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,

\(= 50 \times \sqrt {\frac{{500\; - \;15}}{{500}}} = 49.24\;Hz\)