short circuit calculation

[bjo]

Hello Everyone!
How can I calculate the single phased short circuit current on the primary side of a Yd-connected transformer?
I know that a Dy-connected transformer single phase short circuit turns into a two phased current, but what about the other way around???

 

[peebee]

I'm not quite sure what your question is. . . .

The available fault current on the primary side would be determined by the upstream system.

Unless. . . are you're concerned about motors on the secondary side backfeeding into the primary side through the transformer?

 

[jbartos]

Suggestion: The upstream power distribution system must be known to calculate faults on the transformer primary. The contributions from downstream may be considered; however, they are often neglected since they will be small, i.e. substantially reduced by the transformer impedance.

 

[bjo]

Let me clarify a little.

I have a grid consisting of the utility which has a grounded network. I am a consumer with a Dy-connected transformer with a generator connected to the secondary side. I there is a single phase short circuit on the high voltage side what would the contribution the fault be from my generator??

 

[jghrist]

It will be zero after the utility breaker opens to clear the fault. While there is a utility source, there will be positive- and negative- sequence currents in the secondary. Protection for Unexpected Delta Sources, Ken Behrendt, SEL, has sequence diagrams that will help in calculating the fault contribution from you generator.

http://www.selinc.com/techpprs/6143.pdf

 

[SidiropoulosM]

Your generator will contribute positive and negative sequence fault current, as the zero sequence is blocked. This contribution will be limited by the transformer impedance, as jbartos suggests and it's unlikely to be the utility's main concern. They will be more interested in assuring that energization of their de-energized line is prevented during maintenance, for personnel safety. In any event, these are matters that should be addressed jointly with the electric utility.

 

[rbulsara]

See thread238-47990. It has the correct answers.