Faults: Wye Wye versus Delta Wye

[eeprom]

Hello,
I am working on a project which called for a 1500 KVA delta-wye transformer (25 kV to 480V). Somehow we ended up with a wye-wye. Assuming the transformer is the same size and impedance, would the change from delta-wye to wye-wye have any detrimental effects on the fault current? Is there any reason to re-do the short circuit and arc flash studies?

EE

 

[Skogsgurra]

Absolutely! The secondary short-circuit current will be a lot less in a Y/Y than a D/Y. At least if you do not have the primary star Point grounded. Or a tertiary Winding in the transformer.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[eeprom]

For 1LG, 3LG, or LL?

 

[dpc]

The line-to-ground fault current on the low side will be significantly reduced even if both sides are grounded. If the primary is ungrounded then the fault current will be drastically reduced, as Gunnar says. But the three-phase fault current doesn't change, assuming the same transformer size and impedance. Since the arc-flash calculations are based on three-phase faults, the reduction in ground fault current does not change the required PPE for arc-flash hazards.

But you may want to review your ground fault relaying.

 

[Skogsgurra]

Agree, dpc. I could have been a lot clearer.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[eeprom]

Thanks for your help. I knew it would make a difference, I just didn't know which way. I am not sure arc calculations are based on 3 phase faults; I thought they were based on worst case scenario - which isn't always the greatest fault current. I think it would be best to redo the fault and arc flash study.

Thanks again

 

[waross]

I understood that it was the worst case scenario for a three phase fault. That is; considering both the fault current and the clearing time.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

Arc-flash calculations that are based on the IEEE 1584 guide are always based on three-phase faults. Even if a line-to-ground fault has higher fault current, it is only in one phase, so the energy released is less. In fact, the IEEE 1584 equations only apply for three-phase faults. For outdoor high voltage substations with bare conductors and buswork, the accepted approach is to use line-to-ground faults since the chances of a three-phase fault are considered low. Anything at 480 V should be based on three-phase faults.

 

[eeprom]

Are you certain that ETAP calculates Arc Flash in this way? I had the impression that ETAP used the min and max faults, and respective clearing times, to see which condition caused the worst arc flash incident energy.

 

[dpc]

It is true that the worst-case arc flash may not be at the highest fault current, but only three-phase faults should be considered if the calculation is being done in accordance IEEE 1584. I am sure about that. I don't use ETAP, but I would guess that they follow IEEE 1584 by default. Evaluating minimum and maximum fault currents does not mean that they consider anything other than three-phase faults. Fault current for three phase faults can vary depending on the system conditions and switching scenario. Again, the IEEE 1584 equations are only valid for three-phase faults.

 

[eeprom]

Thanks again for all of your help on this.

EE

 

[ppedUK]

eeprom,
When you say that ETAP calculates min and max faults, if it is based on IEE1584 (most likely) it will be calculating 3 phase fault currents and arcing currents. IEEE1584 recommends calculating trip times at 85% arcing current (for LV systems) as well as 100% to see which scenario gives the highest incident energy, but both are 3 phase values. 85% arcing current results in longer trip times. I am not familiar with ETAP, but SKM PTW calculates the incident energy for both cases and reports the worst case, and I suspect that ETAP does the same.