A transformer is a passive device that transfers electrical energy between circuits at different voltage levels. In the electricity network the same basic machine appears in two roles: the power transformer, which moves large blocks of power over long distances, and the distribution transformer, which delivers the final low voltage to homes and businesses. The design targets are different in a way that surprises many buyers: the power transformer is optimised to run at full load, while the distribution transformer is optimised to run lightly loaded most of the time. This article compares the two types and the steel that goes into them.
Power transformers: transmission duty
A power transformer is installed in a generating station or a transmission substation and is rated in megavolt-amperes at the top of the network, typically above 200 MVA with voltage classes of 400 kV, 200 kV, 110 kV, 66 kV and 33 kV. Its main job is to step the generator voltage up to transmission level, and it runs near full load whenever the plant is operating. Because it runs loaded, it is designed for maximum efficiency at 100% load, and its insulation level and physical size are the highest in the network. Cooling is always forced: large units use oil with radiators, pumps and fans, and the largest use forced oil circulation with water cooling.
Distribution transformers: consumer duty
A distribution transformer connects the medium-voltage network, at levels such as 11 kV, 6.6 kV and 3.3 kV, to the low-voltage consumer supply of 440 V three-phase or 230 V single-phase. Ratings are far smaller, typically below 200 MVA and in practice usually below 2.5 MVA. The load on a distribution transformer follows the daily demand curve of its consumers, so it is lightly loaded most of the time and only approaches its rating at peak hours. For this reason it is designed for maximum efficiency at 60-70% load, not at 100%, and it must show low no-load loss because the core is energised 24 hours a day regardless of load. It is smaller, lighter and simpler to install, with a lower insulation level, and many designs are pad-mounted or pole-mounted.
| Characteristic | Power transformer | Distribution transformer |
|---|---|---|
| Typical rating | above 200 MVA | well below 200 MVA, often below 2.5 MVA |
| Voltage levels | 400, 200, 110, 66, 33 kV | 11, 6.6, 3.3 kV to 440/230 V |
| Design efficiency point | 100% load | 60-70% load |
| Duty | near full load while running | variable, lightly loaded most of the time |
| Size and insulation | large, high insulation level | small, compact, low magnetic loss |
| Main function | step up for transmission | step down for consumers |
Why the efficiency point differs
The core loss of a transformer is constant while it is energised, while the copper loss rises with the square of the load current. Total loss is therefore the sum of a constant and a quadratic term, and it reaches a minimum at the load where the two are equal. A power transformer, running always near full load, is built so that the constant core loss and the full-load copper loss balance at 100% load. A distribution transformer, running mostly at partial load, is built so that the balance falls at 60-70% load; this is achieved with a high-quality core that keeps core loss low. This is why a distribution transformer shows a relatively high no-load current as a percentage and a power transformer shows a higher full-load copper component; neither is a defect, both are design choices.
Steel materials in both types
The core of every transformer is laminated from grain-oriented electrical steel, CRGO, supplied as 0.23-0.35 mm strip with an insulating coating and specified by core loss class to EN 10106 or the equivalent GB/T 2521 series. For distribution transformers, where no-load loss dominates the annual energy cost, a lower-loss core grade pays for itself quickly, because the core is energised day and night. The tank of an oil-immersed unit is welded from structural steel plate to EN 10025, and the enclosures, control cabinets and mounting frames are commonly hot-dip galvanized sheet such as DX51D+Z or S250GD+Z in Z180-Z275 classes. Buyers should request the core loss certificate and the enclosure coating class together with the electrical test report.
Selection guidance
Choose the transformer from the network position, not from the name. If the unit steps voltage up at a plant or substation and runs loaded, a power transformer specification applies. If the unit feeds a local consumer cluster and spends most of its life at partial load, a distribution transformer specification applies, and the no-load loss value becomes the key commercial parameter. In both cases the contract should state the standard, the rated power, the voltage ratio, the impedance, the loss values and the cooling class, and the certificate should confirm the core steel grade and the measured losses.
Frequently asked questions
What is the main function of a power transformer?
To step the generator voltage up to transmission level, for example to 110 kV or 400 kV, and to move large blocks of power over the transmission network with low losses.
Why is a distribution transformer efficient at partial load?
Because it is energised continuously but loaded only by the varying demand of its consumers; balancing core loss and copper loss at 60-70% load minimises its total annual energy loss.
How do I tell the two types apart from the rating plate?
Check the voltage levels and the rating: transmission voltage classes above 33 kV with ratings in the hundreds of MVA indicate a power transformer, while 11 kV or lower input with a 440/230 V output and a small MVA rating indicates a distribution transformer.
Which steel grade is used for transformer cores?
Grain-oriented electrical steel, 0.23-0.35 mm strip with an insulating coating, specified by core loss class to EN 10106 or GB/T 2521; the grade is chosen so that the no-load loss meets the design target.
Can distribution transformers be installed outdoors?
Yes, pad-mounted and pole-mounted distribution transformers are designed for outdoor service, with a sealed tank and a painted or galvanized finish; the enclosure steel is chosen for the local atmosphere.
Why does a distribution transformer have low magnetic loss?
Because its core is energised continuously and its load is usually low, the design deliberately minimises core loss at the expense of accepting higher copper loss at full load; the balance point is chosen at 60-70% load.