Question

Four identical alternators each rated for 20 MVA, 11 kV having a sub-transient reactance of 16% are working in parallel. The short circuit level at the bus bars is:

• A. 700 MVA
• B. 500 MVA
• C. 300 MVA
• D. 200 MVA

Hint:

The short-circuit level at the bus bars is inversely proportional to the sub-transient reactance. A lower reactance leads to a higher short-circuit level.

Answer 1

The Correct Option is D

Step 1: The short-circuit level (SCL) is determined by the total MVA rating of the alternators and their sub-transient reactance. It can be calculated using the formula: \[ SCL = \frac{\text{Total MVA Rating of Alternators}}{\text{Per Unit Sub-transient Reactance (X''})} \] 
Step 2: Given: - Each alternator is rated at \( 20 \, \text{MVA} \). 
- Number of alternators = \( 4 \). 
- Total MVA rating = \( 4 \times 20 = 80 \, \text{MVA} \). 
- Sub-transient reactance (\( X'' \)) = \( 16% = 0.16 \). 
Step 3: Substituting into the formula: \[ SCL = \frac{80}{0.16} = 200 \, \text{MVA} \] 
Step 4: Therefore, the short-circuit level at the bus bars is 200 MVA.