mtwentyc
Petroleum
- Feb 9, 2007
- 10
Ref thread237-178054
Hi all,
back again after a significant absence. Apologies in advance to begin a new thread with such a long ramble, hopefully you'll see how frustrating this is as you read on.
Mid 2006 - The issue referenced in the above thread (Feb '07) related to extreme and selective brush wear on a Top Drive unit fitted with a 1000HP GE 752 traction motor. In short, we experienced multiple cases of extremely rapid erosion of only one brush in each brush holder (each is fitted with three common brushes) - it was eventually surmised that the root cause was selectivity, where (for still unknown reasons) the other two brushes would not commutate correctly, thus saturating/overloading the last brush leading to the rapid wear. Significant discolouration was always apparent on the pigtails, and confirmation on analysis from Morgan carbon was received.
This only occurred at high machine/facility loads, and was presumably when we exceeded the current density of the single brush left commutating. We would then see brushes disappear in less than a shift.
All of the usual checks were done, vibration analysis (OK), spring tension (Ok), incorrect grade of brushes (no), use of acetic acid based sealing compound (no), bad purge air supply (no), replaced the motor with a service exchange unit (No change), bought a brand new motor (no change), installed see-through plexi-glass viewing window to watch commutation (no significant sparking), etc ect. In the end we just made it through the campaign sometimes having to change brushes every 12 hours. The guys got pretty good at it after a while……
Eventually many theories were distilled into two significant and opposing camps:
a) Some sort of drill string or machine induced high frequency vibration that was mechanically affecting commutation, (considered highly unlikely at the time as rig has successfully drilled before) or;
b) Some sort of weird superimposed AC current or harmonic effect (as we had uncovered a previous documented case where ac-ripple on the SCR system caused this uneven brush wear on a similar drilling system) however this was never conclusively proved.
At large expense, the 600V board was temporarily wired up with 32 CT’s and a digital recorder looking for anything anomalous on the AC and DC systems. Unfortunately we ended up with inconclusive results as both time and drilling facility loads ran out when the campaign ended. No answer was found.
Now to the present day:
We have just commenced another drilling campaign on the same rig and again have appreciable electrical loads on the system. We have since replaced the old Top Drive machine (not just beacuse it ate brushes up) to a new VFD driven AC drive unit which is working just fine.
However the problem of uneven brush wear has now re-appeared in one of our similar mud-pump drives.
We now feel confident we can eliminate any mechanical vibration cause as a theory and are certain that the cause is an electrical issue of some sort and not directly related to the motor(s).
It appears to present identically as before, with rapid erosion of only one of the brushes in each set of three and only at high load. Note that the 752 is fitted with four brush holders arranged at 90 deg to each other, the wear occurs on the same brush in each holder, but only on one of the three, 95% of the time it is the brush closest to the armature risers.
These are the questions that we are wrestling with;
a) Could this be the result of harmonic effects from our ABB power quality compensation unit (approx 1200Kw/800A IGBT driven, installed in 2003), an effect that appears only when total drilling load exceeds a threshold as when we run the pump flat out by itself, it seems we have no issue.
b) What are the effects of higher end harmonics (say above the 11th) on commutation and/or DC motor operation?
c) If it was harmonics or a superimposed AC current, how exactly would it cause the symptoms we are seeing – what physically (electrically/magnetically/spiritually?) stops two out of three brushes commutating?
d) Are there any known SCR problems could result in this sort of symptom – we have had the entire system checked prior to this campaign by the OEM, all was reported as good.
e) Could this be a result of circulating or stray earth currents on the platform itself, we are connected to the platform gas turbines via a 6.6Kv (main Earth Neutral Linked) /600V transformers (floating star point)?
There is still lots of frustrated electrical engineers here, any discussion or experience that could help would be gratefully received. Over to you guys.
Cheers
Hi all,
back again after a significant absence. Apologies in advance to begin a new thread with such a long ramble, hopefully you'll see how frustrating this is as you read on.
Mid 2006 - The issue referenced in the above thread (Feb '07) related to extreme and selective brush wear on a Top Drive unit fitted with a 1000HP GE 752 traction motor. In short, we experienced multiple cases of extremely rapid erosion of only one brush in each brush holder (each is fitted with three common brushes) - it was eventually surmised that the root cause was selectivity, where (for still unknown reasons) the other two brushes would not commutate correctly, thus saturating/overloading the last brush leading to the rapid wear. Significant discolouration was always apparent on the pigtails, and confirmation on analysis from Morgan carbon was received.
This only occurred at high machine/facility loads, and was presumably when we exceeded the current density of the single brush left commutating. We would then see brushes disappear in less than a shift.
All of the usual checks were done, vibration analysis (OK), spring tension (Ok), incorrect grade of brushes (no), use of acetic acid based sealing compound (no), bad purge air supply (no), replaced the motor with a service exchange unit (No change), bought a brand new motor (no change), installed see-through plexi-glass viewing window to watch commutation (no significant sparking), etc ect. In the end we just made it through the campaign sometimes having to change brushes every 12 hours. The guys got pretty good at it after a while……
Eventually many theories were distilled into two significant and opposing camps:
a) Some sort of drill string or machine induced high frequency vibration that was mechanically affecting commutation, (considered highly unlikely at the time as rig has successfully drilled before) or;
b) Some sort of weird superimposed AC current or harmonic effect (as we had uncovered a previous documented case where ac-ripple on the SCR system caused this uneven brush wear on a similar drilling system) however this was never conclusively proved.
At large expense, the 600V board was temporarily wired up with 32 CT’s and a digital recorder looking for anything anomalous on the AC and DC systems. Unfortunately we ended up with inconclusive results as both time and drilling facility loads ran out when the campaign ended. No answer was found.
Now to the present day:
We have just commenced another drilling campaign on the same rig and again have appreciable electrical loads on the system. We have since replaced the old Top Drive machine (not just beacuse it ate brushes up) to a new VFD driven AC drive unit which is working just fine.
However the problem of uneven brush wear has now re-appeared in one of our similar mud-pump drives.
We now feel confident we can eliminate any mechanical vibration cause as a theory and are certain that the cause is an electrical issue of some sort and not directly related to the motor(s).
It appears to present identically as before, with rapid erosion of only one of the brushes in each set of three and only at high load. Note that the 752 is fitted with four brush holders arranged at 90 deg to each other, the wear occurs on the same brush in each holder, but only on one of the three, 95% of the time it is the brush closest to the armature risers.
These are the questions that we are wrestling with;
a) Could this be the result of harmonic effects from our ABB power quality compensation unit (approx 1200Kw/800A IGBT driven, installed in 2003), an effect that appears only when total drilling load exceeds a threshold as when we run the pump flat out by itself, it seems we have no issue.
b) What are the effects of higher end harmonics (say above the 11th) on commutation and/or DC motor operation?
c) If it was harmonics or a superimposed AC current, how exactly would it cause the symptoms we are seeing – what physically (electrically/magnetically/spiritually?) stops two out of three brushes commutating?
d) Are there any known SCR problems could result in this sort of symptom – we have had the entire system checked prior to this campaign by the OEM, all was reported as good.
e) Could this be a result of circulating or stray earth currents on the platform itself, we are connected to the platform gas turbines via a 6.6Kv (main Earth Neutral Linked) /600V transformers (floating star point)?
There is still lots of frustrated electrical engineers here, any discussion or experience that could help would be gratefully received. Over to you guys.
Cheers