Question

The duration of sub transient fault current is dictated by _____ time constant and that of transient fault current is dictated by _____ time constant.

Short answer

The duration of sub transient fault current is dictated by direct axis short circuit sub transient time constant and that of transient fault current is dictated by direct axis short circuit transient time constant.

Step-by-step solution

write the formula of fault current.

Write the formula of instantaneous ac fault current.

Here, E_g is line-to-neutral prefault behind terminal voltage, X^"_d is a sub transient reactance, X^'_d is a transient reactance, $\alpha$is phase between i (t) and e (t) , T^"_d and T^'_d is a time constant.

Determine the answer.

In equation (1), T^"_d is direct axis short circuit sub transient time constant and T^'_d is direct axis short circuit transient time constant.

Write the formula of transient and sub transient current.

Here, the duration of I^" is given by T^"_d and the duration of I^' is given by T^'_d .

Therefore, the duration of sub transient fault current is dictated by direct axis short circuit sub transient time constant and that of transient fault current is dictated by direct axis short circuit transient time constant.