Question

A synchronous generator having a subtransient reactance of 0.15 pu and operating at 5 percent above ta rated voltage supplies a synchronous motor having a 0.20 pu subtransient reactance. The motor is cornected to the generator by a transmission line and a transtormer of total reactance 0.305 pu.

A sudden three-phase fault oecurs at the generator terminals. Determine the per unit subtrapsient fault current.
A. -j7079 pu
B. −j8,079pu
C. -j9.079 pu
D. −j10.070pu

Answer 1

To determine the per unit sub transient fault current in a synchronous generator and motor system, we need to calculate the total reactance and use the concept of per unit impedance. By comparing the calculated value with the given options, we can identify the correct answer.

Step-by-step explanation:

In this question, we are given the sub transient reactance values of a synchronous generator and a synchronous motor, as well as the reactance of a transmission line and a transformer. We need to determine the per unit subtransient fault current when a sudden three-phase fault occurs at the generator terminals.

To solve this problem, we can use the concept of per unit impedance and the concept of voltage drop across impedances in a circuit. By calculating the per unit sub transient fault current, we can identify the correct option from the given choices.

Let's go through the calculations step by step:

1. Convert the sub transient reactance's to absolute reactance's using the rated voltage of the generator and the motor.
2. Calculate the total reactance of the transmission line and the transformer.
3. Use the concept of per unit impedance to calculate the per unit sub transient fault current.
4. Compare the calculated value with the given options to identify the correct answer.

The final answer will be one of the given options, specifically the option representing the per unit subtransient fault current, which will be a complex number in rectangular form (real part and imaginary part).