Oil-immersed vs dry-type transformers: how to choose in Tashkent
A practical comparison of oil-immersed and dry-type transformers for buildings, plants and substations in Tashkent: fire safety, losses, price, upkeep.
Choosing between oil-immersed vs dry-type transformers is one of the first decisions in a 10/0.4 kV project, and it is rarely a question of which technology is better in the abstract. It depends on where the unit will stand, what the fire-safety designer will accept, and how losses, purchase price and upkeep add up over 20–30 years of service. This article sets out the practical criteria we discuss with engineers and procurement teams in Tashkent and elsewhere in Uzbekistan, using the S13 oil-immersed and SC(B)13 cast-resin series as reference points.
Oil-immersed vs dry-type transformers: the two designs
- Oil-immersed (S13). Core and windings sit in mineral oil inside a hermetically sealed corrugated-fin tank; the oil is both insulation and coolant (ONAN). Range 30–2500 kVA, 6 / 6.3 / 10 kV to 0.4 kV, Yyn0 or Dyn11, off-circuit taps ±2×2.5 % or ±5 %. Details: S13 oil-immersed distribution transformer.
- Cast-resin dry-type (SC(B)13). The HV coils are vacuum-cast in epoxy resin, the LV winding is usually foil-wound (the "B" in the code), and cooling is natural air (AN) or forced air with fans (AF). Class F or H insulation; state any required IEC 60076-11 environmental, climatic and fire-behaviour classes (for example E2, C2, F1) in the enquiry. Range 30–2500 kVA, 6 / 10 kV to 0.4 kV, delivered with a temperature controller (fan control, alarm, trip). The same page covers the SG(B)13 open-wound (non-encapsulated, class H) variant. Details: SG(B)13 / SC(B)13 dry-type transformer.
Type-test certificates from a national testing centre are available for S13-M-500/10 and S13-M-1000/10 to GB/T 1094 (IEC 60076) and for SCB13-500/10 and SCB13-1000/10 to GB/T 1094.11 (IEC 60076-11); see the certificates page.
Fire safety and permitted locations
- Mineral oil is combustible. An oil-immersed unit indoors normally requires a dedicated transformer cell with fire-rated walls, an oil catch pit or bund sized to the oil volume, ventilation and separation from escape routes. Each item costs floor area and civil work, and in the basement of an occupied building it is often not acceptable at all.
- Cast resin contains no oil. Where fire behaviour class F1 (IEC 60076-11) is required, specify it in the enquiry: it means restricted flammability, self-extinguishing coils and limited emission of smoke and toxic substances. No pit, no oil handling, and the unit can be placed in an electrical room next to the LV switchgear (NKU), which shortens the 0.4 kV busbars.
- Outdoors the picture reverses. A sealed oil tank is indifferent to rain, dust and temperature swings. A dry-type unit outdoors needs an enclosure (IP20–IP54 options), filtered ventilation and periodic cleaning of the coils, which matters during the dusty summer months around Tashkent.
Fire-safety requirements for a specific room are set by the project's fire-safety designer under the applicable norms, which we do not quote here; confirm them before the transformer type is fixed.
Losses and operating cost
Transformer losses have two parts: no-load loss (P0) runs for all 8760 hours a year regardless of load, and load loss (Pk) scales with the square of the load. Datasheet values for the S13 series (Dyn11 load loss shown, referred to 75 °C):
- S13-M-400: P0 = 410 W, Pk = 4 520 W, short-circuit impedance 4 %
- S13-M-1000: P0 = 830 W, Pk = 10 300 W, impedance 4.5 %
- S13-M-2500: P0 = 1 830 W, Pk = 21 200 W, impedance 5 %
A worked example, with the assumptions stated explicitly: a 1000 kVA S13 operating 8760 h a year at an rms-equivalent load factor of 50 %. No-load energy is 830 W × 8760 h ≈ 7 270 kWh; load energy is 10 300 W × 0.5² × 8760 h ≈ 22 560 kWh; total roughly 29 800 kWh a year. Multiply by your own electricity tariff to get the annual cost of losses. At a 30 % load factor the load component falls to about 8 120 kWh, so no-load loss carries more weight in lightly loaded buildings.
The two designs cannot be compared line by line from a generic table. A cast-resin unit of the same rating and performance level usually has a higher no-load loss than a sealed oil unit, while its load loss is referred to a higher winding reference temperature (120 °C for class F rather than 75 °C for oil), so the figures must be compared for the actual rating. Exact SC(B)13 values are stated in the quotation for the chosen rating and vector group. Where losses drive the decision for an indoor unit, consider the SC(B)-NX1 energy-efficiency grade 1 dry-type transformer to GB 20052, or the SC(B)H15 amorphous-alloy dry-type transformer for lightly loaded sites where no-load loss dominates.
Price, footprint and installation
- Purchase price. For the same kVA rating, a cast-resin transformer usually costs more than a sealed oil-immersed one. Compare quotations for the actual rating and options rather than rules of thumb.
- Civil works. The oil unit needs a pit, a cell and fire separation indoors, or a pad and fence outdoors. The dry-type unit needs an enclosure and a ventilated room but no pit. For indoor projects the difference in total installed cost is therefore smaller than the difference in transformer price.
- Complete outdoor solution. For a plant boundary, pump station or yard, an S13 inside a YB prefabricated substation (10/0.4 kV, up to 2500 kVA) gives a compact, factory-tested box-type substation (KTP) that needs only a foundation and cable connections on site.
Maintenance, noise and overload
Maintenance. The S13 is hermetically sealed: no oil top-up, no breather and no silica gel over the service life. Routine work is a visual check of the tank, gaskets, bushings, oil-temperature indicator and pressure-relief device. The SC(B)13 has no oil at all, but the coils and cooling ducts must be kept free of dust, and the fans and temperature controller should be checked at each inspection.
Noise. The oil tank damps core noise, and the manufacturer states that S13 sound levels are about 20 % below the JB/T 10088-2016 limits. A dry-type unit is generally louder at the same rating, and fans add noise in AF mode. Where a transformer room adjoins flats or offices, specify a low-noise design and anti-vibration mounts, or move an oil unit into an outdoor KTP away from the façade.
Overload. Oil has a large thermal mass, so an S13 rides through short peaks with little temperature rise; sustained overload should follow the IEC 60076-7 loading guide for oil-immersed units. A cast-resin unit responds faster to load changes, but AF forced-air cooling adds 40–50 % capacity, with the temperature controller providing alarm and trip; the manufacturer describes this as short-time overload capability, so confirm in the enquiry if you intend to run at the AF rating for extended periods (IEC 60076-12 is the loading guide for dry-type units). This suits shopping centres and PV inverter rooms with predictable peaks, provided the AN rating covers the normal load.
Ambient conditions. IEC 60076-1 assumes a maximum ambient of 40 °C, which Tashkent summers can exceed on the hottest days, so state the site maximum in your enquiry. At around 450 m elevation, Tashkent and most sites in Uzbekistan are well below the 1000 m reference altitude, so no altitude derating applies.
Decision matrix: oil-immersed or dry-type transformer?
| Criterion | Oil-immersed S13 | Cast-resin SC(B)13 |
|---|---|---|
| Outdoor pad or box-type substation (KTP) | Preferred | Only in a ventilated IP-rated enclosure |
| Electrical room in an occupied building | Fire cell and oil pit required | Preferred: no oil |
| Basement, upper floor, next to the load | Usually not permitted | Yes |
| No-load loss | Lower at the same rating | Usually higher; grade 1 (NX1) or amorphous SC(B)H15 available |
| Load loss | Referred to 75 °C | Referred to 120 °C (class F); compare per rating |
| Transformer purchase price | Lower | Higher |
| Civil works | Pit, cell, separation or outdoor pad | Enclosure and ventilation |
| Maintenance | Sealed tank, minimal | Dust cleaning, fan checks |
| Noise | Lower | Higher, especially in AF mode |
| Short-time overload | Good thermal mass | AF adds 40–50 % capacity |
| Dust and humidity | Insensitive when sealed | Needs filtered air; specify E2 and C2 classes |
Typical choices in Tashkent projects
- Residential complex with a built-in substation on the ground floor: cast-resin, in a room with NKU.
- Office block or shopping centre with electrical rooms on several floors: cast-resin near each load centre.
- Industrial plant with a substation at the site boundary: S13 in a YB or similar box-type substation.
- Pump station, agricultural site or remote feeder: sealed oil-immersed unit outdoors.
- Hospital or data centre with indoor distribution and redundancy: cast-resin, N+1 configuration.
Summary and checklist
- Location first: outdoors or in a separate building, oil-immersed; inside an occupied building, cast-resin.
- Obtain the fire-safety designer's requirements for the room before fixing the type.
- Estimate annual hours and load factor, then compare P0 and Pk from the datasheets, not only the price.
- State the site maximum ambient temperature, air quality and noise limits in the enquiry.
- Define peak load and future growth: AF cooling, tapping range, spare capacity.
- Specify vector group and short-circuit impedance so the LV switchgear fault level is correct.
- Ask for type-test certificates, the routine test report and the passport in Russian with the offer.
If you send us the rating, voltage combination, installation location and load profile, we will prepare a comparative quotation for oil-immersed and cast-resin options with datasheet losses, dimensions and delivery time to Uzbekistan. Contact us through the enquiry form.
