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 pu is _____________

To find the resistance that must be placed in the neutral such that the fault current for a single-phase-to-ground fault does not exceed the rated line current in per unit (pu), we can start by determining the rated current of the alternator.

1. Calculate the rated current (I_rated) of the alternator:

– The base power (S_base) = 10 MVA = 10,000 kVA

– The base voltage (V_base) = 11 kV = 11,000 V

Using the formula:

[

I_{rated} = frac{S_{base}}{sqrt{3} times V_{base}}

]

[

I_{rated} = frac{10,000}{sqrt{3} times 11} approx 523.61 text{ A}

]

2. Determine the sequence reactances in per unit:

– Positive sequence reactance (X_1) = 0.25 pu

– Negative sequence reactance (X_2) = 0.40 pu

– Zero sequence reactance (X_0) = 0.10 pu

3. For a single-phase-to-ground fault:

The fault current (I_f) can be expressed as:

[

I_f = frac{V_{ph}}{Z_{total}}

]