Transformer symbol
Also called Power transformer, Two-winding transformer
A power transformer changes voltage between two parts of the system, for example 11 kV to 690 V. The one-line shows its rating, voltages, vector group (such as Dyn11) and impedance.
What the transformer symbol means
A power transformer changes voltage between two parts of the system, for example 11 kV to 690 V. The one-line shows its rating, voltages, vector group (such as Dyn11) and impedance.
ANSI and IEC symbols
Two coils either side of the core lines. Many US one-lines also use the two-circle form.
ANSI SVGWhere you see it
Between every voltage level: generator step-up, main distribution, unit substations and small power transformers.
What is written next to it
On a one-line, the transformer carries its tag and its ratings: rating in kVA or MVA, primary and secondary voltage, vector group (for example Dyn11), impedance in percent and cooling class (ONAN, ONAF).
What gets checked at commissioning
Before the transformer is energised, the test records usually cover: insulation resistance, turns ratio on every tap, winding resistance, vector group, and oil tests on oil-filled units. 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 transformer:
- 87T transformer differential
- 63 sudden pressure
- 49 winding temperature
- 71 oil level
See the full list of ANSI device numbers with their IEC equivalents.
Related symbols
Frequently asked questions
What does the transformer symbol look like?
On ANSI drawings: Two coils either side of the core lines. Many US one-lines also use the two-circle form. In IEC 60617: Two overlapping circles. The coil form is S00842. Winding connections are shown with a star or delta inside each circle (S00858 is star-delta).
Is the transformer symbol the same in IEC and ANSI?
No. Two overlapping circles. The coil form is S00842. Winding connections are shown with a star or delta inside each circle (S00858 is star-delta). The IEC 60617 number is S00841.
Where is a transformer used?
Between every voltage level: generator step-up, main distribution, unit substations and small power transformers.
Which ANSI device numbers go with a transformer?
87T transformer differential, 63 sudden pressure, 49 winding temperature, 71 oil level.
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