Fault in Generator Terminal Box

[fohara]

What is the cause of an arcing fault inside of a generator terminal box? Here is some background info:

Generator system consists of (2) 1500 kW, 13.2 kV diesel generators operating in parallel. The neutral of each generator is grounded through a neutral grounding resistor.

The units were operating for about three hours when one unit tripped. The unit was started again and immediately tripped. The next day, the unit was started again and the operator noticed that there was no current in phase c. He shut the unit down and inspected the terminal box. Phase C connector had burned of the bus bar. Apparently there was an arcing fault between phases which jumped between all three phases and the neutral bus bar. Information downloaded from the protective relay indicated that no ground current flowed through the resistor (a loose connection was found at the resistor). The only damage to the system was to the bolts that connected the lugs to the bus bars - the bolts on each phase and neutral was melted, with material dripping of the ends. There are several marks on the terminal box door apparently from arcing to ground.

After repairing the lugs, the generator was tested and was found to be in good condition. The system is now running.

What could have caused this to happen?

 

[DanDel]

Possibly loose connections at the terminal lugs. Depending on the method of termination, loose bolted or stress cone/hy-press connections.
Any amount of arcing at this voltage could easily cause the damage you describe.

 

[busbar]

Aside — Time spent on visual inspection at maintenance intervals can be worth its weight in gold.

Blind restarts; id est, “burn and learn” can seriously affect revenue streams, and occasionally end careers.

 

[ERaySir]

I had a simular problem with two 1.5 MW units. The cause was vibration. In my particular situation the bus was supported very well and the connections were rigid; vibration worked the winding connection free by snapping the weakening the lugs. We repaired the connections and and noticed that this had occured before. I spoke with two other people in the industry; one said that there should be a transition of more flexable bus between the windings and the connection terminals, the other said that the connections to the cables/busbar should be done in a certain way to allow thermal expansion without overstessing the termination as well as to absorb some vibration. Our units were repaired last year without the mechnical chnges and they're still running. Try placing a handheld vibration meter on the JB and see if it is within acceptable levels.

Best Regards



Ray Micallef, P. Eng.
Power Generation/Utility Industry

 

[ERaySir]

I had a simular problem with two 1.5 MW units. The cause was vibration. In my particular situation the bus was supported very well and the connections were rigid; vibration worked the winding connection free by snapping the weakening the lugs. We repaired the connections and and noticed that this had occured before. I spoke with two other people in the industry; one said that there should be a transition of more flexable bus between the windings and the connection terminals, the other said that the connections to the cables/busbar should be done in a certain way to allow thermal expansion without overstessing the termination as well as to absorb some vibration. Our units were repaired last year without the mechnical changes and they're still running. Try placing a handheld vibration meter on the JB and see if it is within acceptable levels.

Best Regards



Ray Micallef, P. Eng.
Power Generation/Utility Industry