Generator Collector Ring arcing and pitting problem II

[Mbski]

thread237-287242
There was a thread back in 2010 posted by electrageek under same title and ended abruptly without conclusion.
I'm watch electrician in power generating facility and have a similar problem. One of the collector rings developed rough surface ab 1x2.5" and is arcing now.
Does anyone knows cause of the problem

 

[dArsonval]

The collector ring assembly is degraded/[dirty], and allowing potential to track across its surfaces.
It must be removed from the rotor to be thoroughly cleaned/corrected or even replaced.
Not likely the answer one wants to read, depending on the size of the machine.

 

[Mbski]

Turbine was down for cleaning and tested with Megger @250V resistance was 80 Megaohms just prior to this event,rotor(field)was refurbished by GE 2 yrs ago. As I'm writing this new springs were installed and there was slight sparking on one of the brushes but after a day of running it stopped sparking. All readings (volts, amps, trip and alarm relays ) look ok. Hoping for the best till next shut down in the fall.

 

[hitendra_]

Possible arcing could be (1)Carbon dust deposits around collector rings (2)Check the grade of brush (3) check the spring pressure on brush (4)check the brush contact on collector rings (5) check the diameter of collector ring, if it is less minimum permissible.