35kV Oil-Immersed Distribution Transformer Selection Guide

A 35 kV oil-immersed distribution transformer sits between a sub-transmission network and a 400 V distribution board, and that position defines everything about it: the voltage class is fixed by the network, the capacity is fixed by downstream load, and the losses are fixed by how seriously the buyer treats 25 years of energy cost. Get any of the three wrong and the unit is either oversized, under-cooled or economically indefensible.

This guide works through the selection in the order it actually happens — capacity, voltage ratio and taps, losses and total cost of ownership, impedance, cooling and mechanical configuration — using the published 35 kV data table. For the wider context, the oil-immersed power transformer range shows the adjacent voltage classes; if you are still deciding between oil and dry type, start with the oil-immersed transformer selection overview.

What “35 kV” Actually Covers

The published table lists a high-voltage side of 35 kV or 38.5 kV, with a low-voltage side of 0.4 kV. That dual high-voltage rating is not decoration: 35 kV matches nominal 35 kV networks, and 38.5 kV gives the margin used where the network runs above nominal or where the tap range must reach further. The published tap range is ±5% and ±2 × 2.5%, so a 38.5 kV unit with five tap positions can be set to match the actual busbar voltage at commissioning.

It is also worth saying what 35 kV is in market terms. In much of Asia, Africa and Latin America, 33 kV and 35 kV are the standard medium-voltage distribution levels; in North America the equivalent is 34.5 kV. A 35/38.5 kV transformer therefore covers 33 kV, 35 kV and 34.5 kV networks depending on tap setting and the specified insulation level, which is why one catalogue page serves several regions. The same logic applies one level down, where the 10 kV, 11 kV and 13.8 kV class comparison explains why neighbouring markets publish different nominal voltages for what looks like the same network. Confirm the insulation level explicitly: a 35 kV class liquid-immersed unit typically carries a 200 kV lightning impulse level with a power-frequency withstand in the region of 95 kV at the 40.5 kV maximum system voltage, and that is the figure the local standard will be checked against.

Step 1: Capacity From Load, Not From Habit

The published range runs from 50 kVA to 2,000 kVA in the standard steps: 50, 100, 125, 160, 200, 250, 315, 400, 500, 630, 800, 1,000, 1,250 and 2,000 kVA. Select from calculated demand, not from a rule of thumb:

1. Sum the connected load in kVA, using motor input kVA rather than output kW.
2. Apply a demand factor for the actual simultaneity of the loads, which is usually well below 1.0 in an industrial plant.
3. Add a growth allowance — typically 15 to 25% for a first-stage installation with known future load.
4. Check the motor starting case: the largest motor starting across the transformer must not drop the LV busbar below the limit set by the connected equipment.
5. Round up to the next standard rating. A 480 kVA calculated demand becomes a 500 kVA unit, not a 630 kVA unit.

Two constraints push the answer upward regardless of the arithmetic. Impedance limits the fault level the LV switchboard has to withstand, and cooling margin limits how long the unit can carry overload. Both are handled below.

Step 2: Losses, and Why They Decide the Order

This is the part of a 35 kV transformer specification that most purchase orders get wrong, because the losses are invisible on the invoice and enormous over the asset life. The published table gives no-load loss and load loss for every rating:

To turn those watts into money, capitalise them. The standard total-cost-of-ownership form is:

TCO = purchase price + A × P0 + B × Pk, where A = annuity factor × energy price × 8,760 and B = A × load factor².

At 8% discount rate, 25 years and USD 0.12/kWh, the annuity factor is 10.675, so A works out to about USD 11.2 per watt of no-load loss — no-load loss runs 8,760 hours a year. Load loss is capitalised at the same rate multiplied by the square of the load factor, so at 70% loading B is about USD 5.5 per watt.

Run that on the 630 kVA row: 830 W × 11.2 = roughly USD 9,300 of capitalised no-load loss, and 7,860 W × 5.5 = roughly USD 43,000 of capitalised load loss. A lower-loss design that cuts load loss by 10% is worth about USD 4,300 in present value — often more than the price difference between two suppliers. That is the entire argument for specifying an efficiency level rather than accepting the catalogue default, and it is why the published table above should be checked against the efficiency regulation of the destination market.

Note also the load point at which efficiency peaks. For medium-voltage liquid-immersed transformers the reference load point used in efficiency standards is 50% of rating, not the 35% used for low-voltage dry-type units. Comparing an efficiency figure quoted at one load point with a figure quoted at another is a common and expensive mistake.

Step 3: Impedance and Fault Level

The published impedance is 6.5% for the whole range. Impedance does two jobs at once: it limits the LV fault current the downstream switchboard must withstand, and it sets the voltage dip when a large motor starts.

A lower impedance gives a stiffer supply and better motor starting, but a higher LV fault level. A higher impedance reduces the fault level and allows a cheaper LV board, but produces deeper voltage dips and more flicker. On a 35/0.4 kV unit the 6.5% figure is a reasonable compromise for general distribution; where the LV board is being designed at the same time, run both cases and let the fault study choose.

Step 4: Cooling, Temperature Rise and Overload

Oil-immersed distribution transformers are normally supplied with ONAN cooling — oil natural, air natural — which needs no fans and no auxiliary supply. The published temperature rise for this class of unit is 55 K for oil and 55 K for windings, and that figure is what defines the permitted overload.

ONAF cooling adds fans and raises the permissible loading further, but on distribution sizes the gain is modest and the fans become a maintenance item and a single point of failure. Two practical positions:

• Firm capacity needed now — buy the next size up rather than rely on forced cooling to cover a shortfall. A 630 kVA unit running on fans to carry 700 kVA has no margin left for a hot week.
• Load is genuinely cyclic — where the peak lasts two hours a day, the thermal time constant of an oil-filled unit absorbs it, and the standard overload curve for 55 K rise covers it without fans.

Ambient temperature is the other half of that calculation. A unit rated at 55 K rise in a 40 °C ambient derates in a 50 °C environment, and the derating has to be applied to the capacity, not just noted on the datasheet. Where the load is expected to vary across the day and the year, the tap changer arrangement and the voltage regulation question are covered in the off-circuit versus on-load tap changer comparison.

Step 5: Mechanical and Connection Configuration

If the transformer will run in parallel with an existing unit, the vector group, impedance and tap range must all match, not just the capacity and ratio. Parallel units with different impedance share load in inverse proportion to it, and a 6.5% unit beside a 4% unit will carry far more than half the load and trip first.

Six Selection Mistakes

What to Put on the Enquiry

1. Rated capacity in kVA, with the calculated demand and demand factor stated.
2. Voltage ratio, 35/0.4 kV or 38.5/0.4 kV, and the tap range required.
3. Vector group and neutral earthing arrangement.
4. Impedance, or the LV fault level the unit must not exceed.
5. No-load and load loss limits, or the efficiency level required by the destination market.
6. Cooling type and the ambient temperature range at site.
7. Installation: indoor, outdoor, enclosure required or not.
8. Accessories: Buchholz relay, pressure relief device, temperature indicators, surge arresters.
9. High-voltage connection type and cable box details.
10. Standards, routine test reports and any third-party inspection required.

Conclusion

Selection starts with capacity, but the decision that saves or costs the most money is the loss specification. A 35 kV distribution transformer runs continuously for decades; a few hundred watts of no-load loss and a tonne of load loss, capitalised at USD 11 per watt, dwarf the difference between quotations. Fix the capacity from a demand calculation, fix the ratio and taps from the actual network voltage, then specify losses deliberately rather than accepting whatever the catalogue prints.

The published data for the whole 50 to 2,000 kVA range is on the 35 kV oil-immersed distribution transformer page, and the cost breakdown for oil-immersed transformers explains how capacity, losses, accessories and destination market requirements move the price. If the project is a substation package rather than a single unit, the 30 kV substation transformer case shows how the auxiliary and protection scope is normally bundled.

Frequently Asked Questions

What is a 35 kV oil-immersed distribution transformer used for?

It steps a 35 kV (or 33 kV, or 34.5 kV) sub-transmission or distribution voltage down to 0.4 kV for industrial plants, commercial buildings, infrastructure and local distribution. The published range covers 50 kVA to 2,000 kVA with a 6.5% impedance and a 0.4 kV low-voltage side.

Should I specify 35 kV or 38.5 kV?

Match the network. Use 35 kV on nominal 35 kV networks where the busbar voltage stays near nominal, and 38.5 kV where the network runs high or the tap range must reach further. With ±5% and ±2 × 2.5% taps, the 38.5 kV rating gives more room at commissioning.

What is the difference between no-load loss and load loss?

No-load loss is the iron loss, present whenever the transformer is energised, and it runs 8,760 hours a year. Load loss is the copper loss, proportional to the square of the load, and it is zero at no load. No-load loss dominates the capitalised cost for a lightly loaded unit; load loss dominates above about 50% loading.

Why is 6.5% impedance standard for this voltage ratio?

It balances two competing needs: keeping the LV fault current within the withstand rating of standard 0.4 kV switchgear, and keeping the voltage dip during motor starting acceptable. A lower impedance would let the LV board see higher fault current; a higher one would deepen starting dips.

Can a 35 kV transformer be installed outdoors without an enclosure?

Yes, oil-immersed distribution transformers are normally designed for outdoor installation with weatherproof bushings and a sealed or conservator tank. An enclosure is usually added for public-access sites, where safety clearance cannot be guaranteed by fencing alone.

What cooling is used on a 35 kV distribution transformer?

ONAN is the standard, with no fans and no auxiliary supply. ONAF is available where extra capacity is needed from the same frame, but on distribution ratings it adds a maintenance item and a dependency on fan availability, so most buyers prefer the next size up in ONAN instead.