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Public-images-osm logo.svg windings
Osm element key.svg
Number of windings in a power transformer device Edit or translate this description.
Group: Power
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
Status: approvedPage for proposal


A power transformer winding (421-03-01) is an assembly of metallic wire turns forming a circuit associated with some poles of a power transformer. Note on a polyphase transformer, a winding is the combination of phase windings. The number of windings, or voltage levels aren't limited in theory, but won't be over 5 in reality.

Windings counting

The main goal of windings=* is to count them. Common sense would lead us to count them all and each but norms state that a polyphase winding (one actual coil per phase/pole) count as one.
On a 3-phases transformer with 2 sides composed of 3 poles each will get windings=2 instead of windings=6.

windings=* key can be extended like windings:primary=*, windings:secondary=*... to give the amount of windings for a specific interface. 2 windings on secondary interface are often seen on distribution transformers as to ease power sharing between different part of network. windings=* should be equal of the sum of all windings:primary=*, windings:secondary=*,...
See power=transformer for more information.

Windings configurations

There are different ways to arrange and connect windings inside transformers. windings:configuration=* is intended to know how in OSM.
Different connections exists and are called star, zig-zag, open and so on.
This information can't be deduced just by looking at the machine. Engineering documentation have to be publicly available to complete this key. You may not be able to do so at all times.
If you are able to determine how a given transformer windings are connected, you can use windings:configuration=* key as following :

List of possible windings:configuration=* values.
Key Value Comment
windings:configuration star IEC 421-10-01 : All phase windings are all connected to a common point at one side and each one to one pole at the other side as to form an actual star.
delta IEC 421-10-02 : All phase windings are connected to form a closed circuit. Poles are connected between each winding.
open-delta IEC 421-10-03 : The same as delta configuration except the circuit isn't closed at only one corner.
zigzag IEC 421-10-04 :
open IEC 421-10-05 : Each phase winding is independent and not connected to any of its neighbours.
scott IEC 421-10-06 : Configuration used to change the number of phase between primary and secondary. Used in traction substations fed by 3-phases network (railways use to work in 2-phases mode)
leblanc IEC 421-10-07 : Same as Scott, configuration used to change the number of phase between primary and secondary. Used in traction substations fed by 3-phases network (railways use to work in 2-phases mode)

As configurations can be different between several interfaces of a single transformer, conventional introduced prefixes fit perfectly with configuration also, as follow :
windings:primary:configuration=*, windings:secondary:configuration=* and so on...


Here are some special cases to illustrate the key usage. To see more complete examples, see power=transformer page.

Photo Location Tagging Note
Description of a 225kV to 20 kV transformer


This plate can be seen on transformer themselves (picture taken with power grid operator acknowledgement) and gives the windings configuration with several other figures. All tags can be defined with it.

Upside schema on the left informs us that all windings (A, B, C and a, b, c) are connected to neutral point (N), which match the IEC 421-10-01 of star connection definition.
70 MVA rating is taken from the last column of the left table.

See also

Proposed_features/Transformer_extension_proposal : Approved proposal introducing this key