×
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

per unit impedance values of a transformer

per unit impedance values of a transformer

per unit impedance values of a transformer

(OP)
how do you calculate the per unit impedance value of a transformer given the percentage impedance value? How do you also estimate the per unit vales of resistance and reactance from the percentage impedance value?

RE: per unit impedance values of a transformer

Per Unit is same thing as percentage. Only a factor 100 different.

So, if you have 5.6 % that will be equal to 0.056 p.u.

The impedance of a power transformer (10+ kVA) is almost purely reactive. You can safely assume close to 90 degrees impedance. If you need better values, you have to ask the manufacturer for data,

Gunnar Englund
www.gke.org

RE: per unit impedance values of a transformer

I think skogsgurra must have meant (10+ MVA).  Distribution transformers have X/R ratios ranging from 1 to 6.

You can calculate %R from the load (copper) losses divided by the rating times 100.  %X is sqrt(%Z²-%R²).

RE: per unit impedance values of a transformer

Thanks, jghrist. But I actually meant kVA. It all depends on what you mean with "almost purely reactive" - a matter of discussion. Someone?

Gunnar Englund
www.gke.org

RE: per unit impedance values of a transformer

Also if you know actual R in ohms from copper losses, to get the p.u. value, you need to divide it by the base Z which is base ( rated) L-N voltage  divided by the base current. The base current is the rated current.

RE: per unit impedance values of a transformer

Just to summarize,

%impedance means, the ratio of impedance voltage drop at full load to the normal voltage, when multiplied by 100.

Let us take LV side of the trafo as base values,
 
%Z= I Z ÷ V,  where Z is actual impedance or leakage impedance referred to LV side, and  V & I are rated values of LV side.

then,

Z = %Z V  ÷ I  --- referred to LV side of trafo

%R = Full Load Cu losses ÷  trafo rating in VA ,
      (R = Full Load Cu losses /I*I - ref to LV)    

%X = sqrt {%Z*%Z - %R*%R}


P.U. impedance     = % impednace / 100  
             or   
actual impedance / base impedance.

Where,  base impedance = V / I  (rated values of  base side i.e LV side  )  

             or

KV* KV*1000 / KVA, ( where KV is LV side voltage).

hope this help you.

Varri

RE: per unit impedance values of a transformer

See the enclose figure to determine approx. the X/R Ratio

           X~Z  ==>    R~Z/(X/R)  


     

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