Three-winding transformer symbol
Also called Transformer with tertiary winding
A transformer with three windings, for example a primary and two secondaries at different voltages, or a delta tertiary for stability.
What the three-winding transformer symbol means
A transformer with three windings, for example a primary and two secondaries at different voltages, or a delta tertiary for stability.
ANSI and IEC symbols
Where you see it
Generator and grid transformers, and splitting one supply into two buses.
What is written next to it
On a one-line, the three-winding transformer carries its tag and its ratings: rating of each winding, the three voltages, vector group and the impedance between each pair of windings.
What gets checked at commissioning
Before the three-winding transformer is energised, the test records usually cover: insulation resistance, turns ratio between every pair of windings, winding resistance and vector group. Each check is recorded against the tag, so the system can be handed over with its evidence. See commissioning software.
ANSI device numbers
Protective functions you will often find with a three-winding transformer:
- 87T transformer differential
See the full list of ANSI device numbers with their IEC equivalents.
Related symbols
Frequently asked questions
What does the three-winding transformer symbol look like?
On ANSI drawings: Three overlapping circles, as on most US one-lines. In IEC 60617: Drawn the same way: three overlapping circles. The coil form is S00845.
Is the three-winding transformer symbol the same in IEC and ANSI?
Yes, it is drawn the same way in most legends.
Where is a three-winding transformer used?
Generator and grid transformers, and splitting one supply into two buses.
Which ANSI device numbers go with a three-winding transformer?
87T transformer differential.
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