short circuit capacity

[tonyflair]

I'm looking at a system that is supplied from a 1 MW sync. generator. The voltage on the primary line is 600 V (from the gen.) and then stepped up through a transformer to 25kV. The transformer is a 3 phase, 250 kVA, Y-Y, Z=3.42%.
I don't know the generator reactances and I want to calculate the short circuit capacity looking at the primary side of the main transformer mentioned above (preforming a fault simulation on the system and treating this as the utility supply -> which would normally be given). Can someone explain to me how to do this and/or any necessary information that I must get.

Also, can someone tell me how to determine the positive and zero sequence impedances for this above transformer.

Thanks in advance

 

[alehman]

The generator impedance will be the most important factor in determining the fault current. You need to know generator Xd", Xd' and R, and preferrably the transformer X/R ratio.

 

[dpc]

As alehman indicates, you really need the generator reactances along with the transformer impedance and X/R ratio.

The transformer zero sequence reactance depends on the winding configuration. For a Y-Y transformer, if either winding is ungrounded, the zero sequence impedance is essentially infinite, since there is no path for zero sequence current to flow. If both sides are grounded, the transformer zero sequence impedance is equal to the positive sequence impedance (plus the upstream system Z0).

 

[tonyflair]

I have obtained the generator reactances from the manufacturer. What should be my initial first step now ?

 

[luis2107]

Hello,
What you should do now, is to determine the impedance of all elements involved in your "network" taking into account the reactance values that you already got, as well as X/R ratios.

One very important point is the methodology that you may follow, I mean, if your approach will be ANSI C37, IEC or simply superposition.
If you need to verify equipmente ratings you should consider this, only following extrictely the rules your values will be comparable with manufacturer's ratings.

Regards

 

[Rodmcm]

As an example a 1MVA synch genset will have a subtransient reactance between 10 and 20% at 1000kVA. So on a 250kVA base the reactances are between 2.5% and 5%, added to the transformer the total reactance is between 5.4 to 8.4%, hence the fault level is between (250*100)/5.4 to (250*100)/8.4 which is about 4630kVA to 2976kVA.

 

[alehman]

Many references can be found with a quick search on google. If you are needing help at this level, I would strongly suggest consulting someone more familiar with the methods. Incorrect results can prove very dangerous.

 

[jbartos]

Suggestion/Eng-tip:
Reference: IEEE Std 141-1993 Red Book Chapter 4 Short Circuit Calculations beginning by page 109.
1. Step: Select Base kVA, Base Volts, Base Impedance.
It is convenient to transfer all of them to the transformer HV side, i.e. 25kV side. 600V generator voltage can be considered as an approximation of the generator internal voltage, i.e. voltage behind Xd". Then, express it on the HV side.
Positive-phase sequence impedance Z1 is normally equal to the negative-phase sequence impedance Z2. The Zero-phase sequence is different from infinity when the Y-Y transformer is grounded on both the HV side and on the LV side.
If this happens to be a homework example or a take home test, it will be better to check the recommended text book by your professor since the above recommended industry standard is not as good as an average text books.