Convert secondary fault current to the primary
Convert secondary fault current to the primary
(OP)
thread238-163018: secondary fault calculations using utility primary fault current
In reference to Thread 238-163018.
I am having trouble converting a secondary fault current to the primary of a transformer.
I believe you can use either the PU method or the MVA method, however my results are coming out too small.
Here the numbers I am using if it helps:
1500kVA
Vpri = 24,900V, Vsec = 480V, %Z=5.6%
Isc (sec) = 32kA
Thanks, Mike
In reference to Thread 238-163018.
I am having trouble converting a secondary fault current to the primary of a transformer.
I believe you can use either the PU method or the MVA method, however my results are coming out too small.
Here the numbers I am using if it helps:
1500kVA
Vpri = 24,900V, Vsec = 480V, %Z=5.6%
Isc (sec) = 32kA
Thanks, Mike






RE: Convert secondary fault current to the primary
Iprisc= 34.78/0.056pu = 621 A
RE: Convert secondary fault current to the primary
I see you took the FLAs on the primary and divided it by the impedance of the XFMR to get the fault current on the primary. By doing this don't you only take into account the XFMR??
What if there was other contribution from motors or generators?
I am looking for a way/formula to convert a given fault current on the secondary (32kA) to the primary while taking into account the impedance of the XFMR.
thanks.
RE: Convert secondary fault current to the primary
Other contributions from the secondary side for a secondary fault do not contribute to winding current.
RE: Convert secondary fault current to the primary
RE: Convert secondary fault current to the primary
RE: Convert secondary fault current to the primary
Are you asking how many amps will there be on the primary for a 32kA secondary fault? Or, are you asking how much fault current would there be for a primary fault if there is 32kA for a secondary fault?
Two very different questions. The first is well answered already, but from the sound of your follow up questions you might be asking the second.
RE: Convert secondary fault current to the primary
So the second question, a primary fault if there is 32kA on the secondary. Thanks.
RE: Convert secondary fault current to the primary
RE: Convert secondary fault current to the primary
RE: Convert secondary fault current to the primary
If you take an infinite source with your transformer, the SC at the txfo secondary will be:
1500kVA/1.73/480V/5.6% = 32 218 A
So for a 32 kA SC at the secondary, the source is almost infinite, so the SC level at 24.9kV is infinite.
People often do this assumption, it's a conservative way for sizing a breaker at the secondary and you don't have to ask the utility SC current. If you want the real SC current at the primary, ask the utility(or anybody who have this info). With this information, you will be able to calculate a more realistic SC at the secondary too.
RE: Convert secondary fault current to the primary
If they'll give you the figure, that is. Utilities are free to change their system configurations. They don't want to become liable should a customer size equipment based on an outdated figure and then suffer excessive damage following a fault. So the number you'll get will be very conservative.
RE: Convert secondary fault current to the primary
RE: Convert secondary fault current to the primary
RE: Convert secondary fault current to the primary
RE: Convert secondary fault current to the primary
24,900/480=51.875 say 52
If the the short circuit current on the secondary is 32,000A then the primary Isc would be 32,000/52=615.4A
Eric Kench, P.E.
RE: Convert secondary fault current to the primary
The question being asked is "Or, are you asking how much fault current would there be for a primary fault if there is 32kA for a secondary fault?" and the answer is that there is insufficient information to answer the question because 32kA is a nice round number approximately what you get if you have an infinite bus on the high side of the transformer.
Helps to read the whole thread before answering.
RE: Convert secondary fault current to the primary
Mike