Short Cicuit KA

When equipments operate in a large systems, the generators, transformers and feeders have their %age impidences essentially referred to their respective KVA ratings (all of them may have different ratings in KVA).

In order to compare various impidences (percentage resistances and reactances) and to find out the combined effect, it would be necessary for you to refer them to a common KVA (base KVA).

Dear Umair,

Please Mention your System requirement KVA and also Short Circuit KA.

I would like to check

Regard's

Dear!

i hava six generators each of 1445KVA and a tarfo of 3000KVA!!

please provide the Xd" and %Z of the generator and transformer respectively.

also provide the voltage level at which these are connected and voltage level of the generator and transformer.

for generator its 0.11 PU

trafo 17%

11kv system!!

There are two basic methods to calculate the short circuit current (viz.) (i) Ohmic Method or Direct Method or Impedance Method and (ii) Per Unit Method.

Only in per unit method, you need to have some base values like Base Voltage, Base MVA and Base Impedance. In direct method, you don't need to convert anything referring to any base values. It is a simpler method.

would u like to distinguish b/w per unit and impedance method>?

Yes! There are very clear distinctions between the two methods. But, nevertheless, the calculated fault current vales must match.

can u plz give me an example of calculation !!by pasting a link!!it will help me a lot

best option is to use the transformer ratings as base values for your system is basically supplied by the transformer after the generator...or if you want to use the generators nameplate ratings..then transform the impeadances...

Znew = Zunit(MVAbase / MVAunit)(KVunit/KVbase)^2

...just recheck fla, i can't recall it further..

ok!!can u gimme one more answer!!whilr transformin the per unit impedance of transformer!!which voltage should i consider!LT side or HT side???(i mean what will be the base vol)

you can use either of the two...mine, i use the LT voltage for all equipments later on will be supplied from this voltage...

how come?the ans will come different wen i use 0.4 kv as unit voltage adn 11kv as unit volatge for exmaple

i have per unit 0.15

base kv=18

unit kv =0.4

base kva=3000

unit kva 1500

ans will come different if i take unit kv as 11kv!!

hmm...rate some of your components, the order in which they are served..then ill try solving it..include: impeadances, operating voltage, VA ratings...let me check if i could further convey..

consider i m geberating the volateg @ 11kv!!n using the trafo 11.0.4 for auxilaries