It is very clear, one of generators which grounded will generate zero sequence fault current, but this fault current shall be circulated in all of generator winding as positive and negative sequence of fault current as shown in following figure. One of important concerning subject in this grounding way is amount of grounded generator earth fault current. In this case, grounding only one generator will cause that generator to carry twice the current it would have on a three-phase fault approximately. Since mechanical stresses are proportional to the square of the current, they would be equal to four times the three-phase short-circuit stresses.
If Zo is less than Z1 or Z2, as is commonly the case, the ground-fault current will be greater than the three-phase fault current Eg/Z1. If several machines are operated in parallel, and only one machine grounded, this effect is accentuated in the grounded machine.
When different numbers of machines operating in parallel, with all neutrals ungrounded, and with only one machine grounded which illustrated in Fig. It will be observed that the line-to ground fault current is always greater than the three phase fault current, and that the situation becomes particularly serious as the number of paralleled machines increases. In an installation of this kind, some form of neutral impedance device is a necessity.
In order to determine the size of neutral reactor to limit the generator winding current for a single line-to ground fault to the generator winding current on a three phase fault the constants of the generator and the constants of the system must be known. The system constants must include all circuits and sources of supply to the fault except the machine under consideration.