Question

The positive, negative, and zero sequence reactances of a wye-connected synchronous generator are 0.2 pu, 0.2 pu and 0.1 pu respectively. The generator is on open circuit with a terminal voltage of 1 pu. The minimum value of the inductive reactance, in pu, required to be connected between neutral and ground so that the fault current does not exceed 3.75 pu if a single line to ground fault occurs at the terminals is _________(assume fault impedance to be zero). (Give the answer up to one decimal place.)

Answer 1

Given that,

X₁ = 0.2 pu

X₂ = 0.2 pu

X₀ = 0.1 pu

I_f(pu) = 3.75

For single line to ground fault,

\({I_F}\left( {pu} \right) = \frac{{3{E_a}}}{{{X_1} + {X_2} + {X_0} + 3{X_n}}}\)

\(\Rightarrow 3.75 = \frac{{3 \times 1.0}}{{0.2 + 0.2 + 0.1 + 3{X_n}}}\)

⇒ 0.5 + 3X_n = 0.8

⇒ X_n = 0.1 pu