Both types do the same physical job — they transfer energy between two voltage levels through a magnetic core — but where they sit in the network decides almost everything else about them. A distribution transformer is the last step before the customer, converting medium voltage into usable low voltage. A power transformer sits further upstream, moving bulk energy between transmission and distribution levels.
Where each type sits in the network
Distribution transformers step medium voltage, typically up to 36 kV, down to the low voltage used by buildings, factories and the street network. They are the units you find in a substation cabinet on the pavement, in a building basement or on a pole in rural networks. Ratings usually run from about 25 kVA to 4,000 kVA.
Power transformers operate at higher voltages and ratings — often several MVA to hundreds of MVA — at transmission and primary-distribution substations. A unit with a 110 kV or 150 kV primary feeding a 20 kV distribution ring is a power transformer, even though its output eventually reaches distribution networks.
Why the design priorities are different
A distribution transformer is energised continuously but rarely loaded to its full rating. Its losses are dominated by no-load (core) losses, which run 8,760 hours a year regardless of load. That is why EU EcoDesign Tier 2 tightens no-load loss limits so firmly, and why core material choice matters more here than almost anywhere else.
A power transformer is usually loaded much closer to its rating, so load losses in the windings carry more weight in the economic calculation. The design effort shifts towards impedance control, short-circuit withstand, thermal behaviour under sustained load and a detailed type-test programme.
The parameters that differ in practice
- Rating: roughly 25 – 4,000 kVA for distribution units; 5 MVA and upward for power transformers.
- Impedance voltage (uk%): typically 4 – 6 % for distribution units, 8 – 20 % for power transformers to limit short-circuit currents downstream.
- Vector group: Dyn11 is the workhorse in distribution; YNd11 and custom groups are common in transmission.
- Tap changing: off-circuit taps (DETC) on distribution units; on-load tap changers (OLTC) on most power transformers.
- Cooling: ONAN is standard for distribution; power units add ONAF, OFAF or ODAF stages.
Testing and delivery differ too
Both are designed and tested to the IEC 60076 series, but the depth of the programme is not the same. Distribution transformers are covered by routine tests on every unit plus type tests on a representative sample. Power transformers are typically engineered to order, with a witnessed factory acceptance test, a temperature-rise test and lightning impulse withstand testing on the specific unit.
That difference shows in lead time and in how much of the specification is yours to set. A distribution transformer is largely selected from a defined range; a power transformer is designed around your voltages, impedance, loss targets and footprint.
As a rule of thumb: if the low-voltage side feeds equipment directly, you are looking at a distribution transformer, and efficiency at partial load should drive the choice. If it feeds another network layer, you are looking at a power transformer, and impedance, short-circuit strength and tap range matter more. When a project sits on the boundary — an industrial connection of a few MVA, say — it is worth agreeing the specification with your supplier before fixing the rating.
