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Oil-Immersed Transformers
9 Series · 1324 Models
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Oil-Immersed Transformers Overview
Oil-immersed transformers are the workhorse of power distribution worldwide — in IEC markets the 10kV and 11kV classes carry the bulk of distribution capacity — and remain the default choice wherever capacity, overload capability, and cost per kVA matter most. QDTB supplies oil-filled transformers from 10 kVA to 5,000 kVA, up to 35 kV per IEC 60076 and GB 1094, with efficiency grades evolving from S11 through S13 to the latest S20. The insulating oil does double duty - cooling the windings and suppressing partial discharge - which is why oil units handle higher continuous and short-time overloads than dry-type of the same rating. Options include hermetic sealing, on-load tap changers, and amorphous alloy cores for the lowest no-load loss.
Available Series
S Series
The S series three-phase oil-immersed distribution transformers serve North American markets, operating at 60Hz with standard primary voltages of 480V and secondary 240V per IEEE C57 / ANSI C84.1 standards. Rated 10–5000 kVA, 0.48–34.5 kV primary and 0.208–0.48 kV secondary, mineral-oil insulation with 65 K temperature rise. Suitable for utility distribution networks, substations, solar farms, and industrial power systems.
SZ Series
The SZ series oil-immersed transformers integrate an on-load tap changer (OLTC) for automatic voltage regulation under load, available across 6kV, 10kV and 35kV primary voltages per IEC 60076. Rated 10–5000 kVA, 0.48–35 kV primary and 0.208–0.48 kV secondary, mineral-oil insulation with 65 K temperature rise. Suitable for utility distribution networks, substations, solar farms, and industrial power systems.
S11-M Series
The S11-M series features sealed construction with Level 11 energy efficiency, providing enhanced protection against moisture and environmental contaminants. Rated 10–5000 kVA, 0.48–35 kV primary and 0.208–0.48 kV secondary, mineral-oil insulation with 65 K temperature rise. Suitable for utility distribution networks, substations, solar farms, and industrial power systems.
S13-M Series
The S13-M series delivers enhanced energy efficiency with Level 13 rating, combining sealed construction with improved performance. Rated 10–5000 kVA, 0.48–35 kV primary and 0.208–0.48 kV secondary, mineral-oil insulation with 65 K temperature rise. Suitable for utility distribution networks, substations, solar farms, and industrial power systems.
S20-M Series
The S20-M series represents the highest energy efficiency class for oil-immersed transformers, achieving Level 20 rating with exceptional performance. Rated 10–5000 kVA, 0.48–35 kV primary and 0.208–0.48 kV secondary, mineral-oil insulation with 65 K temperature rise. Suitable for utility distribution networks, substations, solar farms, and industrial power systems.
S11 Series
The S11 series represents standard oil-immersed distribution transformers meeting Level 11 energy efficiency standards, widely used in power distribution networks. Rated 10–5000 kVA, 0.48–35 kV primary and 0.208–0.48 kV secondary, mineral-oil insulation with 65 K temperature rise. Suitable for utility distribution networks, substations, solar farms, and industrial power systems.
SL11 Series
The SL11 series features aluminum winding construction with Level 11 energy efficiency, offering cost-effective solutions for power distribution. Rated 10–5000 kVA, 0.48–35 kV primary and 0.208–0.48 kV secondary, mineral-oil insulation with 65 K temperature rise. Suitable for utility distribution networks, substations, solar farms, and industrial power systems.
Renewable Step-Up Transformers Series
Renewable step-up transformers raise the low-voltage output of photovoltaic and wind generation to the medium-voltage grid connection, with low no-load and load losses for high annual energy yield. Rated 100–5000 kVA, 0.4/0.69 kV low voltage to 10–35 kV high voltage, mineral-oil or sealed construction with 65 K temperature rise. Suitable for solar farms, wind farms, and energy-storage interconnection.
Wound Core Transformers Series
Three-dimensional wound core (triangular core) oil-immersed transformers use a continuously wound core forming a triangular (delta) magnetic circuit with no air gaps, giving lower no-load loss, lower noise and a more compact, lighter tank than conventional stacked-core designs. Rated 30–2500 kVA in S13/S20-M·RL grades, 6–35 kV primary, built to IEC 60076 for energy-saving distribution applications.
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The values below are computed from what you selected above. Anything that cannot be computed yet is shown as an em dash — never as zero.
Iₙ = S / (√3·U) · I_sc = 100·Iₙ / uₖ · S_sc = √3·U·I_sc · I_p = k·Iₖ
Still required to complete the calculation: rated capacity S, impedance voltage uₖ, short-time or peak withstand current.
Per-model values (P0 / Pk / uₖ declared by the factory, losses, noise, temperature rise, dimensions, weight) are not in the product library: those rows stay at an em dash until the factory parameter sheet is supplied. An em dash means the value is unavailable — it is never rendered as zero.
Read-only: no price factor, not part of the sum — the loss-grade cost is already carried by the efficiency (subtype) base price, so charging here would double-bill. Values come from the product library type-test data, interpolated between published capacities; grades the library does not cover show an em dash, never zero.
S13 系列（产品库 S13-M / 卷铁芯 S13-M.RL，Class 3）
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Application Scenarios
Utility Power Grids
Primary and distribution transformers for substations up to 35 kV.
Industrial Power
Heavy-duty transformers for steel mills, chemical plants, and mining.
Commercial Distribution
Energy-efficient distribution for large commercial complexes.
Power Generation
Station transformers for thermal, hydro, and nuclear plants.
Agricultural Irrigation
Rural electrification with cost-effective solutions.
Infrastructure
Power for railways, airports, ports, and telecom.
Technical Advantages
S11 to S20 efficiency evolution
Each generation lowers no-load and load losses; S13 is the mainstream low-loss grade and S20 is the latest ultra-low-loss generation.
Superior overload capability
Insulating oil cools and quenches arcs, so oil units carry higher short-time overloads than dry-type of the same rating.
Hermetic sealing option
Fully sealed tanks keep oil from contact with air, extending oil life and cutting maintenance to periodic testing.
Amorphous core option
Combines oil cooling with an amorphous core for the lowest no-load loss in the range.
FAQ - Oil-Immersed Transformers
What are the advantages of oil-immersed over dry-type transformers?
How often does the oil need maintenance?
What cooling methods are available?
What efficiency class do they meet?
Is an on-load tap changer available?
What do the S11, S13 and S20 designations mean?
What is a 10kV oil-immersed transformer?
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Hermetically Sealed vs Conservator vs Dry-Type: How to Choose
| Hermetically Sealed (S-M) | Conservator Tank Type | Dry-Type (Cast Resin) | |
|---|---|---|---|
| Oil preservation | Tank welded shut after vacuum filling — oil sealed from the atmosphere for life | Oil expansion taken up by a conservator tank with breather and silica gel | No insulating oil at all — windings encapsulated in cast resin |
| Routine maintenance | Oil tested only at long intervals; no top-ups in normal service | Breather silica gel and oil level checked periodically, oil tests on schedule | No oil sampling or leak management over the service life |
| Cooling | ONAN, with ONAF available on larger ratings | ONAN, ONAF or OFAF depending on rating | AN or AF |
| Fire safety & location | Mineral oil load needs containment and fire separation | Same oil load, plus headroom above the tank for expansion | Self-extinguishing and oil-free, so suitable indoors |
| Overload capability | Circulating oil carries heat away, giving generous short-time overload | The same oil cooling benefit as a hermetic unit | Lower short-time overload than an oil unit of equal rating |
| Cost per kVA | Lowest of the three at distribution ratings | Comparable to hermetic at the same rating | Higher, driven by resin and the winding process |
| Typical application | Outdoor distribution, pole and pad mounting | Utility and industrial substations with inspection access | Indoor electrical rooms, metro and data centres |
Where QDTB® Oil-Immersed Transformers Are Specified
Oil-immersed units carry the bulk of distribution capacity in IEC markets, so they are specified wherever a 10 kV or 11 kV feeder has to be stepped down to 0.4 kV over a wide area: utility distribution networks, industrial plants, commercial estates and agricultural irrigation feeders. The oil both cools the windings and suppresses partial discharge, which is why one oil unit serves a load that would need a larger dry-type machine.
On renewable projects the same platform works as the step-up transformer at the collector substation. Solar and wind farms in the QDTB® renewable range use oil-immersed or wound-core designs where the transformer stands outdoors and the ambient swings widely across the year.
Petrochemical, mining and heavy-industry plants specify oil-immersed distribution units for motor and process loads that see frequent overload, and move to the S13 or S20 loss grades where the operator buys against total cost of ownership rather than first cost. Those sites also specify the option list — hermetic sealing, OLTC, amorphous core — against their own maintenance capability.
Hermetic sealing is the default where the site has no crew to test oil and change silica gel; conservator designs are chosen where the utility wants a visible oil level and headroom for expansion. An on-load tap changer is quoted on request for networks with wide voltage swings.
Frequently Asked Questions
What are the advantages of oil-immersed over dry-type transformers?
Higher capacity range, better overload, lower cost per kVA, superior cooling. Ideal for outdoor installations.
How often does the oil need maintenance?
Hermetic sealed designs need testing every 3-5 years only. No oil top-ups required.
What cooling methods are available?
ONAN standard up to 2,500 kVA, ONAF for larger units, OFAF for highest capacity.
What efficiency class do they meet?
Standard meets GB 20052 Class 2. Amorphous alloy achieves Class 1 with 60-70% lower no-load losses.
Is an on-load tap changer available?
Yes, with MR and domestic brands for voltage regulation without power interruption.
What do the S11, S13 and S20 designations mean?
They are Chinese efficiency grades for oil-immersed transformers. Each generation lowers no-load and load losses - S13 is the current mainstream low-loss grade and S20 is the newest ultra-low-loss generation.
What is a 10kV oil-immersed transformer?
A distribution transformer that steps 10,000V down to 0.4kV (400V) for commercial and industrial loads. The 10kV class is the standard distribution voltage in China and many IEC markets (11kV in India and the UK), with common ratings from 100 kVA to 2,500 kVA.