Sealed oil-immersed distribution transformers from level 3 (S13) to level 1 (S22) efficiency — the standard answer for outdoor substations, package substations and utility networks.
Where a dry-type transformer is not required or not economic, the oil-immersed unit remains the reference choice. It costs less per kVA, it is available in far larger ratings, and its outdoor siting needs no fire-rated building in most jurisdictions. Choosing between S13, S20 and S22 is essentially a decision about how much the operating cost of lost energy matters to the end user.
The S series is a three-phase oil-immersed self-cooled distribution transformer with a sealed oil tank. The suffix M denotes the fully sealed design, which reduces oil oxidation and moisture ingress and makes the unit essentially maintenance-free. S13 sits at level 3 (NX3) efficiency, S20 at level 2 (NX2) and S22 at level 1 (NX1) under GB 20052-2024.
The laminated core carries the main flux. The three-phase windings are immersed in transformer oil, which performs three jobs at once: electrical insulation, heat transfer and arc quenching. The generated heat moves by natural convection through the oil and out through the radiator bank — that is the ONAN cooling designation. Regulation is by an off-circuit tap changer on the standard build; an on-load tap changer can be specified where supply voltage varies over a wide range.
| Parameter | Typical value |
|---|---|
| Rated power | 50 / 100 / 160 / 200 / 250 / 315 / 400 / 500 / 630 / 800 / 1000 / 1250 / 1600 / 2000 / 2500 kVA |
| Voltage combination | 10 kV / 0.4 kV (6 / 6.3 / 10.5 / 11 kV and 0.23 / 0.415 / 0.69 kV on request) |
| Rated frequency | 50 Hz / 60 Hz |
| Vector group | Dyn11 standard, Yyn0 available |
| Short-circuit impedance | 4% up to 630 kVA, 4.5% to 6% from 800 kVA |
| Efficiency class | S13 = level 3 (NX3), S20 = level 2 (NX2), S22 = level 1 (NX1), per GB 20052-2024 |
| Cooling | ONAN oil natural air cooling |
| Tank type | Sealed oil tank (M suffix) |
| Voltage regulation | Off-circuit tap changer standard, on-load tap changer on request |
| Protective devices | Oil level indicator, temperature indicator, pressure relief valve, Buchholz relay on larger ratings |
| Ambient temperature | -25 °C to +40 °C |
| Altitude | Up to 1000 m without derating |
| Compliance | GB 20052-2024, GB/T 6451-2023, GB/T 1094 series |
Start from the connected load and apply the usual 1.1 to 1.2 headroom, then check the impedance so that the secondary fault level remains inside the withstand rating of the outgoing equipment. Then decide the efficiency class by running the numbers: the difference between S13 and S22 is paid back fastest where the transformer is large, the load rate is high and the tariff is high, because the saving is driven by the no-load loss that runs continuously. Finally, settle the installation type — inside a package substation the transformer rating is often fixed by the kiosk enclosure dimensions rather than by your preference.
They are the same platform with different efficiency levels under GB 20052-2024. S13 is level 3 (NX3), the admission threshold; S20 is level 2 (NX2); S22 is level 1 (NX1), the most efficient. The higher the number, the lower the no-load and load losses.
M denotes the fully sealed oil tank. The sealed design limits oxidation and moisture ingress and reduces the maintenance needed compared with an open conservator tank.
It can be installed indoors in a dedicated transformer room with adequate ventilation and fire separation. Wherever fire rating, floor space or indoor siting is critical, the dry-type SCB series is the better choice.
Typically 55 dB(A) or below for standard oil-immersed units, with the exact figure stated on the nameplate and in the routine test report for the specific rating.
Yes. The standard build uses an off-circuit tap changer; an on-load tap changer is available on request where the incoming voltage varies over a wide range.
GB 20052-2024 for energy efficiency, GB/T 6451-2023 for oil-immersed power transformers and the GB/T 1094 series for general requirements, temperature rise and insulation levels.
10 days for standard ratings and 15 days for non-standard ratios or impedances, counted from deposit plus confirmed technical agreement plus approved drawing. Warranty is 12 months after commissioning.
Send us the rated voltage, rated current or capacity, quantity and destination country, plus any single-line diagram. We reply with a configuration proposal and a quotation — usually within 24 hours of receiving complete technical information.
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