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Public-images-osm logo.svg power = transformer
Trafostation Alter Hellweg IMGP4722.jpg
A static device for stepping up or down electric voltage by inductive coupling between its windings. Large power transformers are typically located inside substations
Used on these elements
may be used on nodesshould not be used on waysshould not be used on areasshould not be used on relations
Useful combination
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Status: approvedPage for proposal

A power transformer is a device for converting electrical power from one voltage to another. A transformer is usually located within a power=substation

Technical definition

A power transformer is a static piece of apparatus with two or more windings which, by electromagnetic induction, transforms a system of alternating voltage and current into another system of voltage and current for the purpose of transmitting electrical power [1]

How to map

A transformer is tagged as a Node. Transformers are usually located within substations and should be tagged as a node within an area tagged as power=substation.

For pole mounted transformers it is suggested to tag the pole as power=pole and add the tag transformer=distribution (or transformer=yes) (see below).

Advanced mapping

Details about the transformer can be added using the following tagging scheme. It is particularly intended for mappers having some knowledge about power systems.


Key Value Comment Recommendation
power transformer Defines this node as a transformer. mandatory
transformer <purpose or type> The purpose or type of transformer (see below for possible values). This tag is not needed for most 'standard' transformers optional
location <location> The location of the transformer. See below for possible values. If the location of the transformer is the same as that of its substation this tag is optional recommended
voltage <high voltage>;<low voltage> The two (or more) voltages of this transformer, highest first, lowest then. A rural 20 kV/400 V is tagged 20000;400 whereas a high voltage 225 kV/400 kV transformer is tagged 400000;225000. Three-winding transformers should have three voltages listed even if two of them are identical recommended
frequency <frequency> The frequency of the transformer. Recommended if the frequency differs from that of the general power grid, for example 16.7 Hz for the German railway traction grid. optional
phases 1, 3, .. Number of phases. Recommended tag for single-phase transformers. Default is 3. optional
rating <rating> kVA/MVA The power rating of the transformer. The value should be in kVA (kilovolt-ampere) or MVA (megavolt-ampere) optional

Transformer values

List of possible transformer=* values.
Key Value Comment
transformer distribution A distribution transformer transfers power from the distribution system to directly connected electricity consumers. Thus the output voltage is that of the low voltage grid in that region. For example 400/230 volt in Europe or 240/120 volt in the US. If the secondary voltage is above 1 kV it is not a distribution transformer. Note for pole mounted distribution transformers you should use this tag together with power=pole (thus no explicit power=transformer tag).
generator A generator unit transformer connects directly to the generator terminals and is used for stepping up the generator voltage to that of the transmission grid. It is almost always located physically close to the generator, typically directly outside the generator building. Unless the generator terminal voltage is known it is sufficient to tag only the secondary voltage.
converter A converter transformer is a special transformer used in HVDC converter plants. It is always located directly adjacent to the valve hall. If the voltage of the valve side is unknown it is sufficient to tag the AC switchgear side voltage.
traction A traction transformer feeds railway overhead contact lines. It is usually a single-phase transformer whose secondary voltage is that of the contact line, typically 15 kV or 25 kV.
auto An autotransformer is a transformer in which part of the winding is common to both primary and secondary circuits. Sometimes used at modest voltage ratios between the primary and secondary, such as 400000/275000 volt.
phase_angle_regulator A phase angle regulator is a special transformer for controlling the flow of power in a three-phase network by shifting the phase angle.
yes This generic value may be used to tag a transformer attached to another power feature such as a power pole (since power=transformer cannot be tagged in such cases). It is however recommended to specify the type of transformer (such as distribution) also in this case but mappers not familiar with the different flavours of transformers may use the value yes.

No transformer attribute should be used for power transformers not belonging to any of the categories above.

Location values

List of possible location=* values.
Key Value Comment
location outdoor A transformer located outside in open air. Use this value even if the transformer is (partly) surrounded by protective walls. This is the default value.
indoor A transformer located inside a building.
underground A transformer located underground.
rooftop A transformer located on top of a building that is used for something else.
kiosk A transformer located inside a small, often prefabricated enclosure that is too small to be considered a "building". However, consider instead tagging such kiosks as power=substation with the attributes substation=minor_distribution and location=kiosk


Photo Tagging Note
380/110 kV Trafo, Kriegenbrunn
Large outdoor power transformer
Pole mounted transformer, Denmark
Typical pole mounted 10 kV transformer.
Modern transformer compact station
Modern transformer station, often used to replace former transformer tower stations. The electrical connection is usually via a ground cable.

See also