On 27th February 2012 at around 08:30 AM the grid fed from gas turbine frame 5, 19.5 MW, 60 Hz, 11.5 KV was subjected to electrical surge due to ground fault on a high voltage motor winding. Some under voltages relays in few substations operated but the generator relays didn’t operate. The vibration on no.3 bearing generator side reached 0.66 in/s for few seconds and then dropped to 0.16 in/s while the machine was making 12 MW. After 23 hours the vibration start to go up to 0.22 in/s after 13 hours reached to 0.26 in/s with same load, after another 18 hours it reached to 0.36 in/s it took another 21 hours to reach 0.49 in/s. the vibration on the reduction load was excessive so that within 3 meters distance from gear box we could feel heavy vibration under our feet. the vibration on the gear inboard was measured and found to be 41.5 mm/s, while no.1&2 bearing reading were 0.07 in/s & 0.09 in/s. we have stopped the machine and when we open gear inspection window we found the high speed pinion 19 tooth out of 61 were broken (towards the generator) and the 2 bearing on low speed side were rubbed. We are suspecting the ground fault on the high voltage motor was the root cause of the gear failure. I appreciate your feed back if you come across similar incident.

 

I have seen similar damage caused by a mal-synchronize of the generator on a Frame 5 many years ago and, from what you describe, I think you had something like a near short circuit on your generator which I guess would cause similar damage.

 

FAM... while the ground-fault may have triggered the event which resulted in damage to the load-gear. I wouldn't say it was the the root cause. More likely, it was mal-operation of the unit's protective relaying

Can you provide details relative to the relaying,such as hardware or software type, targets dropped, sequence, timing, last time tested, etc!

Regards, Phil Corso

 

FAM.....Do you check Rotor Ground fault Relay and brush Ground on the Gen. Rotor?

 

Dear Phil, Thanks for your prompt response

All the protection devices were calibrated in November 2011, over current, earth fault, reverse power, under excitation, synch , over voltage , under voltage, bus differential, negative phase sequence.
All relays are electromagnetic and the excitation system was replaced from GE MVI to ABB unitrol. since excitation system was replaced, the sign of scuff mark shown on some teeth patterns.

 

FAM... Just ignore the thumbs down. C

Can you provide the information in the second part of of my response?

Phil

 

Dear Mohamed,

I am new to this forum rather new to get into the right place in these sort of forums. Any way I think I can get the help.

My question is.
I have 4 Genset ( New) feeding a particular load, and five old Gensets connected with the new ones through bus coupler. Now I want that if the load on the old gensets rise the new genset may feed the load as both the old and new gensets are synched and connected through bus coupler on the same bus bar. BUT if the load on the new gensets rise the old gensets should not feed the load through the bus coupler . What protection I have to use on the bus coupler which trips when the Power flows from Old Gensets to newgensets load and remains closed when the new gensets share the load of old gensets.