Question

For the configuration shown in figure the breaker connecting a large system to bus 2 is initially open. The system 3-phase fault level at bus 3 under this condition is not known. After closing the system breaker, the 3-phase fault level at bus 1 was found to be 5.0 p.u. What will be the new 3-phase fault level at system bus 3 after the interconnection? All per unit values are on common base. Prefault load currents are neglected and prefault voltage are assumed to be 1.0 p.u. at all buses.

$E_g$ = 1.0 p.u., $X_T$ = 0.2 p.u., $X_{Line}$ = 0.3 p.u., $X_d$ = 0.2 p.u.

• A. 11.4285

Answer 1

The correct option is A 11.4285

$I_{3-\varphi }=\frac{E_f}{X_1}$

At bus (1),

$1=\frac{\left(0.4\right)(0.3+X)}{\left(0.4\right)+(0.3+X)}$

$\frac{E_f}{X_1}=5pu\left(given\right)$

or X + 0.7 = 2(0.3 + X)

$\Rightarrow$ X = 0.1 pu

Now at bus (3)

$X_{1eq}=\frac{(0.2+0.2+0.3)\times 0.1}{(0.2+0.2+0.3+0.1)}=\frac{0.7\times 0.1}{0.8}$

$\therefore I_f=\frac{1pu}{X_{eq}}=\frac{0.8}{0.1\times 0.7}=11.4285pu$