A 50 Hz synchronous generator has a stored kinetic energy of 200 MJ in the rotor at 60 MVA. The generator has an internal voltage of 1.2 per unit and it is connected to an infinite bus operating at a voltage of 1.0 per unit through a 0.3 per unit reactance. The generator is supplying rated power. When a three-phase short circuit occurs, all circuit breakers operate. Subsequently, the reactance between the generator and the bus becomes 0.4 per unit. Find the critical clearing angle.