Generally for application of generator-neutral resistances (or the equivalent through a grounding transformer), the current passed through the resistor during ground faults should equal or exceed the capacitive current that would flow during a line-to-ground fault with the resistor disconnected.
The upper limit of current passed through a generator neutral resistor is fixed by the desire to avoid excessive power loss, and is customarily held to 1.5 times full load generator current, or less. The ratio of Xo/Ro should preferably not exceed 1.0, including the reactance of the resistor. Cast-iron grid resistors may have power factors of 0.98 and sometimes less, which means that their reactance may be about 20 percent of their resistance, at system frequency.
There are two scenarios for determining of suitable resistance.
1- Concerning of power swing and stability in low impedance grounding systems.
2- Concerning of over voltage effects and insulation problem in high impedance grounding systems.
1- Power swing and stability consideration
An important consideration in resistance-grounded systems is the power loss in the resistor during line-to-ground faults. The power loss in the grounding resistor is Ir.Er or Ir2
.R. Ir and Er are both in terms of normal values per phase. The power loss obtained by their product is therefore in terms of normal value per phase. Conseqcntly, if the power loss in the resistor is to be expressed in terms of total three-phase system kva rating, it must be divided by three. Thus resistor power loss in percent is R.Ir2
/3, when Ir is in per unit and R in percent. The maximum power loss for this case is 89.3 percent
of the system rated capacity if three times the resistance in the neutral has the same ohmic magnitude as the reactants determining the ground-fault current. If the generator
reactance are lower, still higher power mill occur during grounds and may cause violent swinging of generator phase position or instability. Resistances in this region are to be avoided, and since there is always some additional resistance in the fault, it is preferable that the grounding resistors alone be sufficient to carry well beyond the peak. Effect of system size and operating voltage on size of neutral resistor to limit ground fault current to one-quarter full-load system current.
The actual ohmic value of the grounding resistor required will vary widely depending upon the circuit voltage and system capacity. By “full load system current” is meant the summation of the rated currents of all generating capacity, converted to the voltage base of the line. In terms of the rated line currents, the fault currents may still be several times full load, depending upon the number of lines.
2-Over voltage and insulation consideration
Manufacturers of large generators, 200 MW and above, recommend the ground fault current, Ig to be limited in the range of 5-15 A and a fault clearing time of the order
of 5-30 seconds to protect the machine and avoid overheating of the grounded steel frame.
It is also recommended to limit the TRV by inserting a small resistance into the grounding circuit to make the ground fault current (Ig) somewhat resistive than capacitive due to capacitive coupling between the generator and the associated equipment and the ground as illustrated in following figure.
To achieve this, the ground fault loss represented by Igr2. R (Igr being the active current and R the ground resistance) should be higher than the electrostatic loss to ground, Since a reactor can only offset or over-compensate the capacitive kVA, it will not yield the same result as a resistance. Resistance grounding is therefore preferred to reactance grounding. The GFF, however, will now be higher and may rise to 1.73 times. The phase to neutral voltage in the healthy phases may rise to the line voltage during a ground fault, as in an isolated neutral system.
Machine insulation and all equipment and devices associated with the machine must take care of this. The low-resistance grounding may be achieved through a distribution transformer, with a low resistance on the secondary side in this case.