Fault current 1

[RalphChristie]

On a single transformer, to determine the fault current on the secondary side, you can use the following simple method:

Short-circuit MVA = (100*P) / X% Fault current = Short-circuit MVA / (1.73*Vsec)

My first question is:

What will happen if two or more transformers is in parrallel(assume they have the same vector group) with the same MVA-rating. Is there any methods to determine the fault level?

my second question:

On a single transformer. If there is a earth fault on the secy side of a transformer, how would it look on the primary side? star/star; star/delta; delta/star; delta/delta

 

[dpc]

Your equation gives a maximum fault current (neglecting any downstream contributions) because it assumes an infinite bus on the primary of the transformer.

With this assumption, if you have two identical transformers in parallel, the fault current on the secondary side would be twice that for one transformer. In reality, it will something less than twice, since the primary source will always have some impedance. But it will be close to double.

As for the effect of secondary ground faults, the transformer connection has a big impact on the primary current. If you have access to a Standard Handbook for EE, you can find the information there.

(For ungrounded delta winding, a single line-to-ground fault does not produce any fault current since there is no path for the current to flow).

 

[RalphChristie]

Sorry for not even a thank you in my previous question.
Thanks for the answer on my first question dpc, but how would you do then o/c settings on the secy side of each transformer? Would each relay been set on the fault current, or on half of the fault current? Maybe any other way?

Thanks
Ralph Christie

 

[dpc]

The relays need to be set to protect each transformer. The maximum fault current is good to know, but the relays must pick up at a much lower level. The transformers also must be protected against mechanical and thermal damage due to through-faults.

Without knowing more about your system and protection scheme, it's hard to be more specific.

In the U.S., you must comply with requirements of the National Electrical Code (NEC). Other countries have similar codes.

 

[peterb]

I think that dpc has covered the first question fully. As for the second question -
- Star/star transformer - primary current appears as a ground fault, IFF both primary & secondary neutrals are earthed, otherwise there is no earth fault current
- Star/delta transformer - no earth fault current on either side (other than system capacitance charging current)
- Delta/star transformer - a secondary earth fault appears as a phase-phase fault on the star side
- Delta/delta transformer - no earh fault current on either side (other than system capacitance charging current)

 

[peebee]

You guys all seem to assume that a delta secondary would be ungrounded. That is not necessarily the case, some industrial installations use a b-leg grounding. In that case, a ground fault would look the same as a phase-to-phase fault.

 

[neps3]

Yap peebee is correct.

Depends on the system, the delta side of the transformer can be earthed via an earthing transformer. Refer GEC Protection application guide.

Regards

 

[busbar]

peebee and kantor touch on another aspect of overcurrent protection that sometimes gets overlooked for specification/application of molded-case circuit breakers in 480V ungrounded {delta or wye} systems, AND 480V corner-grounded {delta or wye} systems. It is a serious misapplication not to use MCCB’s Single-Pole interrupting ratings as opposed to their more publicized multiple-pole ratings for fault-withstand duty.

For instance, in the range of 22-65kAIC-rated breakers, single-pole interrupting ratings {in the fine-print} are typically only 8.6-12.1kA. Figuring otherwise for 600A-frame (and less) breakers can cause them to literally “come apart at the seams." The same oversight is sometimes made specifying 300-volt class-T or -G fuses with these system-grounding arrangements.