Mixed H∞/H2 with pole-placement design of robust LMI-based output feedback controllers for multi-area load frequency control

In this paper, mixed H_∞/H₂ control theory with pole-placement is applied to design centralized and decentralized robust output feedback controllers for load frequency control (LFC) of interconnected power systems. In the first stage, the original (full) system is taken as one block, i.e., centralized LFC system. In a second stage, it is decomposed into subsystems using the overlapping decentralization technique to obtain decentralized LFC system. Output feedback controllers are then designed, for both centralized and decentralized LFC, using the mixed H_∞/H₂ that is solved using linear matrix inequalities (LMI) technique. The role of H_∞ is to minimize the disturbance effect on the output represented by the area frequencies whereas H₂ is used to improve the transients in the tie-line powers. Pole-placement technique is used in conjunction with H_∞ and H₂ to place the system poles in a desired region. The separate effects of H_∞ , H₂ besides the mixed H_∞/H₂ together with pole-placement on the system response are carried out for comparison purposes. The effects of the main design parameter of the mixed H_∞/H₂ technique with shifted poles in the complex s-plane are investigated and the results are discussed The system performance is analyzed through simulating severe disturbances and wide parameters variation in the presence on the system inherent nonlinearity. In both centralized and decentralized cases, the system with the designed robust controllers incorporated is found to fulfill the main LFC requirements.