This paper considers the problem of decentralized control of interconnected power systems under large changes in real and reactive loads that cause large structural changes in the system model. In addition to this, small changes in load are regulated by small disturbance controllers whose gains are adjusted for variations in the power system model due to large changes in loads. In this paper small disturbance perturbations are handled using decentralized control. The only feedback needed by subsystem controllers is the state of the subsystem itself. The design is carried out within a large-scale Markov jump parameter systems framework. Using an integral quadratic constraints description for system interconnections and disturbances, we obtain necessary and sufficient conditions for the existence of a decentralized controller which stabilizes the overall system and guarantees its optimal robust performance.