Products/Specification reference
The notation, options and standard values that decide what a liquid-immersed transformer actually is. Published here because a supplier who cannot discuss vector groups and cooling classes precisely is not a supplier you should be evaluating.
Everything on this page is general engineering practice under the IEEE C57 series. Where a value is described as standard or typical, the governing figure is always the one in your own specification.
IEEE C57.12.00 designates cooling with a four-letter code. Each position commits the design to something specific, and the code is the difference between a rating the unit holds passively and one it only reaches with fans running.
Wholly passive. No fans, no pumps, nothing to fail and nothing to maintain. The base rating of almost every liquid-immersed unit.
A unit quoted as ONAN/ONAF carries two ratings: the passive ONAN rating and a higher ONAF rating with the fans running. Specify which of the two your load study is based on. Quoting the forced rating against a requirement that must be met passively is one of the more common ways a transformer bid is compared unfairly.
The vector group states how each winding is connected and the phase displacement between them, expressed as a clock hour where each hour is 30°. Two transformers cannot be paralleled unless both match.
Delta HV, earthed wye LV. Clock 11 — LV leads HV by 30°.
The default for a distribution substation stepping down to an earthed LV network.
Delta HV, earthed wye LV. Clock 1 — LV lags HV by 30°.
Same windings as Dyn11, opposite displacement. Units must match before they can be paralleled.
Earthed wye HV, delta LV. Clock 1 — LV lags HV by 30°.
Where the HV system needs an earthed neutral and the LV side is a generator or delta industrial load.
Earthed wye both sides. Clock 0 — LV in phase with HV.
A neutral on both sides, but no inherent zero-sequence path; a delta tertiary is often specified alongside.
Delta both sides. Clock 0 — LV in phase with HV.
Industrial supplies where no neutral is required on either side.
Adjusted only with the transformer disconnected from supply. Used where the supply voltage is stable and taps are set once at commissioning to suit the actual system voltage at that location.
Commonly specified as five positions at ±2 × 2.5% on the HV winding.
Changes ratio under load, driven by an automatic voltage regulator. Required where the secondary voltage must be held within a band as load or upstream voltage varies through the day.
Commonly specified around ±10% in 32 steps, with a motor drive and control cabinet. It is a mechanism with a maintenance schedule — worth confirming it is genuinely needed.
Tap range, step size, which winding is tapped, and whether the taps are full-capacity or reduced-capacity all change the design. An enquiry that says only “with taps” will come back with a question rather than a price.
Rise is quoted above ambient, not as an absolute temperature. The modern standard for liquid-immersed units is a 65°C average winding rise, with the hottest-spot rise limited above that. Older utility specifications sometimes still call for 55°C, or for a dual 55/65°C rating.
Ambient matters as much as rise. The standard basis is an average ambient over 24 hours with a stated maximum; a site that runs hotter than the standard basis needs that stated at enquiry, because it changes the thermal design rather than just the paperwork.
Loading beyond nameplate is covered by the IEEE C57.91 loading guide. If you intend to plan cyclic or emergency overload, say so at enquiry — it is a design input, not an operating decision to be taken later.
Basic impulse insulation level is what the winding and bushings are built to withstand as a lightning impulse. It follows the voltage class, but more than one BIL is standard at most classes, and the choice is the purchaser’s.
| Nominal system voltage | Standard BIL values (kV) |
|---|---|
| 15 kV class | 95 · 110 |
| 25 kV class | 125 · 150 |
| 34.5 kV class | 150 · 200 |
| 46 kV class | 200 · 250 |
| 69 kV class | 250 · 350 |
| 115 kV class | 350 · 450 · 550 |
| 138 kV class | 450 · 550 · 650 |
Listed as the values commonly specified at each class under the IEEE C57 series, for orientation only. Reduced insulation levels are available at several classes, and the governing figure is the one stated in your specification. We confirm BIL, along with bushing creepage and any altitude derating, before quoting.
Impedance is not a number to leave to the manufacturer. It sets the through-fault current your downstream equipment must interrupt, and it sets how far the secondary voltage sags under load.
Raise it and fault duty falls, but regulation gets worse and losses under load rise. Lower it and regulation improves, but the switchgear downstream has to be rated for more fault current. Transformers intended to run in parallel need matched impedance as well as a matched vector group, or they will not share load in proportion to their ratings.
Give us the impedance and the tolerance your system study requires. If the specification leaves it open we will say so rather than quietly quoting whatever is cheapest to build.
Send these and you get a price and a dated schedule. Send fewer and you get questions.
You will get a clause-by-clause compliance response with exceptions listed explicitly, rather than a blanket statement of conformity.