A 10 MVA, 11 kV alternator has positive, negative and zero sequence reactance of 25%, 40%, 10% respectively. The resistance that must be placed in the neutral such that fault current for a single phase to ground fault will not exceed the rated line current in ohms is __________

To calculate the resistance that must be placed in the neutral for a single-phase-to-ground fault such that the fault current does not exceed the rated line current, we can use the following relationship:

1. Rated Line Current (I_line):

[

I_{line} = frac{S_{rated}}{sqrt{3} times V_{LL}} = frac{10 , text{MVA}}{sqrt{3} times 11 , text{kV}} = frac{10 times 10^6}{sqrt{3} times 11 times 10^3} approx 523.6 , text{A}

]

2. Fault Current (I_f) for a single-phase-to-ground fault:

– For a single-phase-to-ground fault in a generator, the fault current is determined primarily by the zero-sequence reactance (X0) since only one phase is involved.

– The relationship for the fault current is:

[

I_f = frac{V_{LL}}{Z_{fault}}

]

where (Z_{fault} = R + jX_0).

Given:

– (X_0 = 10% text{ of } Z_{base}).

– (Z_{base} = frac{V_{LL}^2}{S_{rated}} = frac{(11