Combined power generation system and control method thereof

A combined power generation system comprising:

• a heat recovery steam generator configured to recover exhaust heat from a gas turbine to produce steam, the heat recovery steam generator comprising a high-pressure superheater and a reheater;
• a steam turbine operated by receiving the produced steam, the steam turbine comprising a high-pressure turbine, a medium-pressure turbine, and a low-pressure turbine;
• a condenser installed under the low-pressure turbine to condense the steam;
• a high-pressure steam line for supplying the high-pressure turbine with the steam produced by the high-pressure superheater;
• a medium-pressure steam line for supplying the medium-pressure turbine with the steam that has passed through the high-pressure turbine;
• a high-pressure bypass line connected from the high-pressure steam line to the medium-pressure steam line;
• a reheat steam bypass line connected from downstream of the medium-pressure steam line to the condenser, and
• an additional high-pressure bypass line connected from the high-pressure steam line to the condenser,
• wherein the additional high-pressure bypass line has a physical capacity which is equal to a value determined by:
• multiplying a difference in the amount of steam that is able to be discharged from the high-pressure bypass line and from the reheat steam bypass line, by a ratio of a design condition flux of the high-pressure bypass line and a discharge flux at startup, and
• wherein the amount of steam is calculated by multiplying a reference cross-sectional area derived by multiplying a reciprocal of a product of density and sound speed under design pressure and temperature conditions by a design flux, and a product of density and sound speed under steam turbine pressure and temperature conditions required at startup.