Question

Match List I (Equivalent circuit parameter) with List II (values) for a 50 MVA three phase alternator and select the correct answer.

	List I		List II
A.	Armature resistance	1.	1 pu
B.	Synchronous reactance	2.	0.1pu
C.	Leakage reactance	3.	0.01 pu

1. A – 1, B – 2, C - 3
2. A – 3, B – 1, C - 2
3. A – 3, B – 2, C - 1
4. A – 1, B – 3, C - 2

Answer 1

Correct Answer - Option 2 : A – 3, B – 1, C - 2

Causes for voltage drop:

(i) drop due to armature resistance i.e. I_aR_a drop

(ii) drop due to armature leakage resistance i.e. I_a X_ℓ drop

(iii) drop due to armature reaction i.e. I_aX_a drop.

Now, ‘I_aX_ℓ’ will be the phase with ‘I_aX_a’

X_ℓ + X_a = X_s

R_a + jX_s = Z_s

In synchronous machine irrespective of its rating always, X_s > X_a > X_ℓ > R_a

We know, Z_s = R_a + j(X_a + X_ℓ)

Since X_s = X_a + X_ℓ

⇒ Z_s = R_s + j(X_s)