What Is a Distribution Transformer?
A distribution transformer is the last transformer in the electricity supply chain. It takes medium voltage from the distribution network, typically 11 kV, 22 kV or 33 kV, and steps it down to a low voltage that equipment can actually use, usually 400 V or 415 V three phase.
If you have ever looked at a factory substation, a building basement plant room, a pad mounted cabinet in a car park or a cylindrical unit on a utility pole, you have seen a distribution transformer. This article explains what it does, how it is rated and how the main types differ.
A distribution transformer is the last transformer in the electricity supply chain. It takes medium voltage from the distribution network, typically 11 kV, 22 kV or 33 kV, and steps it down to a low voltage that equipment can actually use, usually 400 V or 415 V three phase.
If you have ever looked at a factory substation, a building basement plant room, a pad mounted cabinet in a car park or a cylindrical unit on a utility pole, you have seen a distribution transformer. This article explains what it does, how it is rated and how the main types differ.
What a distribution transformer does
The job is simple in principle: change voltage while keeping frequency and (ignoring losses) power the same. A transformer with a primary of 11 kV and a secondary of 415 V has a ratio of roughly 26.5:1. Current changes in the opposite direction, so 26 A drawn at 11 kV becomes about 690 A available at 415 V.
In practice the transformer also determines the fault level at your low voltage board, provides the neutral for single phase loads, and sets how much your installation can grow before you need more capacity.
Main types of distribution transformer
Oil immersed. Core and windings in insulating oil. Excellent cooling, high overload capability, normally installed outdoors or in a dedicated substation.
Dry type (cast resin / VPI). Air cooled with solid insulation. No oil, self-extinguishing, preferred inside occupied buildings.
Pad mounted. A sealed oil immersed unit in a lockable, tamper resistant ground-level cabinet.
Pole mounted. Smaller units mounted on an overhead distribution pole, common in rural and peri-urban networks.
Key ratings on the nameplate
Rated power (kVA). The apparent power the transformer can deliver continuously, commonly 500 to 3150 kVA for industrial and commercial units.
Primary and secondary voltage. For example 11,000 / 433 V.
Frequency. 50 Hz or 60 Hz, depending on the country.
Vector group. Usually Dyn11 for distribution, it determines how the windings are connected and whether units can be paralleled.
Impedance. Typically 4–6%. It limits fault current and affects voltage regulation.
Cooling. ONAN, KNAN, AN or AF depending on construction.
Where distribution transformers are installed
Typical sites are factories, industrial plants, manufacturing facilities, commercial buildings, office towers, hospitals, hotels, shopping malls, data facilities and utility distribution points. In most of these, the transformer is the boundary between the utility’s medium voltage network and the customer’s own low voltage installation.
Frequently Asked Questions
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