Transformer inrush vs short-circuit current

[Gauss2k]

I was reading an interesting document made by S&C about the calculation of a transformer inrush current based on the impedance and the short-circuit available at the primary side of the transformer:

http://www.sandc.com/webzine/2003/012003_1.asp

We usually use a 8 to 12x nominal value for 0.1s to estimate the inrush current, but I saw many transformer protection curves adjusted way below this level, but still allowing energization without any problem. I was wondering how we could have a better estimation of this inrush current instead of the usual 12x.

Do you know any other documents from official organization like IEEE or standards about this calculation?

 

[dpc]

No. The S&C approach has been discussed in this forum before. You might try a search. It works because the voltage drop in the system reduces the inrush. It is not really a controversial issue, I don't think. If the system is weak, the inrush will be less because the voltage is less, so you can get away with smaller fuses.

You can approximate the inrush using any power flow analysis software, modeling the transformer inrush as a very high constant impedance load. But this results in a different value for every transformer on your system, depending on its location in the system and the equivalent source impedance.

It can be useful for small substations on long weak transmission systems - allows smaller fuses to be used providing better through-fault protection.

"An 'expert' is someone who has made every possible mistake in a very narrow field of study." -- Edward Teller

 

[rbulsara]

S&C's recommendation is in line with IEEE recommendations. See IEEE Red Book or Buff Book.