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Harmonics in generator current
5

Harmonics in generator current

Harmonics in generator current

(OP)
Dear All.
One guy was send me this recorder.

Generator 1958year ( one new smile  )
37.5MVA
6.3kV
delta connection

On the high side of GSU-all OK.

3-rd harmonic is OK, but please see this size, is about 40% of 1-st harmonic.

What is a possible reason?
Thanks in advance.
Best Regards.
Slava

RE: Harmonics in generator current

Winding pitch.  Depending on pitch you can get all sorts of harmonics.

RE: Harmonics in generator current

A delta connected generator? That's unusual.

What is the current amplitude? A large percentage of a small current is still a small current: if this is on no load then it could be magnetising current.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: Harmonics in generator current

(OP)
Scotty.
It was our first Q smile.
Generator with nominal load, about 3kA.

For me delta connection is also unusual.
Same pict on all phasaes, voltage is OK, second generator is OK

But. on to first generator directly connected station load, I think..mayne from some nonlinear load.

RE: Harmonics in generator current

Looks like there some sort of rectifier load. What does the voltage look like?

Alan
----
"It's always fun to do the impossible." - Walt Disney

RE: Harmonics in generator current

Well, excitation is a rectifier load, but I've never seen anything like that before. The excitation power won't exceed 10% for a turbo machine, might be a bit higher on a salient pole machine. Wolf will probably give a good estimate if he reads this.

I'm wondering if this is some interaction between the rectifier load and the delta-wound generator, particularly the triplen harmonics.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: Harmonics in generator current

It's almost all third harmonic - I'm going to guess you're looking at core saturation on the GSU or unit auxiliary transformer. Is the terminal voltage somewhere near nominal? I can't imagine any excitation sytem taking that much power from the generator bus.

Can you try reducing the generator terminal voltage and see if the level of third harmonic drops down?

What current is taken when the transformer HV breaker is opened? A heavily saturated transformer will draw a lot of primary (LV) current and the secondary (HV) voltage will be distorted.


Is this definitely a delta connected machine, or a star machine connected in delta? You could just about get rated voltage for star connection out of a delta connected machine if you pushed the excitation high enough, but the terminal voltage will be very distorted.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: Harmonics in generator current


Slava:

Three phase generators with delta windings by nature generate voltages with a strong third harmonic. This is resulting in undesirable equalizing currents. I have no idea why the designer didn't select a wye connection which would compensate the third and all other odd number harmonics having a base of 3. With a winding pitch of 2/3 all third harmonics would have gone also. However, the disadvantage of this solution is a magnetically less efficient stator core.

To get rid of this undesirable situation one can re-arrange the delta winding to a wye winding and to adjust (reduce) the excitation accordingly. The disadvantage of this solution is that the generator output has to be reduced to 57.7% because of the delta winding copper cross section. On the other hand: The generator did serve its purpose sofar, so why change the situation.

As the generator is over 50 years of age, I would recommend to rewind the unit with a proper wye winding (having 57.7% of phase turns compared with delta and with 73.3% larger copper cross section). The original excitation very likely is by rotating d.c. equipment, not by rectifiers.

Regards

Wolf
WWW.HYDROPOWER-CONSULT.COM  

RE: Harmonics in generator current

"The original excitation very likely is by rotating d.c. equipment, not by rectifiers."

Good point Wolf, dunno how I missed that!
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: Harmonics in generator current

You can possibly reconnect the existing winding in two wye, which will be close enough to existing delta connection in performance but without the 3rd harmonic hassles. Need more generator stator data though to confirm the possibility.

Muthu
www.edison.co.in

RE: Harmonics in generator current

(OP)
Thank you.
I was sent your opinion.
We was asked Q, will wait.
Best Regards.
Slava

RE: Harmonics in generator current

Quote:

On the high side of GSU-all OK.

Missed that in the OP. Probably too much harmonic current to be station load. I think Wolf is on the right track.


 

Alan
----
"It's always fun to do the impossible." - Walt Disney

RE: Harmonics in generator current


Muthu is right: Having the stator winding diagram on hand, it may be possible to re-arrange the present winding lay-out into a wye winding without sacrificing on output. It still is my opinion, however, that a new wye winding with modern insulation is the better choice. Less groundwall insulation permits to increase the copper cross section per slot, thus improving efficiency and increasing generator output as well, if turbine permits.

Regards

Wolf
WWW.HYDROPOWER-CONSULT.COM  

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