external fault, internal fuses blowing

[caslon]

The cogen generates power for the plant and excess is exported to the grid. On the external fault, plant is isolated from the utility. Depending on distance/type of the fault it has different effect on the equipment in the plant (sometimes it results in the lost production, sometimes it does not). Last couple of times, 27,6kV fuses blew on the section of the switchgear inside the plant (S&C 150E). The switchgear feeds 5 substations:
4160V - 1250HP motor (2)
4160 - 800HP and 400hp motors
600V - 13x150HP motors
600V - small motors (600HP total).
Any ideas on why this is happening?

 

[Bung]

I don't think your around 5000HP of motors is going to contribute enough to a close in fault to blow the fuse. Assuming 5xFLC for starting, you would only get maybe 200-300A contribution, not enough to take out the fuse in the few cycles the current will be at an appreciable level. But it could be a close call, especially if the fuses have considerable pre-heat. Depends on the motors.

But the fault would have to be very close by. The only other thing I can suggest is some kind of over-voltage as a result of the fault causing a flashover downstream of the fuses. What sort of earthing arrangements do you have, and what kinds of faults resulted in the fuse operation?


Bung
Life is non-linear...

 

[alehman]

How quickly is the plant isolated from the utility when the external faults happen? If the motors have time to slow, simultaneous re-acceleration may be a factor.

With induction motors, if the interruption is brief (less than 30 cycles or so), the motors will retain enough residual field that when the voltage is restored there can be significant overcurrent (and motor damage) due to the out of phase condition.

 

[jbartos]

Suggestion: Try to simulate it by some suitable software.
Total HP=6250
I=.746 x 6250HP / (sqrt3 x 27.6kV x .93PF x .94EFF) = 112A
Depending on nature of the fault, the 112A short interruptions may thermally affect the E150 fuse. Its temperature may be elevated until the fuse blows.
Also, the transformer transient inrushes during very short voltage dips due to outside faults in succession will impact the fuse.