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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
Wikidata-logo.svgSearch Wikidata
Status: approvedPage for proposal

A power Power Transformer (421-01-01) static device which converts a given power voltage to another power voltage. A transformer is usually located within a power=substation.

In more technical terms, a power transformer is composed of 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. The delivery is done at the same frequency than the input.

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.
It is not recommended to mix them with buildings. In case of indoor transformers, place a node inside the building area and add location=indoor on it.

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

Advanced mapping

Many details about the transformer can be added using a more complete tagging scheme. It is particularly intended for mappers having some knowledge about power systems.

Where do I find such data ?

Power transformer datasheet.jpg

First of all, don't go inside power substations if you're not invited to do so please, it's dangerous and probably illegal.
Public information or aerial imagery will help to see outdoor transforming facilities safely.

High detailed information may be found on transformers themselves. A complete datasheet as exposed beside gives many figures on the machine capabilities and structure.
We basically need voltages, windings structure but manufacturer=* and start_date=* may also be completed.
Nevertheless, you may not be able to took picture of those plates and as a reminder : don't go inside restricted perimeters to get the one you couldn't. unless invited


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
frequency <frequency> in Hz 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


<voltage> in Volts The operating voltage of a given interface of the transformer. Don't use voltage=* globally since it's not relevant for a transformer. See Transformer interfaces chapter below for more information optional


1, 3, .. Number of phases of the whole transformer or more precisely on a given interface. Recommended tag for single-phase transformers/interface. Default is 3. optional


<rating> in VA The power rating of the whole transformer or more precisely on a given interface. The value should be given in VA even if seen in kVA or MVA. This ease the process of data without any unit or multiple. optional


1,2,3... Number of windings in the whole transformer or more precisely on a given interface. See Transformers interfaces chapter below for more information 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.
auxiliary Auxiliary transformers intend to supply power to substation internal systems. They're not feeding any subscriber connected to power grid but act as a load on the substation architecture. Use this value on transformer fully designed to this usage, transformers with a particular interface for auxiliary services among many other aren't covered by the value.
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

Transformers interfaces

Power transformer structural example

Transformers are designed to adapt voltage between their windings connected to interfaces. Interfaces use to be called them primary, secondary and so on.
The primary side is connected to the power source, like power generator or distant power plant whereas secondary side is connected to consumers. Tertiary, quaternary and further sides are intended for lower voltages auxiliary services inside power plants.

Complex situations may occur and need extended keys to be accurately described in OSM. phases=*, rating=*, windings=* may be suffixed with :primary, :secondary, :tertiary... to give as specific values as it needs for a specific interface. If no suffix is used, the values will be considered as common to all interfaces of the transformer.
All windings related to an interface (primary, secondary,...) always operate at the same voltage. Different is the voltage, so does the interface.

Voltage tagging

Each interface is defined with its own voltage and transformers won't have a global voltage value. Use of voltage=* without suffix is discouraged on transformers.
You are invited to use voltage:primary=*, voltage:secondary=* or voltage:tertiary=* instead according to the interface's name.

Key Value Description
voltage:primary=* Primary interface's voltage The operating voltage of a given transformer's primary interface.
voltage:secondary=* Secondary interface's voltage The operating voltage of a given transformer's secondary interface.
voltage:tertiary=* Tertiary interface's voltage The operating voltage of a given transformer's tertiary interface.

voltage-high=* and voltage-low=* can be in use in many places. Some mappers may be more confident by using high and low voltages and such information may help to define primary, secondary, tertiary voltages as following :

  • voltage-high=* = voltage:primary=* and voltage-low=* = voltage:secondary=* in case of down transforming, like in power consuming places. The power is taken from high voltage grid to feed some distribution network at a lower voltage
  • voltage-high=* = voltage:secondary=* and voltage-low=* = voltage:primary=* in up transforming places like power plants. Generators produce pretty low voltage power whereas transmission power grids operate at very high voltage. According to definition, the power generator side is definitely the primary interface and here at lower voltage than the secondary.

Windings configuration

Since a transformer is composed of several windings, with many different possible configurations, see windings=* for more information.


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