Dear Coralie,
SAM is not designed to model this kind of a system. It will model a linear Fresnel system with "fossil backup" where heat from a gas boiler supplements solar energy from the field to maintain the field outlet HTF temperature at the design point. The situation you want to model is the opposite, where the solar field reduces the turbine's fuel consumption.
You could use SAM to determine the solar field's hourly output, and then calculate the turbine fuel reduction outside of SAM.
Best regards,
Paul.