Grounding of generator neutral:

[PRSharma]

Grounding of generator neutral:
Generator: 1 MW; 3 phase; 600 V is connected to grid through interface transformer: 1.5 MVA; 27.6 kV star solid gnd/600 V star solid gnd and exports power to grid.

How the generator neutral should be grounded? a) Solid; b)Low impedance; or c) High impedance. If grounded through high imprdance what will restrict the over-voltages if generator islands?

 

[davidbeach]

How about a specific question? All of those are possible, some more relevant to some conditions, others more relevant to different conditions.

 

[cuky2000]

Generators have traditionally been grounded using two methods: high-resistance grounding and low-resistance grounding. In low-resistance grounding, the neutral of the generator is grounded through a grounding resistor that is usually selected to limit the generator ground current to 200-400 Amps. This provides sufficient ground current for proper system relay operation for all operating configurations. Low-resistance grounding is generally used when the generating unit is connected to a bus that supplies system load. This is a common condition on industrial and commercial power systems such as petro-chemical and paper plants, but is also not uncommon on older utility systems.

 

[PRSharma]

The specific question is as under:
If the generator is grounded through high impedance and the interface transformer is star/star solidly grounded at both HV and LV, then after openening of utility breaker for utility ground faults, the overvoltages on healthy phases are unavoidable. The magnitude of the overvoltage will be maximum if there is full neutral shift. Utility is say 3 phase 4 wire. There are 3 phase and single phase load connected in utility distribution system. Loads will moderate the overvoltages.
What would be the ratio of load to generation to keep the overvoltages within acceptable limits?

 

[davidbeach]

There isn't. Load has very little affect, the resistance of your grounding resistor dominates all other affects until it gets very small. If you want to avoid overvoltage issues you need an effectively grounded system. Probably the best solution would be to put the generator behind its own transformer with a delta winding on the generator side of the transformer.