Bases on Relay Setting for a 2 power Transformer

[sendalight]

please assist me on this.

we have two power transformer (25 MVA and 33MVA) with different %impedance but the same voltage rating. these will be connected with a vacuum circuit breaker as tie breaker. This is not designed to operate in parallel. During off-peak, only one will operate but it will be in parallel for a short time during the transfer of load. I have already the fault level of the utility.
My concern is:

1.) What will be my bases on computing the kilo amperes interrupting capacity (KAIC) rating, will i consider the parallel connection or consider only one power transformer.
2.) How will i compute the fault current for the relay setting, consider the big or smaller power transformer? or will consider the parallel connection? What could be the advantages or disadvantages on this.

Your ideas would mean a lot to me. Thank you very much.

 

[Zifkwong]

Can you have separate protection for each transformer? Are there enough CT's?

Since their will be times that both units are in parallel the switchgear should be based on fault current from the parallel set.

You are going to have some crazy var swings when they are put in parallel, because of their impedance difference. I hope the turns ratios are at least the same.

 

[sendalight]

yes there are enough CT's. Each transformer will be protected individually with power circuit breakers (SF6). On the secondary of the transformer or at 13.8kv side, each transformer will have 3 feeders each.

 

[Zifkwong]

If you have two separate circuit breakers and different sets of protection which settings are you concerned with the feeders?

 

[sendalight]

my concern is the setting of both. since they can run in parallel for a short time and loads from one will be transferred to the other during off-peak.

if we consider protection individually, yes it can but im taking consideration of transferring of load. the setting of one will be different from the other.