A distribution transformer is the last transformation step before electricity reaches the user. It takes medium voltage from the local network — typically 10 kV, 20 kV or up to 36 kV — and steps it down to the 400/230 V used by buildings, machines and street lighting. Ratings usually run from about 25 kVA to 4,000 kVA.
Where you find one
In a street cabinet or a compact substation on the pavement, in the basement or plant room of a building, on a pole in rural networks, or inside an industrial site feeding a factory's low-voltage boards. Increasingly they also sit at EV charging hubs, solar parks and datacentre connections, where the load is new but the function is unchanged.
How it works
Two windings share a laminated core of grain-oriented electrical steel. Alternating current in the medium-voltage winding creates a changing magnetic flux in the core, and that flux induces a voltage in the low-voltage winding in proportion to the ratio of turns. There is no electrical connection between the two — the energy crosses magnetically.
That magnetic coupling also isolates the two networks galvanically, which is what allows the low-voltage side to have its own earthing arrangement. The vector group — Dyn11 on the great majority of European distribution units — describes how the windings are connected and the phase shift between them.
The parameters that define one
- Rated power in kVA — the continuous load it can carry under rated conditions.
- Voltage ratio, for example 20 / 0,42 kV, including the tap range, commonly plus or minus 2 x 2,5 %.
- Impedance voltage uk%, typically 4 % or 6 % — it sets the short-circuit current on the low-voltage side and how well units can be paralleled.
- No-load and load losses, both capped by EU EcoDesign Tier 2.
- Vector group, cooling class (usually ONAN for oil, AN for cast resin) and sound power level.
Why losses dominate the specification
A distribution transformer is energised every hour of the year but often loaded to well under half its rating. No-load losses therefore run continuously and, over a thirty-year life, frequently cost more than the transformer itself. That is the reasoning behind the EcoDesign limits, and the reason it is worth calculating total cost of ownership rather than comparing purchase prices.
Load losses matter where loading is high and steady — an industrial site, a datacentre, a charging hub. The right balance between the two depends on your load profile, and a supplier can quote alternative loss combinations against it.
Oil or cast resin
Both are built and tested to the IEC 60076 series and both must meet EcoDesign Tier 2. Oil-filled units give the lowest losses and the lowest cost per kVA, but need a containment pit and either an outdoor location or a fire-rated room. Cast-resin units are self-extinguishing and can stand inside a building without those measures.
If you are specifying one, start with the load profile and the site rather than the kVA number. The rating follows from the load plus a margin for growth; the choice of oil or cast resin follows from where the unit has to stand; and the loss combination follows from how many hours a year it will actually be loaded. Get those three right and the rest of the datasheet largely writes itself.
