JIMGEN
Electrical
- May 28, 2004
- 25
We are puzzled regarding (#2 generator) a 400kw gen set on a oil rig.
The generator is 480 wye ungrounded neutral. The generator is a WEG which has a separate power circuit for the AVR and it senses off of the 240volt inter connections. The AVR is single phase.
There are many 480volt three phase circuits and a three phase transformer 480X208/120. And a couple of single phase 480 v x 240/120 systems.
The main generator house is grounded and bonded to the drill rig. All power cords (3 pole ,4wire,480 volt)have the green bonded to the generator house and to the plugs and receptacles. All components are bonded back to the generator house or if that is not possible a separate ground stake is driven and that component is bonded to the separate
stake.
There are two generators connected to a manual transfer switch. (Generator #1 has had no problems at all.)
The rig toolpusher called and stated that #2 generator was down. The toolpusher said something had taken out the drilling computers but they were now up and running on generator #1. (Computers were replaced and everything was up on gen set # 1) On site rig tech said voltages at the manual transfer switch on set #2 was something like 380v,380v,8v to ground. (Or something similar, two voltages were equal and one leg was very different)
We checked gen set #2 AVR generator fuses and both the power circuit fuse and the AVR fuse were blown. The generator appeared to be ok so we replaced the fuses and the gen set seemed ok.
So the rig ran on both sets( using the manual transfer switch) for a few days and then #2 went down again with the AVR fuse blown and computers blown, and main breaker tripped.
We installed a new AVR and again everything was running.
Please note that we could not drop power on any of the circuits as there was over 9000 ft of intermediate casing weighing about 250,000 lbs hanging on the rig and the pumps and computer had to stay up. At this time we were running on gen set #1.
The rig electrician had showed us that the line to neutral voltages were uneven on gen set #2. (again as I recall they were 360v,360v, 8v but will have to ck with the technicians).
After our people left the rig,(we advised the rig electrician to try to isolate the problems by disconnecting the various circuits one by one after they got the casing cemented.
This rig has run fine with no major problems for about 6 years when we built the electrical system. But has been modified some by their field personnel.
So later the rig owner and his electrician said they now remembered what they had changed recently and they remembered they had changed a power cord on the low side of a 45kva single phase transformer 480v x 240/120. They claim ed that they reconnected the wires on the x terminals (low voltage) they same way that they think we had originally connected them years ago. We are not sure what they corrected but I think they had some sort of a ground fault but the transformer breaker did not open.
Now they say they have readings of 277v on all three 480v legs to ground on gen set #2. They think the problem is solved. I am not sure.
My question is what made the avr fuse blow and the main breaker open on gen set #2. I think one can
say that there was some high voltage involved since the computers were destroyed. Did the gen set saturate? But the motor starter coils did not open at the time. All motors and starters were functional after these episodes.
I would appreciate any comments or questions. TIA
The generator is 480 wye ungrounded neutral. The generator is a WEG which has a separate power circuit for the AVR and it senses off of the 240volt inter connections. The AVR is single phase.
There are many 480volt three phase circuits and a three phase transformer 480X208/120. And a couple of single phase 480 v x 240/120 systems.
The main generator house is grounded and bonded to the drill rig. All power cords (3 pole ,4wire,480 volt)have the green bonded to the generator house and to the plugs and receptacles. All components are bonded back to the generator house or if that is not possible a separate ground stake is driven and that component is bonded to the separate
stake.
There are two generators connected to a manual transfer switch. (Generator #1 has had no problems at all.)
The rig toolpusher called and stated that #2 generator was down. The toolpusher said something had taken out the drilling computers but they were now up and running on generator #1. (Computers were replaced and everything was up on gen set # 1) On site rig tech said voltages at the manual transfer switch on set #2 was something like 380v,380v,8v to ground. (Or something similar, two voltages were equal and one leg was very different)
We checked gen set #2 AVR generator fuses and both the power circuit fuse and the AVR fuse were blown. The generator appeared to be ok so we replaced the fuses and the gen set seemed ok.
So the rig ran on both sets( using the manual transfer switch) for a few days and then #2 went down again with the AVR fuse blown and computers blown, and main breaker tripped.
We installed a new AVR and again everything was running.
Please note that we could not drop power on any of the circuits as there was over 9000 ft of intermediate casing weighing about 250,000 lbs hanging on the rig and the pumps and computer had to stay up. At this time we were running on gen set #1.
The rig electrician had showed us that the line to neutral voltages were uneven on gen set #2. (again as I recall they were 360v,360v, 8v but will have to ck with the technicians).
After our people left the rig,(we advised the rig electrician to try to isolate the problems by disconnecting the various circuits one by one after they got the casing cemented.
This rig has run fine with no major problems for about 6 years when we built the electrical system. But has been modified some by their field personnel.
So later the rig owner and his electrician said they now remembered what they had changed recently and they remembered they had changed a power cord on the low side of a 45kva single phase transformer 480v x 240/120. They claim ed that they reconnected the wires on the x terminals (low voltage) they same way that they think we had originally connected them years ago. We are not sure what they corrected but I think they had some sort of a ground fault but the transformer breaker did not open.
Now they say they have readings of 277v on all three 480v legs to ground on gen set #2. They think the problem is solved. I am not sure.
My question is what made the avr fuse blow and the main breaker open on gen set #2. I think one can
say that there was some high voltage involved since the computers were destroyed. Did the gen set saturate? But the motor starter coils did not open at the time. All motors and starters were functional after these episodes.
I would appreciate any comments or questions. TIA
