Electric power generating system

In an electric power generating system capable of absorbing a variable load of the type comprising a boiler, a steam turbine, an electric generator driven by said turbine, said boiler and said turbine being interconnected through a steam conduit including a control valve which adjusts the flow amount of the steam in said steam conduit, and a variable pressure operation control device which maintains a constant degree of opening of said control valve at a time when a steam pressure in said steam conduit lies in a range between predetermined pressure values, the improvement in which said variable pressure operation control device comprises means for comparing an actual steam pressure in said steam conduit with a pressure predetermined in accordance with variation of a load of said generator for generating a signal to determine a variable pressure operation characteristic line for changing the degree of opening of said control valve in response to a difference between said compared pressures.