@sreeindia: As i said use LM 317/similar adjustable voltage regulator to make your changing input to constant 24 V and improve its current gain by amplifier and step it up with transformer,but i think it is very in-efficient.
But if u do little math,assuming ideal transformer for stepping up,
Power at primary (Vp * Ip) = Power at Secondary (Vs * Is)
U said Vs = 600 V & Is = 12A(max.) Vp = 24V(after regulating made constant)
u need Ip = 300 A !!!!!!!!!
Output max. current of LM 317 = 1.5A , so u need a current gain of 300/1.5 = 200. That power amplifier even if u design it would be a very poor model as it would consume 300*24 = 7200W = 7.2kW power. Highly inefficient.
See if u can do ur job efficiently with buck-boost converter,i'm not good at those and can't provide u with math.