Combined power generation system and driving method thereof

A combined power generation system includes a gas turbine, a heat recovery steam generator (HRSG) generating steam using combustion gas from the gas turbine, a vaporizer vaporizing liquefied ammonia, an ammonia decomposer section decomposing ammonia with the combustion gas, a first exhaust gas line through which exhaust gas from the gas turbine is transferred to the HRSG, a steam turbine generating a rotational force with the steam from the HRSG, a decomposed gas supply line through which decomposed gases generated in the ammonia decomposer section are supplied to a combustor, and a cold heat transfer line absorbing cold heat of the liquefied ammonia and supply the cold heat to the condenser section, and a condenser section condensing the steam from the steam turbine.