Short Circuit Analysis

[kingtutley]

Ok. New grad here working in power generation industry.

I have been given the task of calculating the short circuit fault current in a bus system given the following information:

4-Hydro generators rated at 38.5MVA, 13.8kV : grounded wye conneced (I am given no further info about the generators, so I have been assuming a 0.12 p.u. subtransient reactance)

2 of these generators will be fed into a bank of three tripple winding transformers 15/15/30MVA, 13.8/13.8/161kV (delta:delta:wye)

Transformer impedances are 10% primary:secondary and primary:tertairy. secondary:tertiary is 20% (This is the only information I am given about these transformers).

If I analyze this using a simplistic approach and say that the generator component to the fault on the 13.8kV bus is (38.5M/(13.8k*sqrt(33)))/.12 and the second generators component to this fault is (38.5M/(13.8k*sqrt(3)))/.2 and the system component is (30M/(161k*sqrt(3)))/.1 then my fault current is 22.553kA.

I have been told that the fault current has been calculated to be 31.953kA Symmetrical and 41.225kA Asymmetrical.

What am I missing here? I should be at least CLOSE to this.

Please, be kind.

 

[dpc]

If you don't have the generator sub-transient reactance, you're basically wasting your time.

But you do need to review how compute faults for three-winding transformers.

Is this a homework problem, by any chance?

 

[kingtutley]

No. Not homework. I actually graduated and am now working in a EE consulting firm. I needed some help. I know this is probably very simple, and I know I need the Xd", that is why I said I didn't have it and am assuming a standard of 0.12 p.u.

I have reread the textbook I used in class on this subject to no real avail. That is why I came here.

If my method is incorrect. PLEASE, explain the correct method.

 

[dpc]

I'm not clear on how the four generators are connected. You say two are connected to a three-winding transformer. Where are the other two connected, what else is connected to three-winding transformer and for what location are you trying to calculate fault current? You will also need the system impedances.

The ratio of symmetrical to asymmetrical is a function of the X/R ratio looking back into the system at the fault point.

If you are doing this for a living, I'd suggest purchasing a good power system text such as Elements of Power Systems Analysis by Stevenson, Analysis of Faulted Power Systems by Anderson or Practical Guide for Short-Circuit Calculations by St. Pierre.

 

[waross]

Possibly the first and quite important part of your task is to find the information that you need.

(I am given no further info about the generators, so I have been assuming a 0.12 p.u. subtransient reactance)

This is homework wording and thinking. This is great for practicing methods of calculating and teaching/learning. In the real world you must have quite a bit of experience to make such assumptions and when assumptions are made they are usually noted as assumptions and accompanied by several actual instances of very similar equipment to justify the assumption.
If your superior wants an accurate figure, he may not be happy with a figure based on assumptions. (And don't forget that your answer didn't correlate with existing calculations).
This may be a test to see if you are able to find accurate figures on your own.
Kindly, as well as reviewing your calculation method, which others here are more qualified than I to assist with, you may want to try a little harder to find hard numbers to replace your assumptions.
respectfully

 

[Bahram7]

Kingtutley,

0.12 pu of X'd for Hydro units is low, I will go more for 0.20 to 0.25 pu.

This is a fairly simple problem that can be solved by a standard short-circuit software or by hand calculation.

Bahram7

http://www.Simtech-Intl.com

 

[JJHorak]

Without a one line, it is hard to figure out what is going on, and you did not mention what voltage the reported fault duty was referring to. However, if you have 4 gens in parallel at 13.8kV, the basic calc is:

4*38.5E6/(13800*sqrt(3)*0.12) = ~53kA. If you raise the Xd" to maybe .2, you get numbers closer to what was reported to you.

 

[sslobodan]

More accurate formula is Xd"=xd"*Un^2/Sn

I"=1.1*Un/Xd"

Is=2.45*I" (maximum short circuit current that can show up in generator system on stator)
When you multiply by 4 with you values you get around 25KA
Factors numeric factors are from a long calculus, so no need to put those equitations here
Generally you should count on 25KA max on 0.12 xd"
and 15KA for 0.2 factor (hydro generator usually have that factor, and smaller values are for different kind of generators)

 

[JMatt]

kingtutley,

Check your math. You've summed the system component amps before converting them to amps at the 13.8kV level. Multiply the system amps that you calculated by 161/13.8. Then, add that figure to the generator fault contribution that you calculated. You'll get around 34kA. That's closer to the 32kA symmetrical that you were looking for.