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generator avr

generator avr

generator avr

(OP)
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

RE: generator avr

Is the AVR connected phase to phase or phase to neutral?
Have you checked the generator for internal ground faults?
Regarding the transformer that was changed, is this transformer fed from either generator or just from the #2 generator?
Have you checked the wiring from the generator to the transfer switch for grounds? Has anyone installed a load connected from phase to ground(neutral)?
Is the wye point of the #1 generator grounded?
Why are you running ungrounded?
Why are you running ungrounded without ground detection? This is a code violation in many areas.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: generator avr

In other words, a stupid system will have stupid problems.  Ungrounded is stupid.  And uncalled for.  And an extreme hazard to life and property.  And did I say stupid?  Fix the system grounding, then find the problem.

RE: generator avr

(OP)
Actually I had a lot of misinformation at the time.  We did not install the generators.  Upon opening the units we found that the one unit that did not fail did have a small conductor neutral to ground.  The one unit that did have a problem with the AVR did not have the neutral grounded.  We installed 2 x 4/0 conductors neutral to ground.  We had no further problems.  We were not able to determine the fault because the rig could not be shut down.  

RE: generator avr

Thank you for the follow-up. It is much appreciated.
Yours
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

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