single phase transformer in arc flash study

[tem1234]

Hi everyone, It's my first post here.

I have to do an arc flash study where the utility have a lot of 1 phase transformer 25kV to 120/240V. The client want us to calculate the short circuit current and the arc flash hazard at the secondary of theses transformers.

I red that the calculation must be done for an equivalent 3 phases system for the arc flash, but what's the equivalent? If I want the same short circuit current as the single phase, i must take a transformer with a bigger kVA, but is it realistic? Does the equivalent system is the one with the same short circuit current or the same power?

And for a 3 phase system, you can stop the arc flash if the transformer is 125kVA or less (at 240V), but if it's a single phase, do you stop at 125 kVA or 125/3 = 42 kVA?

Thank you

 

[dpc]

IEEE 1584 and the other arc-flash calculation methods are based on three phase systems. For these single-phase panels, I would use the Arc-flash hazard table in NFPA 70E.

For three-phase systems, IEEE 1584 says calculations should be done for ALL 240 V systems.

For 208 V systems, it recommends calculations be done for systems 125 kVA and larger.

 

[tem1234]

Thanks dpc,

But i would like to do it via the arc flash software we are using (who work in 3 phase system only). I red in "a practical solution guide to AFH" that IEEE-1584 recommend that calculations be done for an equivalent 3 phases systems and states that this will yield conservative result.

But i have some difficulty to figure what's the equivalent system. Is it three single phase transformer with the same rating of the one i am studying who will form a 3-phase transformer.

If i have a single phase transformer 14.4kV/120-240V 167kVA, then i would take a three phase transformer 25kV/416V 501 kVA for the study? (that the 416V which make me perplex)

Anybody have an idea?

Thank

 

[dpc]

I would use a 167 kVA three-phase transformer in the model.

 

[jghrist]

How about a different opinion to muddy the works? I'm not familiar with the arc flash software that you're using, but I suspect that the transformer size is used to determine the fault current. Using a 167 kVA three phase transformer with the same % impedance as your 167 kVA single phase transformer would result in fault currents that are too low because the ohmic impedance would be three times a high. I'd use a 500 kVA three phase transformer as the equivalent. This will give the correct fault current for faults at the transformer secondary.

If you are looking at equipment some distance from the transformer, the three phase analysis may not be acceptable. Zero-sequence cable impedance will be significanty higher than the positive-sequence impedance. A three phase analysis will result in fault currents that are too high and fault clearing times that are too low.

 

[tem1234]

Just to be sure, am I ok when i calculate the fault current at the secondary of the single phase transformer (if i consider a perfect source and no cable)

i take a 167kVA 14.4/120-240 V with 5% impedance

the nominal current of the transformer is 167kVA/240V = 696 A

And the fault current will be 696 A / 5% = 13 916.66 A

is it right?

 

[dpc]

jghrist,

That method may give you a single-phase fault current that matches the three-phase, but what about the arc-energy?

Just setting the single-phase fault current equal to the three-phase fault current does not mean the energy will be calculated correctly, since all IEEE equations are based on three-phase faults.

This is really a moot point anyway, since IEEE-1584 explictly says it does NOT apply to single-phase systems.