Oil Immersed Power Transformer: What It Is and Why It Still Dominates

An oil immersed power transformer is a transformer whose core and windings are mounted inside a sealed steel tank filled with insulating oil. The oil does two jobs at the same time: it insulates live parts and current-carrying conductors, and it carries heat away from the coils by convection to the radiator fins or corrugated tank walls, where that heat escapes into the surrounding air. Because this cooling loop is simple, reliable and inexpensive to build, the oil immersed power transformer remains the default choice for 10kV, 20kV and 35kV distribution networks and for most industrial power supply projects.

Compared with dry-type units, an oil immersed power transformer usually delivers lower no-load and load losses at the same capacity, stronger short-time overload capability, and a lower cost per kVA. The trade-off is that it needs a dedicated installation room or fire separation plus periodic oil testing, so the two technologies often appear side by side in one project, each doing the job it does best.

How an Oil Immersed Power Transformer Works

Every unit is built around four building blocks. Understanding them makes specification and price comparison far easier.

• Core: laminated grain-oriented silicon steel, sometimes wound as a three-dimensional magnetic path wound core to shorten the magnetic circuit and reduce no-load loss.
• Windings: copper or aluminium conductors in layer, disc or foil form. Copper windings cost more but tolerate higher current density and short-circuit forces better.
• Tank and cooling: a plain tank, corrugated tank, or tank with radiators. Cooling classes are normally ONAN (oil natural, air natural) or ONAF (oil natural, air forced).
• Insulation system: transformer oil plus insulating paper, pressboard and spacers. Oil quality decides dielectric strength, so it is tested for breakdown voltage, moisture and gas content.

Because the oil both insulates and cools, an oil immersed power transformer is compact for its rating and can be built up to very large capacities. In distribution service, the 30-6300 kVA band covers most demand.

Common Series: S11, S13, S20, S22 and More

Model codes tell you the loss level and the construction. Generally speaking, the higher the number, the lower the energy loss and the more efficient the design.

• S11 oil-immersed transformer: the classic workhorse, widely used in distribution networks, with mature manufacturing and competitive pricing.
• S13 oil-immersed transformer: a lower-loss upgrade of S11, often specified where electricity cost matters across the whole service life.
• S20 and S22 oil-immersed transformers: higher-efficiency designs that meet stricter energy efficiency grades, suitable for projects with tough loss targets.
• S11-MR and S13-M.RL: sealed and fully sealed corrugated tank versions that eliminate the oil conservator and cut maintenance on outdoor poles and pads.
• SH15 amorphous alloy transformers: amorphous metal cores reduce no-load loss dramatically, which suits lightly loaded and rural distribution lines.
• Special transformers: mining transformers for underground service, rectifier transformers for electrolysis and smelting, capacity-regulating transformers for seasonal loads, and lightning arrester transformers for testing and protection duties.

Key Parameters to Confirm Before You Order

A quotation is only comparable when the technical data matches. Check these items line by line.

• Rated capacity: 30 kVA to 6300 kVA in distribution service; confirm load factor and future expansion.
• Voltage class and tapping: 10kV or 35kV primary with off-circuit taps such as plus or minus 5 percent, or plus or minus 2 times 2.5 percent.
• Vector group: Dyn11, Yyn0 or Yd11, chosen to match your switchgear and protection scheme.
• Loss levels: no-load loss and load loss must be verified against the applicable energy efficiency grade, not just against the model number.
• Impedance voltage: typically around 4 percent for small distribution units and higher for large power units; it affects short-circuit current and voltage regulation.
• Tap changer: off-circuit for most distribution work, or an on-load tap changing transformer where voltage must be adjusted while energised.
• Accessories: Buchholz relay, pressure relief valve, oil level gauge, dial thermometer, breather and terminal arrangement.

Buying Pitfalls an Inexperienced Buyer Usually Misses

• Aluminium windings sold as copper. Ask for the winding material in writing and check it against the nameplate and test data.
• Loss values that do not match the certificate. Measured no-load and load loss on the routine test report should be at or below the declared values.
• Thin or recycled core steel. It lowers the price, raises temperature rise, and shortens service life.
• Missing documentation. A serious supplier ships manuals, inspection reports, test certificates and lifelong quality tracking as standard.
• Paying middlemen margins. Factory-direct purchasing removes one or two layers of markup and gives you a single point of contact for warranty and spare parts.

Where Oil Immersed Power Transformers Are Used

• Utility and industrial distribution networks, pole-mounted or pad-mounted.
• Mining, cement, steel and chemical plants, including underground mining transformers.
• Solar and wind farms, where a renewable energy transformer steps generation voltage up to the grid.
• Packaged power solutions built around a compact substation, European-style or American-style, with high/low-voltage switchgear.

Installation and Maintenance in Brief

Mount the unit on a level foundation with adequate clearance for airflow and inspection. Check oil level, silica gel in the breather and all gaskets before energising. During operation, monitor oil temperature, oil level and gas relay alarms, and take an oil sample at least once a year. A well-maintained oil immersed power transformer commonly serves 20 to 30 years, and most failures trace back to moisture ingress, loose connections or long-term overload rather than to the core and coils themselves.

Buying Factory Direct from a Transformer Manufacturer

Runsheng Transformer Co., Ltd. manufactures S22, S20, S11 and S13 oil-immersed transformers, SCB13, SCB14 and SCB18 dry-type transformers, SH15 amorphous alloy transformers, 10kV three-dimensional magnetic path wound core units, and special designs such as mining transformers and rectifier transformers. The company also supplies 35kV and below preassembled substation equipment, including European-style and American-style compact substations and high/low-voltage switchgear. Because units ship direct from the factory, buyers get assured quality at factory-direct prices, with manuals, inspection reports, test certificates and lifelong quality tracking included. Its quality policy, customer satisfaction is Runsheng's mission, keeps the focus on matching each project with the right transformer rather than selling the most expensive one.

For specifications, drawings or a quotation on any oil immersed power transformer or dry-type unit, contact the sales team directly: phone +86 13406394988, e-mail runsheng@7813transformer.com, WhatsApp 008613508930968.