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Low frequency oscillation is becoming a serious bottleneck limiting power system stability enhancement, especially in the conditions of interconnections of regional networks. Because of the global properties, most of the units are involved in this problem. The controllers of these devices must be designed coordinately. In this paper, an optimal controller design approach is proposed for the applications in large scale power system. A special injection signal is used to excite the dynamics in certain frequency range, and then prediction error method (PEM) is employed to identify the reduced order system based on multiple inputs and outputs. Traditional optimal control techniques are improved to include the lead-lag blocks, so the popular supplementary damping controllers are compatible. Finally, this approach is applied in the design of two HVDC modulation controllers in China Southern Grid, and the results demonstrate the advantages.