×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

3 phase Fault Currents
3

3 phase Fault Currents

3 phase Fault Currents

(OP)
Hi,

I've recently carried out short circuit analysis for the initial symmetrical 3 phase short circuit current for embedded generation.[ I"k = Cmax . Vn / (1.732 . Zk) according to IEC 60909].

Looking a some grid application forms, they require the initial short circuit current I"k as well as the short circuit current for a given times Ik(t) i.e.

Ik(t=0 sec,)
Ik(t=10ms)
Ik(t=50ms,)
Ik(t=100ms) etc....

Can anyone provide some information on how to calculate the short circuit current for the given times Ik(t) or any literature/books that may have the relevant information.

Thanks in Advance.
   
 

RE: 3 phase Fault Currents

I don't know much about IEC terminology, but I think you will need the generator time constants and the system X/R ratio.  

Do a web search for "Generator Decrement Curve".  The fault current out of a generator is not constant - it decays over time.  This is determined by the generator time constants and also the excitation system.  

Any power system analysis text (Stevenson, Anderson, etc) has at least some discussion on this.

 

RE: 3 phase Fault Currents

It seems that you have a near-to-generator short circuit. If you are interested in the AC-component only, you can use the (well known?) equation for the current as function of time, see for example http://www.iea.lth.se/eks/L7.pdf , or the book of Nasser Tleis. (The book contains a thorough discussion of the current as function of time.) Or you can let the computer do the job, see http://pp.kpnet.fi/ijl .

But if you need the breaking current, that is, the sum of the AC- and DC-components, then the IEC-standard should be used. It gives (complicated :) instructions for the  calculation of the breaking current at different times. The calculation method is also described in several books, see the books of Kasikci or Schlabbach, for example.

 

RE: 3 phase Fault Currents

You have to to read Factors for the calculation of short-circuit currents in a short circuit book based on IEC standard.

book: Short-circuit currents - J. Schlabbach

search: gigapedia.com/ libray Nu
 

RE: 3 phase Fault Currents

xxx23

Some terminology correction:

The actual fault current waveform is based on the analysis of an RL circuit with no pre-fault current.

The instantaneous current is given by:

i(t) = idc(t) + iac(t) - note this is an instantaneous value

The symmetrical value of i(t) at t  = 0 is the initial symmetrical fault current, Ik" (rms value). Due to AC decay (from rotating plant) the symmetrical current decays to a steady state value called Ik (rms). Thus one does not calculate Ik at different time values as you have stated above. Note that Ik" is a theoretical rms value (cannot be measured) whilst Ik is an actual rms value (can be physically measured). Exception is when there is no AC decay and then Ik" = Ik.

The instantaneous/(theoretical)rms current can be calculated at various points of the transient waveform.  

i(0)or I(0)
i(0.01) or I(0.01)
i(0.05) or I(0.05)
i(0.1) or I(0.1)

As a first pass you can calculate the above by hand or using Excel IF you know what Ik and the X/R is using the following formula:

i(t) = 1.414*Ik*[sin(w*t+90)-e^(-t/T)]  


where T  =  X/(w*R)

Of course you need to use the same voltage factors (usually 1.1p.u.) to calculate Ik as you would have done doing the full IEC 60909 analysis.

The above can be used if you have an analysis package that only  determine the steady state fault currents.

Please note that the above does not consider AC decay (only DC decay) so may not be much use in your case but could be helpful to get ballpark figures.

The formula for considering AC as well as DC decay is to cumbersome to write out here with the limited text abilities. The others posts will undoubtedly be more useful in this regard.

Regards.

 

RE: 3 phase Fault Currents

veritas,
Can you check the equation for i(t). A printing error or two, perhaps?

RE: 3 phase Fault Currents

The formula I have stated is not the general form of the equation but the form of the equation where maximum offset is assumed.

In the attachment, I have stated equation 2 and not equation 1 which is the general form.

If this does not answer your question, then do let know if there is an actual typo.

Regards.

RE: 3 phase Fault Currents

Maybe
  Ik", not Ik
In addition, I would prefer -pi/2 (not +90) and +exp(-t/T), to be consistent with the normal usage (positive DC).

   

RE: 3 phase Fault Currents

ijl

Yes, you are correct. It should be Ik" and not Ik unless of course there is no AC decay in which case the two are always the same. Thanks for that.

With regards to +ve or _ve DC I guess it depends on the convention you're familiar with. I've come across a derivation of the fault current equation using calculus and the fault current is all in terms of cosines rather than sine resulting in the first peak being positive rather than negative. I tend to work with both.

Regards.  

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources