Digital PID based load frequency control through open communication networks In this paper, a delay-dependent method is adopted to deal with the continuous-discrete operation mode and time delays induced by communication through open networks in load frequency control (LFC) scheme. By using a novel technique, the design problem of digital PID-type controller is converted into a design problem of static output feedback controller based on the discrete-time system model. The controller gains are obtained using networked control system theory, linear matrix inequality technique and cone complementarity linearization algorithm. By taking the developed systematic procedures,digital PID-type controllers for LFC scheme of a three-area interconnected power system are designed. Simulation results show that the designed controllers provide better robustness against certain time delays than the existing controllers.