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Power transformers
The machine that turns transmission voltage into distribution voltage. It is a different class of equipment from anything that sits on a pad, and it is worth being clear about which one you are actually looking for.
What a power transformer is
A power transformer steps voltage between transmission and distribution levels. Typical primaries are 69 kV, 115 kV, 138 kV and 230 kV, stepping down to 12.47 kV, 13.2 kV or 34.5 kV for the distribution network. It lives inside a substation, it is rated in MVA rather than kVA, and it is engineered for one site rather than built to stock.
Almost every one is oil filled. Mineral oil, or increasingly a natural ester, both insulates the windings and carries heat out to the radiators. That is what makes ratings in the tens or hundreds of MVA physically possible.
Power transformer against distribution transformer
This is the question behind most searches for the term, and the answer is position in the network rather than anything about how the machine works.
| Power transformer | Distribution transformer | |
|---|---|---|
| Rated in | MVA | kVA |
| Typical size | 5 MVA to 500 MVA and above | 15 kVA to 5000 kVA |
| Primary voltage | 69 kV to 765 kV | 4.16 kV to 34.5 kV |
| Where it sits | Inside a substation | On a pad, a pole or a plant floor |
| Loading | Near continuous, close to rating | Varies through the day |
| How it is bought | Engineered to order, long contract | Catalogue build or from stock |
| Typical lead time | Often measured in years | Weeks to months |
A pad mounted transformer is a distribution transformer. If you are feeding a building, a site or a piece of plant from a utility service, that is the class of equipment you are looking for, not this one.
Typical ratings and cooling classes
Cooling class tells you how much of the rating is available without help. ONAN is natural oil and natural air. ONAF adds fans. OFAF adds forced oil. A unit is commonly given two or three ratings, one per stage, written as something like 20/26.7/33 MVA.
| Class | Meaning | Where it is used |
|---|---|---|
| ONAN | Oil natural, air natural | Base rating, no moving parts |
| ONAF | Oil natural, air forced | Fans add roughly a third more capacity |
| OFAF | Oil forced, air forced | Larger units where pumps are justified |
| ODAF | Oil directed, air forced | Directed flow through the windings |
Why the lead times are what they are
Grain oriented electrical steel, high voltage bushings and load tap changers each come from a small number of plants worldwide. Every unit is designed for its site, tested individually, and shipped as an oversize load. None of that scales quickly.
Demand from data centres, ageing grid replacement and renewables interconnection has outrun that capacity. Quoted lead times moved from months to years and have not come back. If you are planning around one, treat the lead time as the constraint that sets the programme, not a line on the schedule.
We do not sell power transformers. We are a distributor of three phase pad mounted distribution transformers, and saying so is more useful than pretending otherwise. If a substation class unit is genuinely what the job needs, an EPC or the manufacturer direct is the right route.
Where the line actually falls
Plenty of jobs that people describe as needing a power transformer are served by a large distribution unit. Pad mounted transformers are built up to 2500 kVA as standard and beyond that on request, which covers most commercial and light industrial loads including a lot of what gets called a substation on a drawing.
Not sure which class the job needs. Send the load, the utility voltage and the service size and we will tell you straight, including when the answer is somebody else. Send the details or call 786 220 2801.