Generator Harmonics

[NicNac]

I hope someone can help.

I have been carrying out a harmonic survey in my place of work and have recently proven that our 32MVA generator is the source of harmonic voltages. The third harmonic was found to be 1.5% of the total voltage. There is no noticeable change in harmonic current when the generator is brought online.

We have 5th and 7th harmonic capacitor banks that are online at present. I will switch them off in the near future to get the full results with the generator on.

I am finding it hard to get any information on this type of problem. Some websites touch on harmonics from generators but I can't seem to get any in depth information, therefore any information or book recommendations would be greatly appreciated.

 

[waross]

If the source of the harmonic voltages is the generator the harmonics will be present when the generator is up to voltage but not connected to any load.
Generators do not produce a perfect sine wave. The actual waveform depends on the winding pitch. I know enough to know when to ask for help. Others on this forum may be able to suggest the actual winding pitch based on the harmonic content.
What is the magnitude of the third harmonic current from the generator?
Is the presence of the harmonic causing issues?

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

 

[dpc]

Why do you think that 1.5% 3rd harmonic voltage from the generator is a problem? Sounds fairly typical to me.

Just because you measure harmonic currents and voltages doesn't mean there is a problem or that any remedial action is required.

I suspect you could have "proven" the generator harmonic voltage by asking the manufacturer for the data.

 

[7anoter4]

As waross already said it is acceptable to ignore the space harmonics produced in a generator by "armature reaction", but in an actual machine there are space harmonics in the air gap. Also the mmf of the inductor magnetic field produces space harmonics [see the Attachment]. In some windings, short pitched coils (the distance between two sides of coil is smaller than that between two adjacent magnetic poles) are used to eliminate a certain harmonic. If the number of slots per phase and per pole[q=total slot no/no.of phases/no.of poles] is fractional sub-harmonics are also possibles. The harmonics influence will be less for an elevated q. However by the eddy currents produced by these harmonics in the inductor laminate [or massive] will be reduced but a minimum of these harmonics still exists. The saturation and the hysteresis in the laminate produce also harmonics.
If we take a single loop of coil under the pole N and continued to the adjacent pole S with another loop the mmf will be in the form of two adjacent rectangles.
Using Fourier series the harmonic function will be F(n,t)=4/pi*H/n*sin(n*w*t) so the amplitude will be H=4/pi*H(1)/n H(1)the fundamental amplitude. Further, using the method of superposition we have to divide by q the fundamental amplitude

 

[cranky108]

A good deal of the harmonics should be reduced through the generator step up transformer. And some third harmonic is used for some protection of the generator (larger ones anyway). If it is a noticable problem on the high side of the step up, you can place filtering capacitors there. But usually it is not a problem.

I expect with some loads you will also see harmonics.

 

[NicNac]

Thank you all for your help.

The reason I have asked is because I got unexpected results (obviously due to my inexperience). The purpose of the survey was to check the existing installation and confirm that the filtering capacitors we have (installed before the generator) are adequate for an upgrade we are planning.

It is a 24/7 manufacturing facility and I will not be able to disconnect the generator from the load to get the results from only the generator (for financial reasons also). But as you have said this should not be a problem and is expected from generators. Now I can explain my findings as I have had trouble getting the manufacturers information.

I have attached a copy of my results for one phase. The first page shows the time plot for THD in the voltage and the step change is when the generator is brought online. I plan to switch off our capacitor banks mid January to get the full extent of this. The second page shows the current in the same phase. There is no step change with the generator and the higher harmonics are the 5th and 7th. We have many VSDs onsite.

I have "Electric Machinery" by Fitzgerald that I found last night so this should help also.

Thanks again.

 

[wolf39]

NicNac:

A synchronous generator excited to rated voltage - but not connected to the grid - does generate a sine wave with a certain amount of harmonics. However, the third harmonic will NOT be present if the stator winding is symmetrical phase-wise, i.e. if the number of turns per phase is identical.

A suitable winding pitch can suppress the 5th and 7th harmonics to a certain extent. A winding pitch of 80% would take care of the 5th harmonic but would not be very effective to reduce the 7th harmonic. A winding pitch of 85.7%, in turn, would eliminate the 7th harmonic but is not capable to reduce the 5th harmonic considerably. The best compromise is a winding pitch of 83.3% for fighting both, the 5th and 7th harmonics.

The 3rd harmonic you've mentioned certainly has it's origin in the grid (unsymmetrical load distribution in the system?).

Wolf

http://www.hydropower-consult.com

 

[wolf39]

NicNac:

I have to make one important clarification:

A third harmonic in a generator sine wave will NOT be present if the stator winding is of the WYE connection type. A delta connected stator winding will have a substantial third harmonic. Such a winding configuration therefore must be avoided.

Wolf

http://www.hydropower-consult.com