Title Robust Bi-level Model for Optimal Allocation and Design of Power System Stabilizer in MultiMachine Power Systems Type of Research Article Keywords Power System Stabilizer, Expanded Single-Machine Infinite-Bus, Optimal Allocation, Controllability Gramian, COA Abstract This paper presents a new optimized and systematic bi-level approach for the optimal robust allocation and design of Power System Stabilizers (PSS) in order to enhance system stability and damping of low frequency oscillation (LFO) modes in the multi- machine systems. The proposed approach provides full ranked optimal conditions for the PSS allocation which provide complete controllability of the system. The proposed technique is based on the use of controllability gramian and participation factor. Moreover, this study proposes a new linear power system model for optimal PSSs design which can consider both local and inter-area modes of the system that the proposed linear model is called Expanded Single-Machine Infinite-Bus (ESMIB) system. The tuning problem of stabilizer parameters is formulated as an optimization problem which is solved by Cuckoo Optimization Algorithm (COA). The eigenvalues based objective function involving damping ratio and damping factor of LFO modes are considered for the PSSs design problem. The stabilization performance of the designed PSSs has been verified in two case study standard models (3 and 10 Machines). The simulation results show that the proposed model can be considered as an efficient tool in optimal allocation and design of PSS in the multi machine environment. Researchers Farhad samadi gazijahani (First Researcher)، ata ajoulabadi (Second Researcher)، Amin Safari (Third Researcher)