×
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

Estimating the source inductance of a 3ph. generator

Estimating the source inductance of a 3ph. generator

Estimating the source inductance of a 3ph. generator

(OP)
I'm working on a rectifier for a drive system that will be supplied by a 625KVA diesel genset. The existing rectifier being used has the usual 3% impedance line reactor as normally specified when a drive is supplied by the mains and this strikes me as both counterproductive (more voltage drop) and superfluous, as even a 625KVA synchronous generator will exhibit a much higher source impedance than the mains. I know that to determine the interrupt capability of the breakers required in the event of a short one uses the subtransient reactance, X"d, to determine the initial current spike (lasting ~ a half to one full cycle), but for the purposes of reducing harmonic distortion and improving rectifier power factor, which reactance value is appropriate? Using the synchronous reactance, Xd, gives what seems like an absurdly high value of inductive reactance (e.g. - 1.2mH). Using the armature time constant, and knowing the resistance of the stator windings, gives an eminently plausible value of 70.3uH. Finally, using X"d gives a somewhat lower than expected, though still plausible, inductance of 38.5uH.

For reference, the following specifications for this 480VAC generator are known:

KVA base rating for reactance values = 825
Xd = 2.80 PU
X'd = 0.13 PU
X"d = 0.09 PU
Ta = 0.019 seconds (armature time constant)
Stator winding resistance = 3.7 milliohms


RE: Estimating the source inductance of a 3ph. generator

According to the reference that 7anoter4 just posted[1], the most appropriate value is the sub-transient. This seems to make sense, since the distortion due to the non-linearity of a rectifier is definitely within a cycle (ie. at multiples of the mains frequency), so in the region of sub-transients.

http://www.idc-online.com/technical_references/pdf...

By the way, I tend to agree with your premise that additional line reactance would be counter-productive. Ultimately the reactance is normally there to reduce harmonics for other loads on the line side. If you're running off a generator then you want what you can get from it.

[1] over in http://www.eng-tips.com/viewthread.cfm?qid=339052

RE: Estimating the source inductance of a 3ph. generator

(OP)
Thanks for the response LiteYear. I was leaning towards the subtransient reactance as the one most appropriate, but I was having a hard time grasping this on an intuitive level. Poking around on IEEE Xplore netted me a few promising papers on the matter (by Bonwick, dating June 1973 and January 1980) and at least one Master's level thesis...


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