Oil Immersed Transformer

STELLAR Oil Immersed Transformer Manufacturer

STELLAR builds oil immersed transformers for distribution and power systems up to 35 kV. The core and windings sit in insulating oil that does two jobs at once: it insulates the live parts and carries their heat out to the tank. The range runs through the energy-saving S11, S13, and S20 series in ratings from 30 kVA to 2500 kVA, with larger power units on request.

Each unit uses a low-loss silicon-steel core, copper windings, and a sealed corrugated tank that needs no breather, built to IEC 60076 and GB 1094. Where a site rules out oil, the dry-type transformer is the alternative; where no-load loss is the priority, an amorphous core cuts it further. The matching medium voltage switchgear comes from the same base.

Oil Cools and Insulates

The core and windings sit in oil that both insulates them and carries heat away.

Low-Loss S-Series

S11, S13, and S20 series cut no-load loss generation by generation.

Sealed, Leak-Free Tank

Sealed corrugated tanks flex with the oil, so there is no breather and no leak.

Outdoor and Utility Ready

Built for outdoor substations and utility grids in any climate.

Oil Immersed Transformer Series

Series	Class	Rated Power	HV / LV	Cooling
S11	Low-loss distribution	30–2500 kVA	≤35 kV / 400 V	ONAN
S13	Lower-loss distribution	30–2500 kVA	≤35 kV / 400 V	ONAN
S20	High-efficiency distribution	30–2500 kVA	≤35 kV / 400 V	ONAN
SZ11	On-load tap-change	50–2500 kVA	≤35 kV / 400 V	ONAN

Voltage classes 10 kV, 20 kV, and 35 kV; vector groups Yyn0 or Dyn11; off-circuit tap changer ±5% or ±2×2.5%, or an on-load tap changer on the SZ series. Standard tank is a sealed corrugated type; a conservator with radiators is available for larger units. Copper windings standard, aluminium on request. Built to IEC 60076, GB 1094.1-2013, and GB/T 6451-2015.

Oil Immersed Transformer Product Range

Oil Does Two Jobs, Insulation and Cooling

In an oil immersed transformer the core and both windings are submerged in mineral insulating oil. The oil holds off the voltage between windings and to earth, and at the same time it picks up the heat from the core and copper and moves it to the tank wall, where the air takes it away. Because oil carries heat far better than air, an oil unit handles more load and more overload in a smaller body than a dry-type transformer of the same rating.

· Oil insulates and cools in one medium
· Higher load and overload capacity than dry-type
· Natural ONAN circulation, no fans needed

Low-Loss Cores, Sealed Tanks, Long Life

The core is low-loss cold-rolled silicon steel, and each new series, S11 to S13 to S20, lowers the no-load loss the transformer draws every hour it is energised. Copper windings braced against short-circuit forces give a long, reliable life. The sealed corrugated tank flexes as the oil expands and contracts, so there is no breather to service and no path for moisture. For the lowest no-load loss, an amorphous core is available.

· Low-loss silicon-steel core, falling loss by series
· Sealed corrugated tank, no breather, no leak
· Copper windings braced for short-circuit strength

Distribution and Power, Built to Your Grid

We build distribution units from 30 kVA to 2500 kVA and larger power transformers on request, for the 10 kV, 20 kV, and 35 kV classes. You choose the vector group, the impedance, the tap range, and the fittings, and we set them from your specification. The transformer pairs with the medium voltage switchgear that feeds it, so the package comes from one supplier.

· 30 kVA to 2500 kVA distribution, larger power units
· Vector group, impedance, and taps to your spec
· Conservator or sealed tank, with your fittings

About STELLAR

STELLAR is a Singapore-headquartered manufacturer of switchgear, transformers, and complete power distribution equipment, serving utilities, contractors, and distributors in 50+ countries. From single-line diagram to commissioning support, we deliver project-configured electrical systems built to IEC and GB standards.

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FAQs

What is an oil immersed transformer?

An oil immersed transformer, also called an oil-filled transformer, is a transformer whose core and windings are submerged in insulating oil. The oil insulates the live parts and carries their heat to the tank, where it dissipates to the air. This is the most common construction for distribution and power transformers, especially outdoors and at higher ratings.

What is the difference between oil immersed and dry type transformers?

The difference is the cooling and insulating medium. Oil immersed units use oil, which cools better, so they carry more load and overload, reach higher ratings, suit outdoor use, and cost less per kVA at large sizes, but the oil needs containment and adds a fire consideration. Dry-type transformers use air and resin, so they carry no oil and are favoured indoors and where fire safety rules, at a higher cost and lower overload. Many projects use oil outdoors and dry-type inside buildings.

What does the oil do, and is the tank sealed?

The oil does two jobs: it is the main insulation between windings and to earth, and it is the coolant that moves heat from the core and windings to the tank. Most of our distribution units use a fully sealed corrugated tank, where the corrugated fins flex as the oil expands and contracts, so there is no breather and no way for moisture to get in. Larger units can use a conservator with a silica-gel breather instead.

What ratings and voltages do you offer?

We build distribution transformers from 30 kVA to 2500 kVA and larger power transformers on request, for the 10 kV, 20 kV, and 35 kV classes, stepping down to 400 V or another low voltage you specify. Vector groups are typically Yyn0 or Dyn11. See the series table above for the standard range, or tell us the rating, ratio, and impedance you need.

Which series should I choose, and can I get an amorphous core?

Pick the series by how much you value lower running loss against first cost: S11 is the standard low-loss choice, S13 lowers no-load loss further, and S20 is the high-efficiency tier. For the lowest no-load loss of all, an amorphous transformer uses an amorphous alloy core that cuts standing loss sharply, which pays back on lightly loaded grids.

Oil Immersed Transformers Explained

An oil immersed transformer is a transformer in which the magnetic core and the high- and low-voltage windings are submerged in insulating oil inside a sealed steel tank. The oil is the heart of the design: it insulates the live parts from each other and from the tank, and it carries the heat they generate out to the tank surface, where it passes to the surrounding air. Oil immersed transformers, also called oil-filled transformers, are the most widely used type for distribution and power across utilities, industry, and infrastructure.

What the Oil Does

Insulating oil has two roles. As an insulator, it has a far higher dielectric strength than air, so the windings can sit close together and still hold off the voltage, which keeps the transformer compact. As a coolant, it absorbs heat from the core and copper and circulates, by natural convection, to the tank wall and the cooling fins or radiators. This dual role is why oil units run cooler and carry more overload than an equivalent air-cooled design. The oil is normally mineral oil; less-flammable ester fluids are available where fire risk must be reduced.

How It Works

The transformer steps voltage up or down by magnetic coupling between two windings on a shared core. In a distribution unit the high-voltage winding connects to the medium voltage grid, commonly 10 kV, 20 kV, or 35 kV, and the low-voltage winding delivers 400 V for local supply. As load current flows, the windings and core heat up; the oil carries that heat away so the insulation stays within its temperature limit. The whole core-and-coil assembly is dried under vacuum and the tank is vacuum-filled with oil to keep moisture out.

The S-Series: S11, S13, and S20

Chinese-standard oil immersed distribution transformers follow the S-series, where each generation lowers the loss. The S11 cut no-load loss by roughly 30% against the older S9 it replaced. The S13 lowers no-load loss again, and the S20 and S22 reach the top national efficiency grades. The choice is an economic one: a higher series costs more to buy but draws less standing loss every hour it is energised, which adds up on a transformer that runs for decades. We build S11, S13, and S20, plus the SZ series with an on-load tap changer for grids that regulate voltage under load.

Sealed, Conservator, and Corrugated Tanks

Oil expands and contracts with temperature, and the tank design handles that in one of two ways. A conservator type carries a small expansion tank above the main tank, breathing through a silica-gel breather that dries the incoming air. A hermetically sealed type has no conservator: the tank is fully sealed, and a corrugated tank wall, with flexible fins, expands and contracts with the oil. The sealed corrugated design is now standard for distribution because it needs no breather maintenance, keeps moisture out completely, and resists leaks, while the fins also serve as the cooling surface. Larger power transformers more often use a conservator with separate radiators.

Cooling: ONAN and Beyond

The standard cooling class for distribution oil transformers is ONAN, oil natural air natural: the oil circulates by convection and the air cools the tank without fans. Larger units can add fans (ONAF) or pumps and forced-oil circulation (OFAF) to push more heat out of the same tank when load is high. Most distribution ratings never need more than ONAN.

Oil Immersed vs Dry Type

The main alternative is the dry-type transformer, which uses air and cast resin instead of oil. Oil immersed units cool better, so they carry more overload, reach higher voltages and ratings, suit outdoor substations, and cost less per kVA as size grows; the trade-off is the oil itself, which needs a bund or containment and adds a fire and environmental consideration. Dry-type transformers carry no oil, so they are the safer choice inside buildings, basements, and crowded plant rooms, at a higher price and with less overload headroom. The usual split is oil outdoors and dry-type indoors.

Amorphous Core for Lower Loss

For grids where transformers sit lightly loaded for long hours, the no-load loss, the power drawn just to keep the unit energised, dominates the energy bill. An amorphous transformer replaces the silicon-steel core with an amorphous metal core, which can cut no-load loss by around 70% against a standard unit. It costs more and is a little larger, but on a rural or standby network the saved standing loss pays it back. Amorphous cores are available in oil immersed form, so the cooling and sealing are the same as a standard oil unit.

Ratings, Sizes, and Standards

Our oil immersed distribution range covers 30 kVA up to 2500 kVA, the sizes that suit substations, industrial parks, commercial buildings, and renewable plants, with larger power transformers built to order. A common reference point, a 1000 kVA 11 kV to 400 V unit, sits comfortably in the middle of the distribution range. High-voltage classes are 10 kV, 20 kV, and 35 kV, with low voltage usually 400 V. Every unit is designed and tested to IEC 60076, equivalent to GB/T 6451-2015, with GB 1094.1-2013 for the assembly and GB 20052 for the energy-efficiency grade.