Two and Three Winding Xformer 2

[Power2all]

Ok.. I am not very experienced on this subject.. can somebody explain the difference between these? or is it just another way of saying 1-phase and 3-phase xformer?

 

[rcw retired EE]

Usually "2-winding" refers to either a three-phase or single-phase transformer that has two voltage connections, a primary winding and a secondary winding: Example 480 - 120V, 69kV - 13.8 kV. A three-winding transformer could also be single-phase or three-phase and has three voltage levels, three windings: primary winding, secondary winding and tertiary winding. (Note that the voltage levels do not have to be different).

A common use of a three-winding transformer is in generating stations to tie two generators to the same high voltage bus while limiting short circuit current and reducing HV switchyard costs since there is only one HV breaker. Typical ratings might be: Primary 345kV, 200 MVA. Secondary 18 kV 100 MVA, Tertiary 18 kV 100 MVA.

Another common example is the 480 - 240 x 120 dry type transformer with two identical but separate 120 V windings that can be connected in parallel for full power at 120V or in series for the "standard" 240/120 V grounded neutral single phase connection.

Three winding transformers are prevalent as the front end for rectifier systems. By using different winding connections to get multiple phases the rectifer can be fed with 6 or 12 phase instead of three.

Just to make it confusing, is a 230kV-115kV-13.8kV wye-wye, 3-phase autotransformer with a 13.8 kV delta tertiary winding a two- or three-winding?

 

[mikekilroy]

perhaps comment refers to the common practice of using TWO windings in a delta arrangement (missing one winding) for 'open delta' 3 phase system? as opposed to all 3 windings being there?

 

[davidbeach]

Nope Mike, rcwilson nailed it.

 

[prc]

Wilson, answer to your confusing question: There will be three windings( common-series-tertiary) and three circuits in the auto transformer mentioned.It may be more appropriate to say two or three circuit rather than 2 winding or three winding. There are cases when there will be 4 concentric windings( two LVs as split concentric + Hv+ Hv regulating) but effectively only two circuits outside.( large GTs)

 

[waross]

I would consider an auto-transformer a single winding transformer with taps.

Bill
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"Why not the best?"
Jimmy Carter