×
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

X/R variation with transformer rating?

X/R variation with transformer rating?

X/R variation with transformer rating?

(OP)
Hi

Suppose a given transformer has a given X/R rating for a given MVA value. Is this X/R value the same at a different rating value for the same trfr?

As an example I have a 20/25MVA trfr where 20MVA is the ONAN rating and 25MVA the ONAF rating.

I would tend to think that the resistance of the primary and secondary windings do not change appreciably due to the increased current flow? One can allow for slightly increased resistance due to the conductor heating up due to increased current flow but this is offset by the increased forced fan cooling.

Secondly I cannot see the leakage reactance changing due to increased current flow so I tend to then the X/R remains the same.

Any comments?

RE: X/R variation with transformer rating?

Your conclusion is correct.

RE: X/R variation with transformer rating?

The impedance stays the same. The PU impedance changes when the base KVA is changed.
The impedance is tested with the transformer at operating temperature.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: X/R variation with transformer rating?

(OP)
Thanks for that.

Regards.

RE: X/R variation with transformer rating?

I agree with all was said in above posts practically. But theoretically I doubt a bit. If the monitored oil temperature will be the same that means the winding temperature will increase so the conductor. In this case the resistance will grow-up. Skin effect will be less but it does not reduce the resistance back to the latter ONAN value.
Now, if the resistance is increased the voltage drop will increase and the e.m.f   will decrease, so the magnetic flux density. That means less saturation and then less field losses then less reactance. In turn, the voltage drop will decrease and so on.
An equilibrium occurs somewhere but it could be a slight offset-less X/R.
Pure theoretically.
 

RE: X/R variation with transformer rating?

The fans are normally controlled by winding temperature indicators, not oil temperature.  There will be some difference because the winding temperature indication is based on heating oil in a well with CT current and is not an actual direct measurement, but it will be close.  There are probable differences in stray losses also.

I certainly wouldn't worry about any X/R differences for normal fault current calculations.  If you're doing some esoteric analysis that requires a real accurate X/R ratio, then you might worry.
 

RE: X/R variation with transformer rating?

Of course, jghrist you are right. I don't argue about the actual situation. I only explain that what veritas was concern about could be very logical.

RE: X/R variation with transformer rating?

In practical world,there is no difference for X/R with load.But theoretically there is variation.% X will vary linearly with load .% R will have some difference.%R consists not only DC resistance, but also loss from AC resistance, circulating currents in winding and stray losses in metallic parts.These components will form neraly 25 % of total load losses. This component will decrease with temperature and DC rsistance will increase with temperature. But in power transformers, X is very much larger than R and hence  the result of these variations on X/R is negligible.

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