×
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

generator derating for unusual harmonic load

generator derating for unusual harmonic load

generator derating for unusual harmonic load

(OP)
thread237-278719: quiz: neutral current = sqrt(3)*phase currrent  described a generator with an unusual load that draws current from each phase for only 1/3 of the cycle (all current returns on the neutral).

what kind of derating would be applied to the rms current to account for harmonic effects

Generator Nameplate information
500KVA, pf=0.8, 400KW, 260vac, 60hz, 1110Amps.
 

=====================================
(2B)+(2B)'  ?

RE: generator derating for unusual harmonic load

(OP)
More specifically, should the true rms phase current be limited below 1110Amps and if so by how much?

=====================================
(2B)+(2B)'  ?

RE: generator derating for unusual harmonic load

The simple answer is to consider this as three single phase loads. True RMS describes the heating effects. The current in each phase winding will be equal to the current in each line. The neutral current is the current in the neutral conductor back to the wye point. Inside the generator the neutral current resolves into the line currents in each phase winding.
The harmonics are probably inducing currents in the rotor but this size of machine is much more forgiving of rotor heating than larger machines.
The issues that I see are:
Possible AVR instability that may be resolved with a different model of AVR.
Possible rotor heating: If this machine is in service I would attempt to check or monitor rotor heating.
Oversizing? As a suggestion, this load has a lot in common with a UPS load or a VFD load. Possible 50% to 100% oversize. (Or 50% to 67% of rated load.)

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

RE: generator derating for unusual harmonic load

(OP)
Thanks Bill.  

=====================================
(2B)+(2B)'  ?

RE: generator derating for unusual harmonic load

(OP)
This is similar to ups or vfd input, except that those would draw current during both positive and negative half-cycles, whereas this only draws current during positive half cycle (for example).  As a result we expect to see significant even harmonic content of the spectrum.   I don't know if that creates any concerns but I just mention it since it seems fairly unique to this application.  


 

=====================================
(2B)+(2B)'  ?

RE: generator derating for unusual harmonic load

(OP)
One thing I tried to look at is the temperature of existing machine.  We don't have temperature indication on the stator.  Attached is a thermal image of the exterior of the generator under normal conditions.    It shows temperatures of approx 150F up to 160F hot-spots on the outside surface of the machine, which I believe is in intimate thermal contact with the core backiron (otherwise I wouldn't expect it to be this hot... and yes I have verified these numbers with contact temperature probe).  The machine nameplate states "80C rise" but does not state insulation class.  Most of our machines of this vintage (early 1980's) have class F, but conservative assumption would be class B.

Note this normal condition shown in the attached corresdponds to 100 – 125A true rms phase current steady state.   Current can double from the existing load for long periods under worst case conditions and there are discussions of possibly higher.   If I take a conservative approach, I would say the temperature rise above ambient will increase with load squared.  Under that assumption, the existing ~50F rise (150F on skin minus 100F ambient) would quadruple to 200F when we double load, which would put the temperature close to 300F... above class B limits, even on the outside of the machine.  Is that a reasonable or conservative conclusion?
 

=====================================
(2B)+(2B)'  ?

RE: generator derating for unusual harmonic load

(OP)
From what little we know about suspected relatively high temperature rise for a very lightly loaded machine....if there really is abnormal heating due to this current pattern, it must be due to core losses (eddy and hysteresis) in rotor and stator.  Would you agree? If that is the case, these losses are proportional to local B^2*F^2.... but there is a limit how high B can go based on local saturation.  We may already be at saturation for all I know.... or get get to saturation long before 200A... in which case the temperature rise ~ load^2 projection is way too conservative. Any thoughts?

By the way, we have four identical machines at same load level and same temperature pattern. But it may be nothing to do with the load.... there may be some recirculation of cooling air going on.  These machines are s single-end ventilated at the inboard end.  Looking from inboard end, the air discharges at 9:00 position, has intake at 7:00 position, discharges at 5:00 position, and has another intake at 3:00 position.  FWIW From this end the machine would be rotating CCW.   The support directly below the machine forms a sort of hollow pocket... I tend to think the 5:00 hot discharge air is going into that pocket and getting sucked right into the 7:00 intake.  Would like to try to put a barrier in there to check if it causes improvement, but it's not easy for various reasons.

====================
Also, I don't think even harmonics are an issue here any more than odd (accounted for simply be rms). But I googled to see what I could find:

http://www.schneider-electric.us/tasks/sites/us/assets/File/mechanical-design-applications/Power.pdf

IEEE Table 10.3 Current Distortion Limits for General Distribution Systems
(120 V through 69 kV).... [Note] 1 Even harmonics are limited to 25% of the odd harmonic limits above.


Does anyone know why they have a lower limit for even harmonics?

=====================================
(2B)+(2B)'  ?

RE: generator derating for unusual harmonic load

(OP)
So now another question.  Forget about the neutral and focus on the phase conductors (input side of rectifier) that carry the signal which is positive for 1/3 of every cycle.

Those phase conductors are protected by breaker which has an  internal CT feeding an amp-tector unit for time overcurrent.

I assume the dc portion of the wave is lost.  I can calculate the parameters of the CT secondary current (rms, peak, rectified average etc).

What I'm not sure about: what parameter does the amptector respond to?  The rms?
 

=====================================
(2B)+(2B)'  ?

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