Question

A 10 MVA, 13.8 kV alternator has positive, negative and zero sequence reactances of 30%, 30% and 10% respectively. The value of reactance (in per unit) that should be placed in generator's neutral so that the fault current for a line to ground fault (with zero fault impedance) will not exceed the rated line current is ____ (Asuming system to unloaded prior to fault).

• A. 0.80
• B. 0.60
• C. 0.90
• D. 0.85

Answer 1

Answer: (A), (C), (D)

0.85

Given that, $X_1=X_2=0.3p.u$
$X_0=0.1p.u$
$I_F=I_{\text{a(rated)}}=1.p.u$
${\overline{E}}_a=1\angle 0^o$
For LG fault, $I_F=\frac{3.E_a}{X_1+X_2+X_0+3X_n+3X_f}\le I_a$
$\frac{3}{0.3+0.3+0.1+3X_n}\le 1p.u$
$3-0.7\le 3X_n$
$X_n\ge \frac{2.3}{3}=0.766p.u$
$X_{\text{n(min)}}=0.766p.u$
So from $X_n>0.766$ magnitude of fault current will be less than rated fault current.