The two types sit at different points in the network and are built to different rules: a power transformer moves bulk power between transmission voltage levels, while a distribution transformer steps voltage down to the level end users consume. This guide compares them on the six specifications that usually decide which one you need.
What is a power transformer?
A power transformer is a large, high-voltage unit installed at transmission and sub-transmission substations. Its job is to step voltage up or down between levels of the transmission network.
Power transformers are typically found at:
- Generating stations (generator step-up transformers)
- Transmission substations (220 kV, 400 kV, 500 kV, 765 kV)
- Grid interconnection points
- Large industrial plant in-feed substations
What is a distribution transformer?
A distribution transformer is a smaller, lower-voltage unit that steps distribution voltage down to the end-user level, for example 11 kV to 400 V. They sit on utility poles, in pad-mounted cabinets or inside building substations.
Distribution transformers typically range from 10 kVA to 5 MVA and operate at voltages up to 36 kV.
Key differences
1. Voltage level
| Parameter | Power Transformer | Distribution Transformer |
|---|---|---|
| Voltage Range | 33 kV to 765 kV and above | Up to 36 kV |
| Typical Applications | Transmission & Grid | End-user Supply |
2. Power rating
| Parameter | Power Transformer | Distribution Transformer |
|---|---|---|
| Power Range | 10 MVA to 1000 MVA and above | 10 kVA to 5 MVA |
| Load Factor | High (near 100%) | Lower (50 to 70%) |
3. Operating principle
Power transformers run at their highest efficiency near full load. They are designed for continuous operation at high load factors, so the design point sits at about 100% load.
Distribution transformers see a load that changes through the day, and they spend much of their life at partial load, so efficiency is tuned to a lower load factor, typically 50 to 70%.
4. Cooling method
Power transformers use forced cooling systems:
- ONAN (oil natural air natural)
- ONAF (oil natural air forced)
- OFAF (oil forced air forced)
- ODAF (oil directed air forced)
Distribution transformers are small enough to run on ONAN cooling alone.
5. Tap changer
Power transformers are almost always fitted with an on-load tap changer (OLTC) so voltage can be regulated while the unit carries load, usually over a range of ±10%.
Distribution transformers usually have off-circuit tap changers, which means the transformer has to be de-energised before the tap is moved.
6. Size and weight
A 500 MVA power transformer can weigh 300 to 500 tonnes, needs special transport and installation, and stays in a permanent substation.
A 1000 kVA distribution transformer weighs around 2 to 3 tonnes and can be pole-mounted or pad-mounted.
Which one do you need?
Choose a power transformer if:
- You’re building or expanding a substation
- Your voltage level is 33 kV or higher
- Your capacity requirement exceeds 10 MVA
- You need voltage regulation under load (OLTC)
- You’re connecting to the transmission grid
Explore our range of large power transformers and grid transformers.
Choose a distribution transformer if:
- You need to supply end users directly
- Voltage level is below 36 kV
- Capacity is below 5 MVA
- You don’t need on-load voltage regulation
Conclusion
The power transformer vs distribution transformer question comes down to voltage level, rating and where the unit sits in the grid. Power transformers carry bulk power through the transmission network; distribution transformers deliver it to end users.
If you are unsure which type fits your project, send us your voltage level, rating and load profile and we will confirm the specification. Contact our engineering team.