Fault current contribution (upstream of transformer)

[supra33202]

This is from Fault Current Analysis quiz at http://www.electricalpereview.com. @Zach Stone, P.E.

The fault is between the generator and the transformer. Does the transformer to the fault current? If yes, why?

My solution:

Since the fault happens at the upstream of the transformer, only the generator contributes to the fault current. In this case, power (or current) flows from upstream to downstream. Generator generates power. Transformer doesn't generate power. Transformer transforms voltages. The power at primary side of the transformer is equal to the power at the secondary side of the transformer. There is only one KVA rating (power) in a transformer. Voltage and current at the power transformer (autotransformer is different) might/will change, but the power input and power output will not change.

MVA method:

Sg= 50MVA/7.9%= 632.9 MVA

Isc=6329.9MVA/(sqrt(3)*4160V) = 87.9KA

Please advise.

Thanks!

 

[supra33202]

Question: Does the transformer contribute to the fault current in this case? If yes, why and how?

Thanks!

 

[Zach Stone P.E.]

Question: Does the transformer contribute to the fault current in this case? If yes, why and how?

Thanks!

Hi Tommy,

Worse case three phase faults operate on the assumption that all machines and devices are at rated conditions. Meaning in this case, the generator is supplying 50MVA of power and the transformer is stepping down 1.5MVA worth of power. 

During a fault between the generator and the transformer, the low impedance would cause all power from the generator to temporarily bypass the transformer, and all power from the transformer to temporarily change direction and 

Your formula would be correct if we were looking only at the generator, however, since the transformer is included in the circuit we are analyzing for a worse case three phase fault, it needs to be included in the calculation as well, as the solution to that particular quiz question shows. 

The answer is 91.5kA.

For anyone wondering, this is Question #9 on the Fault Current Analysis chapter included in the Free Trial of Electrical PE Review's Online Review Course for the Electrical PE Exam.

 

[Zach Stone P.E.]

This is from Fault Current Analysis quiz at http://www.electricalpereview.com. @Zach Stone, P.E.

The fault is between the generator and the transformer. Does the transformer to the fault current? If yes, why?

My solution:

Since the fault happens at the upstream of the transformer, only the generator contributes to the fault current. In this case, power (or current) flows from upstream to downstream. Generator generates power. Transformer doesn't generate power. Transformer transforms voltages. The power at primary side of the transformer is equal to the power at the secondary side of the transformer. There is only one KVA rating (power) in a transformer. Voltage and current at the power transformer (autotransformer is different) might/will change, but the power input and power output will not change.

MVA method:

Sg= 50MVA/7.9%= 632.9 MVA

Isc=6329.9MVA/(sqrt(3)*4160V) = 87.9KA

Please advise.

Thanks!

Also, I just want to point out that it looks like you are access an much older version of the fault current analysis quiz that was updated. 

If you refresh your browser window when you access the online course you will see the latest updated graphics and solutions:

 

[supra33202]

@Zach Stone, P.E.

Thanks!

 

[rg1]

Question: Does the transformer contribute to the fault current in this case? If yes, why and how?

Thanks!

I remember I posted answer to it somewhere in this thread but I did not find it . So I do it again.

Your are right. Transformer is a passive device and does not have power of its own. You will get an indication in the question whether transformer will feed fault current or not. Say if the other side of the transformer is connected to a grid of generators the transformer will feed the fault . If the transformer is on radial feeder with only loads on the other side it will not feed the fault. However be careful of synchronous motor loads they dissipate their energy back into the system in such cases.

 

[rg1]

I remember I posted answer to it somewhere in this thread but I did not find it . So I do it again.

Your are right. Transformer is a passive device and does not have power of its own. You will get an indication in the question whether transformer will feed fault current or not. Say if the other side of the transformer is connected to a grid of generators the transformer will feed the fault . If the transformer is on radial feeder with only loads on the other side it will not feed the fault. However be careful of synchronous motor loads they dissipate their energy back into the system in such cases.

In this question the hint is they show a grid ( The straight line on the other side of the transformer). So this transformer feeds the fault. Generally radial feeder will be shown by an arrow going away from the transformer. That means Transformer is only feeding the loads.