Generally transformer symbol is bisects of two circle. in some case three circle are there why,?

Question

in the case of transformer only two windings are there.so that we represent bisects of two circle as its symbol .i have observed in some of single line diagram that designed three circle bisect each other, why and what is the reason of this three circle,especially in the case of high voltage side(power transformer)

Answer 1

3 coils for 3 phase power.  These tend to be shown a triangles or "chicken foot." So this may not be what you are looking at.

Another possibility is a single phase transformer that has one voltage IN and 2 voltages OUT.  That kind of transformer has 3 coils also

Answer 2

In electrical single line diagrams, especially those using IEC standard symbols, the 3 overlapping circles indicate the presence of a tertiary winding.  This is common on 12-pulse rectifiers, or for the station service power in a transmission switching station.

In 3-phase systems, the circles are also usually provided with an indication of the vector group (delta or star), and the phase angle difference (e.g. Dy1d would indicate a three-phase 3-winding transformer, with the primary have a delta connection, the secondary having an unearthed star lagging 30°, and a delta-connected tertiary).