High-resistance grounded system. 1

[beyond86]

Hi gents!
We have islanded power station with some GTG 10.5 kV, each GTG works with step-up transformer 10\110 kV.
There is a resistor installed in neutral each GTG, which limits earth fault current to 10A.
My question is for which purpose need this resistor? Why we can't work with an ungrounded neutral?
I have only one answer: to suppress transient overvoltage when earth faults occur.
Any additional ideas?

 

[pwrengrds]

limit damage due to over current to the stator during a ground fault, giving time to clear the fault with minimal damage.

 

[dpc]

I think you've basically got it. Line-to-ground overvoltage is the biggest problem with ungrounded systems. Even during normal operation, the neutral can be displaced from ground, creating continuous overvoltage conditions. In addition, trapped charge and repetitive restrikes can cause L-G voltage that are many multiples of normal voltage. These overvoltages create failures in the system components with the weakest insulation - transformer, motors and generators. Also, if the system is left in service with a single ground fault, if another phase goes to ground, you are left with a very nasty phase-to-phase fault.

Remember that ungrounded systems are actually grounded through the capacitive coupling between phase and earth. this can result in several amps of current even during normal operation.