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The convention OSM reviewed makes primary on the original producer side, like power generator or distant power plant whereas other sides including secondary sees consumers.
Most producers usually step up their voltages for transmission (generator) toward consumers which makes by convention primary > secondary anywhere else in the infrastructure even if energy can actually flow in all directions on many intermediate nodes.
The most important for OSM is the consistency between transformer=* and voltages, particularly for facilities that both produce and consume power at different times. No choice is wrong when values are consistent.
voltage-high=* and voltage-low=* are in use in many places. Some mappers may be more confident by using high and low voltages. However these high and low collide with high and low voltages (like high voltage for transmission and low voltage for domestic usage) and transmission transformers deal with two different high voltages for instance. Not to mention there are often more than two voltages in transformers used in transmission networks. Such information may help match both ways as following:
- voltage-high=* = voltage:primary=* and voltage-low=* = voltage:secondary=* in case of down transforming, anywhere outside a power generating facility. The power normally flows from higher voltage grid to lower voltage grid towards consumers.
- voltage-high=* = voltage:secondary=* and voltage-low=* = voltage:primary=* in up transforming places like power plants, for generator only. Generators produce pretty low voltage power whereas transmission power grids operate at very high voltage. According to convention, the power generator side is definitely the primary interface and here at lower voltage than the secondary connected to power grid.