer, and high and low voltage bushings.

Main types you will meet on the market

Buyers usually compare a handful of well-established series. Knowing the naming logic helps you read a quotation quickly:

• S11 oil-immersed transformer: a classic energy-efficient distribution series, widely used in 10kV networks and general industrial loads.
• S13 oil-immersed transformer: lower no-load losses than S11, a common choice where long-term running cost matters.
• S20 oil-immersed transformer: a newer generation with further reduced losses, often selected for grid and renewable energy projects.
• S22 oil-immersed transformer: high efficiency design aimed at the strictest loss limits and lowest operating cost over the service life.
• SH15 amorphous alloy transformer: amorphous metal core with very low no-load loss, ideal for light-load or long-idle conditions.
• Wound core and three-dimensional magnetic path designs, plus single-phase units for specialized distribution systems.
• Special units such as mining transformers, rectifier transformers, capacity-regulating transformers and other custom special transformers.

Dry-type alternatives such as SCB10, SCB13, SCB14, SCB18 and SGB10 are used where oil is not permitted, for example indoors in high fire-risk areas. For outdoor and standard distribution duty, an oil cooled transformer generally offers better overload capacity and a lower price per kVA.

Key parameters to check before you order

The specification sheet is where money is made or lost. Always confirm these values in writing:

• Rated capacity, typically anywhere from 30 kVA to 6300 kVA for distribution applications.
• Voltage class and ratio, such as 10kV or 35kV primary with 0.4kV secondary, including tap range.
• No-load loss and load loss, which must meet the applicable energy efficiency standard; low-loss designs cost more up front but save power year after year.
• Impedance voltage, vector group, insulation level and temperature rise.
• Cooling method, tap changer type (off-circuit or on-load), and whether a conservator is included.
• Material details: copper or aluminium windings, core grade, and whether the core is a stacked, wound core or three-dimensional magnetic path design.
• Accessories: Buchholz relay, pressure relief device, silica gel breather, oil level gauge, radiators, terminal box layout and cable entry direction.

Where an oil cooled transformer is used

These units serve power plants, urban and rural distribution networks, industrial plants, commercial buildings, water treatment facilities and infrastructure projects. They are also common in renewable energy transformer applications, connecting wind and solar generation to the grid, and in mining and heavy industry where dust, moisture and continuous load are normal. In compact substations and preassembled substation packages, the oil cooled transformer sits alongside high and low voltage switchgear as a factory-built, site-ready solution.

How to avoid overpaying

Price differences between suppliers are rarely random. They usually come from winding material, core material, loss level, tank construction, accessory package and test scope. A cheap quotation often hides aluminium windings where copper was expected, losses that only just pass or even miss the standard, thinner tank steel, or a reduced accessory set.

• Compare on full technical data, not headline price. Ask for loss values, impedance and material declaration on the official quotation.
• Request routine test reports for every unit and confirm that type test reports exist for the design.
• Check that manual, inspection report, test certificate and nameplate data match the ordered specification.
• Buy from a manufacturer rather than a trading layer where possible; factory-direct pricing removes intermediary margins and shortens the response time on spare parts.
• Look at service life cost: a slightly higher purchase price with lower losses can repay itself in two to four years at typical load profiles.

As a power transformer manufacturer with a broad catalogue covering oil-immersed, dry-type and amorphous alloy transformers, Runsheng Transformer Co., Ltd. supplies S11, S13, S20 and S22 oil-immersed transformer series, SCB10/SCB13/SCB14/SCB18 dry-type units, SH15 and SCBH15 amorphous alloy transformers, 35kV transformers, mining and rectifier transformers, plus compact substations and high and low voltage switchgear. Products are manufactured to national standards and delivered with manuals, inspection reports, test certificates and lifelong quality tracking.

Installation and maintenance basics

Correct installation extends life significantly. Mount the unit on a level foundation with adequate ventilation and clearance for radiator airflow, follow the earthing requirements, and check oil level and tightness before energizing. During operation, periodic oil sampling for dielectric strength and moisture, inspection of the silica gel colour, leak checks on gaskets and weld seams, and verification of the breather and pressure relief device are the core routine tasks. With proper care, a well-built oil cooled transformer will run for decades.

Getting a fast, accurate quotation

To receive a precise offer, prepare the following: rated capacity in kVA, primary and secondary voltage, phase and frequency, vector group, impedance, loss level or efficiency class, cooling method, tap changer type, indoor or outdoor installation, ambient and altitude conditions, and any special requirements such as mining, rectifier or capacity-regulating duty. Sharing this information avoids re-quoting and keeps delivery on schedule.

For technical selection, pricing or customization of an oil cooled transformer, contact Runsheng Transformer Co., Ltd. at phone +86 13406394988, e-mail runsheng@7813transformer.com, or WhatsApp 008613508930968.