Effect of fault contribution from motors going through multiple xfmrs
Effect of fault contribution from motors going through multiple xfmrs
(OP)
Im looking at a fault analysis on one system and need to consider fault contribution from an adjacent system. From the adjacent system I have aproximately 1.4kA fault contriubution at the 138kV level made up of contribution from motors and generators down in the adjacent system. I want to know or determine what effect if any this 1.4kA will have downstream at the 4.16kV level on the second system. This 1.4kA contribution will have to go through two levels of transformation 138kV-43.5kV and 34.5kV-4.16kV as well as go through cabeling in the system. Can this 1.4kA fault contribution at the 138kV level be considered neglegible by the time it reaches the 4.16kV level?
There is other utility fault current that is being considered but I dont have all the information yet as far as motors loads from the adjacent system so I want to determine if any fault contribution from adjacent system could be neglected anyway due to the fact it goes through multiple transformers. I beleive I heard a rule of thumb somewhere thaa motor contriubtion will not have much effect two transformers away.
Secondly on a slightly different note within a ring bus configuration will the fault contribution on any one of the branch feeders off of the ring bus be the greatest when all the breakers in the ring bus are closed. I am almost positive this is the case but wanted to double check.
Thanks!






RE: Effect of fault contribution from motors going through multiple xfmrs
For the ring bus, use the ring bus closed as the maximum fault case. Having the ring open won't necessarily reduce the fault current - it depends on the location of the sources, but the closed ring should be the maximum.
RE: Effect of fault contribution from motors going through multiple xfmrs
You have confirmed what I though regarding motor contribution being negligable with two transformers in series. And yes I do understand that this simply a result of transformer impedance.
My only other concern is that part of the 1.4kA contribution from the adjacent is from generator sources. Again this 1.4kA is made up of both motor contribution and the generator sources. I wanted to determine if this whole 1.4kA contribution can be neglected after going through two transformers in series. Is there a way to roughly calculate if this 1.4kA contribution from the adjacent system will be neglidgable after going through two transformers in series absent of any other utility contribution on the system?
I guess I could simply use this 1.4kA similar to a utilty or fault source contribution and then calculate what the resulting fault current would be after two transformers?