Correct Answer - Option 3 : 49.24 Hz
Kinetic energy stored = SH
Where S is the kVA rating
H is inertia constant
Given that,
inertia constant (H) = 5.0 kW-s per kVA
kVA rating (S) = 100 MVA
The energy stored at no load = 5 ´ 100 = 500 MJ
Before the steam valves open, the energy lost by the rotor = 25 ´ 0.6 = 15 MJ
Stored kinetic energy is directly proportional to the square of the frequency.
The frequency at the end of 0.6 seconds is,
\(= 50 \times \sqrt {\frac{{500\; - \;15}}{{500}}} = 49.24\;Hz\)