In the paper we propose a numerical procedure to determine the load-frequency control of a power system composed of several interconnected areas. In order to decrease the associated implementation cost, the control law is constrained to have two different special structures: decentralised feedback and/or output feedback control. The procedure is based on a new property of the classical Riccati equation which is analysed in two different aspects: closed-loop asymptotic stability and suboptimality degree. An example of two interconnected areas is solved and comparisons are made in order to evaluate the performance of the closed-loop system.