Transformer ou-of-phase synchronization

[Bronzeado]

Hi folks,

Considering a two winding, 13.8/230kV step-aup transformer feed by a generator under a conditions of a 180^o out-of-phase synchronization:

1. What about the directions of the windings currents? Are there oposite to each other as during a short-circuit?

2. Is this condition worse (electromechanical forces) than a three-phase short-circuit on transformer terminals?

Best Regards,

Herivelto S. Bronzeado
Brasília, Brazil

http://www.linkedin.com/profile/view?id=46319837&trk=tab_pro

 

[Bronzeado]

Sorry!
I forgot that had already done these questions before.
Anyway, if somebody wants contribute with the discussion ....

Best Regards,

Herivelto S. Bronzeado
Brasília, Brazil

http://www.linkedin.com/profile/view?id=46319837&trk=tab_pro

 

[rcw retired EE]

Short, semi-technical answer. Yes, because for a short circuit you have one source pushing current through the transformer to the solid fault. Driving voltage is phase to neutral voltage for each phase.

For the 180 out synch condition you have two sources, one pushing and one pulling the current. Driving voltage is twice phase-neutral voltage. (There is more impedance in the circuit but not enough to compensate). Current through the transformer will be much larger than a bolted 3-phase fault on one side.

Draw your positive sequence network diagram. Since this is a balanced fault, there is no negative or zero sequence (at least until something melts or shears). The two voltages will be 180 degrees out of phase, so driving voltage will be approximately 2.0 p.u.

 

[ScottyUK]

Here's the other thread: thread238-378765