×
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

PARALLEL TRANSFORMERS - LOAD/IMPEDANCE CALCULATIONS

PARALLEL TRANSFORMERS - LOAD/IMPEDANCE CALCULATIONS

PARALLEL TRANSFORMERS - LOAD/IMPEDANCE CALCULATIONS

(OP)
I want to parallel two transformers with the same voltage ratios, kva rating, and vector group.  
Can anyone tell me how to calculate the load carried by parallel transformers with difference impedances. In this case 5.97 and 6.1 %
What aout transformers with different kva ratings ?

RE: PARALLEL TRANSFORMERS - LOAD/IMPEDANCE CALCULATIONS

From the J&P Transformer book, for two transformers, A and B having the same or different kVA and the same or different impedance voltages AND THE SAME X/R ratio:

Current in A (Ia), in Amps:

Ia = (Il x Na)/(Na + Nb)

Current in B (Ib), in Amps:

Ib = (Il x Nb)/(Na + Nb

Na = kVAa/Vza

Nb = kVAb/Vzb

Il = total load current (in Amps)
kVAa = Transformer A kVA rating (in kVA)
Vza = Transformer A impedance voltage at full load (%)
kVAb = Transformer B kVA rating (in kVA)
Vzb = Transformer B impedance voltage at full load (%)

There is a more complicated formula (with drawings) for transformers with different X/R ratios but it is a bit of a pig to write!

Regards

RE: PARALLEL TRANSFORMERS - LOAD/IMPEDANCE CALCULATIONS

I suggest to study the equivalent 1ph circuit of your system in which you consider the absolut (ohm. nor p.u. neither %) impedance values. After you solve this circuit, you will know the power flowing in each transformer.

RE: PARALLEL TRANSFORMERS - LOAD/IMPEDANCE CALCULATIONS

Since both transformers have the same rating, to figure out the distribution between the two, simply divide the impedance of the other by their sum (.0597 + .061 = .1207). The transformer with 5.97% impedance will carry .061/.1207 = 50.538% of the load; the 6.1% transformer will carry .0597/.1207 = 49.47% of the load; ie. the transformer with the lower impedance will carry more of the load.

For transformers of different kVA, you'll need to convert them to the same kVA

Znew p.u. = (Zold p.u.) x (kVA newbase/ kVA oldbase)

Keep in mind that in paralleling transformers with different impedances that you will need to be aware of circulating currents.

RE: PARALLEL TRANSFORMERS - LOAD/IMPEDANCE CALCULATIONS

Tagott has a good point since parallel transformers could be closely modelled as a current division circuit with two reactance in parallels.

The above method is simple and provide sufficient accuracy, particularly if X/R are the same with small deviation otherwise.

See the enclose site for additional info.

http://www.usbr.gov/power/data/fist/fist1~8/1~8_1.htm






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