Few purchasing decisions in a power distribution project carry as much long-term weight as choosing an oil immersed power transformer. A well-matched unit quietly delivers decades of stable, low-loss service. A poorly chosen one brings overheating, inflated no-load losses, unplanned outages and, all too often, a supplier that stops answering the phone the moment a fault appears. This overview covers the fundamentals: what an oil immersed power transformer actually is, how it works, which model series dominate the market, which parameters matter, and how to buy factory-direct instead of paying a layer of middleman margin.

What Is an oil immersed power transformer?

An oil immersed power transformer is a static electrical device that transfers electrical energy between two or more circuits through electromagnetic induction, using insulating oil as both the dielectric medium and the cooling agent. The active part, consisting of a magnetic core and copper or aluminium windings, is fully submerged in transformer oil inside a sealed steel tank. Heat produced by core losses and copper losses is carried away from the windings by natural oil convection (ONAN cooling) or forced circulation (ONAF and OFAF), then dissipated through the tank walls, radiators or corrugated fins.

Oil immersion delivers three practical benefits: dielectric strength far higher than air, excellent heat transfer, and reliable long-term operation in humid, dusty or fully outdoor environments. This is precisely why oil immersed distribution transformers dominate medium-voltage networks from 10 kV through 35 kV.

How an oil immersed power transformer Works

• Magnetic core: laminated silicon steel or amorphous alloy carries the magnetic flux. Core grade and lamination thickness largely determine no-load loss.
• Windings: copper or aluminium conductors wound in concentric layers form the high-voltage and low-voltage circuits. Conductor cross-section and winding geometry drive load loss and short-circuit withstand strength.
• Insulating oil: mineral oil, or ester fluid in fire-sensitive locations, provides insulation, cooling and arc quenching.
• Tap changer: off-circuit (no-load) or on-load tap changing equipment adjusts the turns ratio so secondary voltage stays stable as primary voltage fluctuates.
• Protective accessories: breather, oil level gauge, pressure relief valve, Buchholz relay, temperature sensors and silica gel dehumidifier safeguard the unit over its service life.

Model Series You Will See in Every Tender

Model numbers are not marketing noise; they encode loss class and construction. The letter S indicates a three-phase unit, and the following number indicates the energy-efficiency tier. The higher the figure, the lower the no-load loss and the lower the running cost over a 20 to 30 year lifespan.

• S11 oil immersed transformer: the long-established workhorse, widely stocked and economical for general distribution duty.
• S13 oil immersed transformer: lower no-load loss than S11, a common specification for utility and industrial projects.
• S20 oil immersed transformer: a higher-efficiency class suited to continuous-duty industrial loads.
• S22 oil immersed transformer: the current top-tier energy-efficient design, with markedly reduced no-load loss and strong performance in renewable energy and 35 kV applications.
• Three-dimensional wound core types: 10 kV triangular wound-core construction that cuts no-load loss further.
• Derived and special types: sealed-tank and conservator designs, S11-MR and S13-M.RL variants, plus mining transformers, rectifier transformers, lightning arrester transformers, capacity-regulating transformers and fully custom special transformers.

Standard capacity coverage runs from 30 kVA to 6300 kVA, with voltage classes at 10 kV, 20 kV and 35 kV.

Where Oil Immersed Units Are Used

• Utility and industrial distribution substations
• Preassembled and compact substations, both European-style and American-style
• Mining, metallurgy and heavy manufacturing sites
• Solar and wind renewable energy step-up and collection systems
• Rail, tunnel, water treatment and municipal infrastructure projects

Key Parameters to Verify Before Ordering

• Rated capacity (kVA) and rated voltage ratio, for example 10 kV / 0.4 kV
• No-load loss and load loss, which together determine lifetime energy cost
• Impedance voltage percentage and short-circuit withstand capability
• Cooling method and permissible temperature rise
• Tap range, such as plus or minus 5 percent in two steps, and tap changer type
• Insulation level, tank construction and ingress protection rating
• Applicable national and IEC standards, plus routine and type test reports

Oil Immersed Versus Dry Type

Oil immersed power transformers generally cost less per kVA, handle higher capacities more economically, run cooler outdoors and tolerate harsh environments. Dry-type units, such as SCB10, SCB13, SCB14 and SCB18, avoid oil and suit indoor locations with strict fire requirements. Choosing between them comes down to capacity, installation environment, fire code and total lifecycle cost.

What Actually Drives the Price

• Conductor material and purity, where copper versus aluminium and conductor weight are decisive
• Core steel grade, whether conventional silicon steel or amorphous alloy
• Oil volume and quality, plus tank and radiator construction
• Accessories and whether a tap changer is on-load or off-circuit
• Routine tests, type tests and third-party inspection requirements

A quotation that looks dramatically cheaper almost always reflects a substitution somewhere in this list. That is the single most common pitfall buyers encounter.

How to Avoid Costly Purchasing Mistakes

• Ask for the actual core steel grade and conductor cross-section, not just the loss figures on paper
• Insist on routine test reports, a type test report and a third-party inspection certificate
• Confirm oil specification and, where relevant, oil test data before shipment
• Check whether the supplier manufactures or merely assembles, since assembling often hides outsourced cores and windings
• Confirm warranty terms, spare part availability and technical support response times
• Buy factory-direct where possible, because each distribution layer adds cost without adding quality

Sourcing Factory Direct

As a power transformer manufacturer, Runsheng Transformer Co., Ltd. produces S22, S20, S11 and S13 oil immersed transformers, SH15 amorphous alloy transformers, three-dimensional wound core units, mining and rectifier transformers, plus 35 kV and below preassembled substation equipment and high and low voltage switchgear. Products are supplied at factory-direct prices with manuals, inspection reports, test certificates and lifelong quality tracking, so the documentation trail stays intact from delivery through to end of service.

For procurement teams, that means one less mark-up in the chain and a direct technical contact when specifications need adjusting.

Contact

Phone: +86 13406394988 | Email: runsheng@7813transformer.com | WhatsApp: 008613508930968