Transformer Bushing Short-time withstand current

[kssschsekhar]

Dear Folks,
As per IEC60137 standard, transformer bushing short-time withstand current should be 25 times the rated current for 2-sec. If a fault occurs just after the bushing (inside the transformer), the fault current through the bushing is almost equal to connected supply bus fault current (e.g.31.5kA). In this case, which current we have to select as short-time withstand current of the bushing? Assumption: Transformer bushing is equally important as transformer winding for faults inside the transformer...
Advanced thanks for your participation...

 

[7anoter4]

From IEC 60137 para 4.3:
"Unless otherwise specified, the standard value of Ith shall be 25 times Ir, tth being 1 s. For bushings with Ir equal to or greater than 4 000 A, Ith shall always be 100 kA.
For transformer bushings, tth shall be 2 s, unless otherwise stated, with reference to IEC 60076-5.
For durations of tth greater than 1 s, the relationship between current and time is assumed to be in accordance with:
Ith^2 × tth = constant"
The fault current through the bushing it is not involved here but the rated current of the bushing only [has to be more than transformer rated].
Let's say the transformer Sr=100 MVA at 160 kV Irtransformer=100/sqrt(3)/160=0.361 kA.
The bushing Irated could be 1000 A. So Ith=25 kA for 1 sec. For 2 sec.Ith2sec^2*2=25^2*1 Then Ith2sec=25/sqrt (2) =17.7 kA.
If the bushing rated current will be 4kA or more Ith=100kA for 1 sec. and 100/sqrt (2) =70.7 kA for 2 sec.

 

[jghrist]

If a fault occurs just after the bushing (inside the transformer), the fault current through the bushing is almost equal to connected supply bus fault current (e.g.31.5kA).

In this case the transformer has failed. If the bushing fails also, add that to the repair/replacement cost of the transformer. The 25 times rated is to ensure that the bushing will withstand through faults, not internal faults.