A transformer nameplate carries everything needed to verify a unit against its specification — provided you can decode it. The two fields most often misread are the rating (it is kVA, not kW) and the vector group (it decides whether you can parallel or replace the unit at all). This page decodes every field, including the Chinese model codes that have no direct international equivalent.
Nameplate fields at a glance
| Field | What it states | Why it matters |
|---|---|---|
| Rated power | Apparent power in kVA | Current-carrying capability; kW depends on load power factor |
| Rated voltages | HV and LV, often with tap range | Insulation coordination and ratio verification |
| Rated frequency | 50 Hz or 60 Hz | Not interchangeable; affects flux, losses and cooling |
| Number of phases | Three-phase or single-phase | Connection and application |
| Vector group | e.g. Dyn11 | Decides paralleling, earthing and harmonic behaviour |
| Impedance voltage | Percentage | Fault level and voltage regulation |
| Cooling method | ONAN, ONAF, AN, AF | Rating headroom and noise |
| Insulation class | Class A, F or H | Thermal limit and insulation life |
| Temperature rise | e.g. 60 K / 65 K winding rise | Actual thermal duty under load |
| Connection diagram | Terminal markings and links | Installation correctness |
| Insulation levels | LI / AC withstand values | Coordination with surge protection |
| Mass and oil volume | Total, oil, transport | Foundation, handling, transport, environmental planning |
| Standard | e.g. IEC 60076 | Which limits and tolerances apply |
| Serial number and year | Unit identity | Ties the unit to its test certificate and warranty |
Vector group: the field that decides compatibility
The vector group notation looks cryptic but is strictly structured. Take Dyn11:
- D — capital letter, the high-voltage winding is connected in delta.
- y — lowercase, the low-voltage winding is connected in star (wye).
- n — the neutral of the LV winding is brought out to a terminal.
- 11 — the phase displacement, in units of 30 degrees, so 11 means 330 degrees (equivalently, minus 30 degrees) between HV and LV.
Dyn11 is the most common distribution vector group. The delta-connected HV winding gives triplen harmonics a circulating path, which keeps them out of the supply, while the star-connected LV side provides a neutral for single-phase loads.
The practical rule: units intended to run in parallel must share the same vector group, a closely matched impedance voltage, and the same ratio and tap position. Mismatched vector groups put a phase displacement between the two secondaries, which drives circulating current even with no load connected. On a replacement unit, check the vector group before anything else.
Impedance voltage
Impedance voltage is the voltage that must be applied to one winding to circulate rated current in the other with that winding short-circuited, expressed as a percentage of rated voltage. It does two jobs:
- It limits fault current. A higher impedance reduces the current the transformer contributes to a downstream fault, which also reduces the mechanical stress on the windings.
- It sets voltage regulation. The same impedance causes voltage drop under load, so higher impedance means poorer regulation.
Typical distribution values sit around 4–6%, with 6% commonly chosen for parallel operation. Furnace and special transformers run deliberately higher — 6–12% typical for electric arc furnace duty — to limit current and stabilise the arc.
Decoding the Chinese model codes
| Code | International wording | Core |
|---|---|---|
| S11 / S13 | Oil-immersed distribution transformer, baseline efficiency tiers | Silicon steel |
| S20 / S22 | Oil-immersed distribution transformer, higher efficiency tiers | Silicon steel |
| SH15 | Oil-immersed amorphous alloy distribution transformer | Amorphous metal |
| SZ11 | Oil-immersed OLTC power transformer (on-load tap changing) | Silicon steel |
| SCB | Cast resin dry-type transformer | Silicon steel |
| SCBH15 | Cast resin dry-type transformer, amorphous core | Amorphous metal |
| YBW | Prefabricated substation — also package substation or kiosk substation | n/a |
These are GB designations and have no international equivalent, so a buyer outside China cannot verify an “S20” against any standard they hold. Always restate the designation in international terms on the drawing and nameplate. Note also that GB 20052 is the Chinese minimum energy performance standard: outside China it is not a compliance route on its own and should be quoted alongside IEC and EU Ecodesign or DOE parameters — and because it is revised, confirm the current version rather than relying on a legacy code.
Cooling designation
For oil-immersed units the four letters read in pairs: the first pair is the internal cooling medium and the second the external medium, with the circulation method as the second letter of each pair.
- ONAN — Oil, Natural circulation; Air, Natural. The baseline self-cooled rating.
- ONAF — Oil natural, Air Forced. Fans over the radiators give additional rating headroom.
- OFAF / ODAF — Oil forced (or directed), air forced. Higher ratings again.
- OFWF — Oil forced, Water forced. Where water cooling is available.
- AN / AF — dry-type: Air Natural and Air Forced.
Each step up gives more rating, and each forced-air step raises noise — so a noise-sensitive site must be specified at the peak cooling condition, not the natural-cooled figure. See the cooling classes page for the full table.
Insulation class and temperature rise
Insulation thermal class describes the insulation system: Class A to 105 °C (mineral-oil-immersed), Class F to 155 °C and Class H to 180 °C (typical dry-type classes). A higher class permits a higher temperature rise, or the same rise with more thermal margin and longer insulation life.
Two cautions. The class describes the insulation system, not the whole transformer. And service life depends on the actual operating temperature, not the class alone — which is why stating ambient temperature and altitude matters.
Five misreadings worth avoiding
- Reading kVA as kW. A 1000 kVA unit at 0.8 power factor delivers 800 kW.
- Ignoring the vector group on a replacement. It decides whether the unit can be paralleled at all.
- Assuming the rating applies at any altitude. Above 1000 m, IEC 60076-2 requires correction and possibly derating.
- Comparing impedance figures across standards. IEC and ANSI/IEEE define and measure differently, and tolerances differ by edition.
- Treating the model code as an international specification. Restate S20, SH15 and SCBH15 in international terms before ordering.
Frequently asked questions
Is the rating on the nameplate kW or kVA?
kVA, apparent power. The real power in kW depends on load power factor: a 1000 kVA transformer at 0.8 power factor delivers 800 kW. This is the most common nameplate misreading and it leads to undersized enquiries.
What does Dyn11 mean?
The vector group: D for delta HV, y for star LV, n for brought-out neutral, 11 for 330 degrees phase displacement. It is the most common distribution group, giving triplen harmonics a circulating path and providing an LV neutral.
Can I parallel two transformers with different vector groups?
No, not safely. Mismatched groups produce phase displacement between secondaries, driving circulating current even unloaded. Parallel units need the same vector group, matched impedance and the same ratio and tap position.
What is impedance voltage and why does it matter?
The voltage required to circulate rated current with the other winding short-circuited, as a percentage. It sets fault contribution and voltage regulation, and must be matched across parallel units.
How do I decode ONAN, ONAF, AN and AF?
Medium then circulation, twice. O is mineral oil, A is air, N is natural, F is forced. ONAN is self-cooled, ONAF adds forced air, AN and AF are the dry-type equivalents. Forced steps add rating and noise.
What do S20, SH15, SCBH15 and SZ11 mean?
GB designations with no international equivalent: efficiency tiers of oil-immersed silicon steel (S11/S13/S20/S22), oil-immersed amorphous (SH15), cast resin dry-type (SCB) and its amorphous version (SCBH15), and oil-immersed OLTC (SZ11). Restate them internationally before ordering.
Why is altitude not on the nameplate, and does it matter?
The plate gives rating at reference conditions. Above 1000 m, IEC 60076-2 requires temperature-rise correction and possibly derating. A site at 2500 m changes the usable rating — state altitude and ambient at enquiry stage.