why there is an sparking in carbon brushes of synchronous generator

hello
we ahve a synchronous generator of 20MW. last day there was a problem of sparking on the carbon brushes fixed on two slip rings. there are two rows of carbon brushes. and in each row there are five carbon brushes on each slip rings. we cleaned the on row of carbon brush with instrument air , foe the time being the sparking minimize, but on the next days again it increases on one row of carbon brushes, the health of the carbon brush are ok . what is the reason of this sparking . is the load may effect. normally the loads on our generator is 12MW.
THANKS FOR RESPONSE

 

[TheBlacksmith]

Brush spring tension incorrect, slip ring out of round, slip ring surface slightly rough or etched from having brushes in contact and stationary (machine not turning) for a prolonged period, chemical/lubricating oil contamination, inferior, not specification compliant brushes, just some things that come to mind.

Blacksmith

 

[abeltio]

Let me add to theblacksmith list: improperly seated carbon brushes.
Sparks may develop before they are even noticeable to the naked eye... a very smart way to detect developing sparks is one I've found in an old Chemical Engineering Magazine number:
have an AM radio set between two stations (AM!!! not FM) extend the antenna and pass it near the suspect equipment (you don't even have to open covers of switchgear or mcc's!!!) if you hear a crackling noise it is most likely due to sparking.
Hope this helps.
a.

 

[benray]

I have encountered the same problem with 2 and 4 pole generators. The 6 and 8 pole do not have this problem as often.

All the above is very good information. The brushes are not properly seated. This can be from spring pressure, quality of brush composition, condition of slip rings, and particles in the surrounding air. Any and all of these conditions can be present.

Air filtration in facility is important. A small particle can start the arcing, it then takes on a life, or death, of its own.

 

[busbar]

Cuttlebone and a GOOD vacuum cleaner.