<p>This paper presents a novel decentralized controller design approach for synchronous generators’ voltage and power control in complex power systems. Unlike traditional methods that require detailed modeling of interconnection dynamics, our method exploits the relationship between the stability of isolated subsystems and that of the overall system by transforming the plant model into a descriptor form with a block-diagonal (decentralized) structure. This transformation enables independent controller synthesis at the subsystem level through a method consisting of three key steps. First, the overall stability of the complex power system is evaluated using a Lyapunov function approach. Second, stability conditions at the subsystem level are derived such that ensuring each subsystem’s closed-loop stability guarantees the stability of the entire system. Third, a decentralized controller is designed using any suitable method (e.g., via extended LMI/BMI conditions), with the guarantee that the closed-loop subsystem stability measure is enhanced relative to the open-loop. The robust controller design is carried out via extended LMI/BMI conditions, ensuring improved dynamic performance and robust performance under bounded disturbances by incorporating variations in subsystem parameters. Simulation results on a multi-generator power system demonstrate improved transient response, robust disturbance rejection, and scalability, making the proposed method an attractive solution for large-scale power system control.</p>