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Oil-Immersed Transformer

Oil Immersed Transformers from 30 to 3,150 kVA

An oil immersed transformer places the core and winding assembly inside a tank of insulating liquid, so the same fluid does two jobs at once: it insulates the live parts and it carries heat away from them. The terms oil filled transformer and liquid filled transformer describe the same construction, and American specifications usually use the liquid-filled wording. Talite builds this construction from 30–3,150 kVA on the 6, 6.3 and 10 kV classes, and from 50 to 2,500 kVA on the 35 and 38.5 kV classes.

Send Your Electrical Schedule Your drawings, single-line diagrams and load schedules are handled as confidential.

Engineer-reviewed quote within 24 hours

Standard ratings ship in 20–30 days

Single-unit orders accepted

Published loss and impedance schedule

Oil-Immersed Transformer Capability At A Glance

Published Rating Envelope

30–3,150 kVA at 6 / 6.3 / 10 kV, 0.4 kV secondary. 50 to 2,500 kVA at 35 / 38.5 kV.

Quotation Turnaround

We return an engineer-reviewed quotation within 24 hours on business days.

Standard-Rating Lead Time

Standard-rating units ship in 20–30 days after order confirmation and drawing approval. Freight is quoted separately.

Minimum Order

The minimum order quantity is one unit. We accept single-unit orders on standard ratings.

Trade Terms

EXW, FOB, CFR and CIF terms are standard. DAP and DDP delivery are quoted on request.

Warranty

The warranty period is 12 months from site acceptance, and no more than 18 months from dispatch, whichever comes first.

Buyers Write This Product Several Different Ways, And The Enquiries That Reach Us Use All Of Them.

What The Enquiry Says	What It Means On This Page
Oil Immersed Type Transformer	The same construction, named from a different specification tradition
Oil Filled Transformer, Dielectric Liquid Filled Transformer	Also the same construction; the liquid-filled wording is the American habit
Oil Filled Distribution Transformers	The same units described by their grid role rather than by their insulation, which is how most utility and contractor enquiries arrive
Three Phase Oil Immersed Transformer	The three-phase member of the family, which covers almost every distribution order placed from this page
Oil Immersed Self Cooled Transformer	Cooling by natural circulation with no fan and no pump, carrying the ONAN designation

Five Oil-Immersed Transformer Families and the Duty Each One Answers

Buyers arrive at this page with a kVA number and a voltage pair, not with a product name. Below, the Five-Family Duty Router turns those two inputs into the right page in one step. Each card answers the same five questions so the families can be compared side by side rather than read one at a time.

Sealed distribution transformers

Duty Three-phase distribution from a 6, 6.3 or 10 kV network down to a 0.4 kV secondary. These are the oil filled distribution transformers most enquiries start from

Published band 30 kVA to 3,150 kVA

Mounting Indoor or outdoor, corrugated sealed tank, no conservator

Data published Full per-rating loss, impedance, mass and outline schedule

Next step oil immersed distribution transformer

Higher-voltage project units

Duty 35 and 38.5 kV class supply into a project substation

Published band 50 kVA to 2,500 kVA on the published 35 kV schedule

Mounting Outdoor, three-phase tank with cooling surfaces

Data published Per-model loss and impedance schedule

Next step oil immersed power transformer

Hermetically sealed units

Duty Sites where the fluid must stay isolated from the atmosphere

Published band The same 30 kVA to 3,150 kVA family, sealed configuration

Mounting Corrugated wall absorbs fluid expansion in place of a conservator

Data published Sealed-versus-conservator decision boundary

Next step hermetically sealed transformer

Single-phase vertical units

Duty Single-phase supply where no three-phase feed reaches the site

Published band Small ratings for rural, remote and distributed supply

Mounting Vertical tank; orientation and mounting are separate selection fields from the cooling method

Data published Mounting and interface fit check

Next step single phase vertical oil immersed transformer

Custom-engineered units

Duty A voltage combination, vector group or impedance that no published row matches

Published band Defined by the project rather than by a catalogue row

Mounting Set by the site interface drawing

Data published The evidence register that has to close before release

Next step custom power transformer

Where the Five-Family Duty Router sends you next

Our rule of thumb is that the site decides the family and the schedule decides the rating: fire rules, containment and access settle which of the five you are buying, and only then does the kVA number pick the row. Read the Five-Family Duty Router as a product-organisation map rather than a regulatory classification. Which family a unit belongs to in Talite’s catalogue and whether it is covered equipment in a destination market are two separate determinations, and the second one is made per project.

Rating, Voltage and Vector Group Envelope for Oil-Immersed Transformers

Once the Five-Family Duty Router has narrowed the family, this section fixes the numbers inside it. You’re likely going to be sizing against a peak load the network operator has already fixed rather than against a number you are free to choose, because the operator wants the rating set as close to real peak load as the connection rules allow. That means the envelope below is a screening tool, and the binding numbers come from your electrical schedule.

Field	Published value	How it is fixed
Rated capacity, 10 kV class	30–3,150 kVA	Selected from the published per-rating schedule
Rated capacity, 35 kV class	50 to 2,500 kVA	Selected from the published 35 kV schedule
High voltage	6, 6.3 or 10 kV; 35 or 38.5 kV	Stated in your enquiry
Low voltage	0.4 kV	Stated in your enquiry
Vector group	Yyn0 or Dyn11	Model dependent, confirmed on the data sheet
Tap range	±5% or ±2 × 2.5%	Model dependent, confirmed on the data sheet
Rated frequency	Confirmed per project	Stated in your enquiry and fixed at drawing approval
Normal service conditions	Altitude up to 1,000 m, ambient −25 °C to +40 °C	From the supplied normal-service data

Rating, Voltage and Vector Group Envelope for Oil-Immersed Transformers

Why frequency sits on the enquiry, not on the nameplate promise

Frequency is the field that most often separates a specification written for one market from equipment built for another. It is also one of the five tests the United States uses to decide whether a unit is covered equipment at all. We fix it with you in writing before the design is released, and we do not treat it as a default.

Service conditions are the standard band, not a narrow one

Altitude up to 1,000 m and ambient −25 °C to +40 °C is the normal-service band published for this construction class, which is the band set out in IEC 60076-1, published in Europe as EN 60076-1

Conditions outside that band trigger an engineering review of the temperature-rise basis rather than an assumed derating factor

Humidity, contamination, coastal salt and seismic duty are separate inputs and are handled on the drawing, not by a footnote

Iron Core, Winding, Conservator and Radiator Construction

Ask about the oil immersed transformer working principle and the answer splits into two jobs the fluid does at once: it carries heat from the winding surfaces out to the tank wall, and it insulates between live parts at a dielectric strength air cannot reach across the same clearances. Core, windings, tank, fluid, oil filled transformer bushings and protection devices are the oil transformer parts that carry those two jobs out.

Core and windings generate the loss, and are where the heat has to leave from
Tank wall, corrugated panel or radiator bank is the surface that moves that heat into the surrounding air
Fluid, bushings, protection and monitoring devices are service items with an inspection interval, not fittings that are installed and forgotten

Our iron core is built from cold-rolled grain-oriented silicon steel with 45-degree fully mitred joints, which is the geometry that keeps the magnetic flux distribution even at the joints and holds no-load loss and magnetostrictive noise down. Low-voltage windings use copper foil in a cylindrical arrangement on all but the smallest ratings, and high-voltage windings use a multi-layer cylindrical structure so the ampere-turn distribution stays balanced and short-circuit strength stays high.

Two tank configurations, two accessory sets

Choosing between a hermetically sealed transformer vs conservator type construction is the question buyers raise most often, and site access to the fluid settles it rather than price.

Fully sealed corrugated tank

The corrugated wall flexes to absorb the volume change of the fluid as temperature moves, so no conservator is fitted and overall height drops
Because the fluid never meets outside air, oxygen and moisture ingress is removed as a degradation path
A pressure relief valve operating at 35 kPa releases pressure on an internal fault and reseats when pressure returns to normal

Conservator-equipped tank

An expansion vessel above the tank takes the fluid volume change instead of the corrugated wall
This is the configuration that carries a gas relay in the connecting pipework, which a fully sealed tank has no pipework for
Units of 800 kVA and above are fitted with a signal thermometer and a gas relay for temperature and fault alarms

Radiators are a third option rather than a default. Smooth-walled tanks with bolted-on radiators are used in some regions, and the choice between corrugated wall, radiator bank and conservator changes the footprint, the height and the accessory schedule, so it is confirmed on the order rather than assumed. Loss, temperature rise and the tolerances that apply to the figures in this section are defined by IEC 60076-1, published in Europe as EN 60076-1[3], which is also the document a bidder should be quoting its declared losses against.

Fluid and vacuum processing

Coils and core are vacuum dried, and the mineral oil is vacuum filtered and vacuum filled, which drives the residual moisture inside the unit to a minimum

The insulating fluid both cools and insulates, so its condition is a service parameter and not a fill-and-forget item

Fluid type and transformer oil grade are confirmed with the order; mineral oil is the standard fill, and alternative approved liquids are quoted on request against the grade your specification names

The corrugated wall is not a cost-saving shortcut. It removes the breather, the conservator and one external interface, and in exchange it fixes the fluid volume at the factory. That is a good trade for a sealed distribution unit and a poor one for a duty that needs field oil sampling, which is exactly why we ask which of the two you need before we draw anything.

— Talite Engineering Team

Oil-Immersed Transformer Loss, Impedance and Dimension Schedule

Two loss schedules are published for the 10 kV class: the S11-M sealed series and the S13-M energy-saving series. Subtracting one from the other rating by rating gives the S11-to-S13 Loss Delta Curve below, which is the number that decides operating cost rather than the number printed largest on a brochure.

S11-M Series

S13-M Series

S11-to-S13 Loss Comparison (30 to 315 kVA)

Rated capacity	S11-M no-load loss	S13-M no-load loss	Difference	Reduction
30 kVA	100 W	80 W	20 W	20.0%
50 kVA	130 W	100 W	30 W	23.1%
63 kVA	140 W	110 W	30 W	21.4%
80 kVA	150 W	130 W	20 W	13.3%
100 kVA	200 W	150 W	50 W	25.0%
125 kVA	240 W	170 W	70 W	29.2%
160 kVA	290 W	200 W	90 W	31.0%
200 kVA	330 W	240 W	90 W	27.3%
250 kVA	400 W	290 W	110 W	27.5%
315 kVA	480 W	340 W	140 W	29.2%

S11-to-S13 Loss Comparison (400 to 1600 kVA)

Rated capacity	S11-M no-load loss	S13-M no-load loss	Difference	Reduction
400 kVA	570 W	410 W	160 W	28.1%
500 kVA	680 W	480 W	200 W	29.4%
630 kVA	810 W	570 W	240 W	29.6%
800 kVA	980 W	700 W	280 W	28.6%
1000 kVA	1150 W	830 W	320 W	27.8%
1250 kVA	1360 W	970 W	390 W	28.7%
1600 kVA	1640 W	1170 W	470 W	28.7%

What the S11-to-S13 Loss Delta Curve says that a headline percentage does not

No-load loss reduction from S11-M to S13-M at each rating, computed from the two schedules in the table above. The dashed red line is the 30% figure the category markets on; the dotted black line is what these two schedules actually average. Sixteen of the seventeen ratings sit below the red line.

Across the seventeen ratings both schedules share, the reduction averages 26.3% and spans 13.3% to 31.0%
Only one rating, 160 kVA, clears 30%, so a single round figure would misdescribe sixteen of the seventeen rows
The 80 kVA row is the weakest at 13.3%, and a buyer sizing at 80 kVA should know that before paying an energy-saving premium
No-load loss is incident to excitation, so it runs for all 8,760 hours a year whether or not the unit is carrying load
That last point is why the delta curve matters more than it looks. Load loss varies with loading and a distribution unit is rarely at full load, so on a lightly loaded feeder the no-load column is the one that dominates lifetime energy cost. The total owning cost method used in utility procurement handles this by capitalising the two loss categories separately before comparing bids.

The 800 kVA Impedance Threshold

Short-circuit impedance in the published S11-M schedule steps from 4.0% to 4.5% at 630 kVA. United States rules draw their own line in a different place, and the two are often confused.

Rating band	Published impedance	Federal normal range, three-phase liquid-immersed	Position
30 to 63 kVA	4.0%	1.0–4.5% for 15 ≤ kVA < 75	Inside the range
80 to 100 kVA	4.0%	1.0–5.0% for 75 ≤ kVA < 112.5	Inside the range
125 to 400 kVA	4.0%	1.2–6.0% for 112.5 ≤ kVA < 500	Inside the range
500 to 630 kVA	4.0–4.5%	1.5–7.0% for 500 ≤ kVA < 750	Inside the range
800 to 3,150 kVA	4.5%	5.0–7.5% for 750 ≤ kVA ≤ 5000	Below the lower bound

The 800 kVA Impedance Threshold is where the federal band floor rises to 5.0% while the published impedance stays at 4.5%
A transformer built to operate outside the normal impedance range for its rating meets the special-impedance definition, and special-impedance transformers are one of the thirteen types excluded from the federal distribution-transformer definition[1]
If your specification needs covered equipment at 800 kVA or above, the impedance has to be designed into the 5.0–7.5% band, which is a design change and not a selection option
Below 750 kVA the published impedance sits inside the normal range, so impedance is not what decides coverage there
Reading that table as a compliance conclusion would be a mistake, because impedance is only one of five tests. Input voltage, output voltage, frequency, capacity and impedance all have to be assessed together against the definition, and frequency is the field we confirm with you rather than assume.

Oil mass and the site containment threshold

The published schedule gives oil mass for every rating, and that number feeds a calculation most transformer pages never mention. United States facilities count qualifying oil-filled equipment toward their aggregate aboveground oil capacity, and only containers of 55 gallons or more are counted at all.[2]

Rated capacity	Oil mass	Approximate volume	Counts toward the 55-gallon threshold
100 kVA	115 kg	about 35 US gallons	No
200 kVA	170 kg	about 52 US gallons	No
250 kVA	190 kg	about 58 US gallons	Yes, first rating to cross
630 kVA	320 kg	about 98 US gallons	Yes
1000 kVA	570 kg	about 175 US gallons	Yes
1600 kVA	750 kg	about 230 US gallons	Yes
3150 kVA	1180 kg	about 362 US gallons	Yes

Volumes are screening estimates converted at a fluid density of 0.86 kg per litre; the delivered fluid data sheet gives the density that applies to your order
250 kVA is the first published rating whose fluid charge crosses the 55-gallon counting line
Four 3,150 kVA units hold roughly 1,450 gallons between them, which is above the 1,320-gallon aggregate figure on its own
This is a screening calculation for the design team, not a compliance determination, which belongs to the facility owner