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Increasing penetration of intermittent and variable renewable energy, varying loads, and system contingencies significantly contribute to voltage stability challenges of power systems. Although unified power flow controllers (UPFCs) can provide mitigation, it is necessary that they be sized and located optimally, which is a concern addressed in this study. Specifically, the UPFC is located based on the weighted voltage stability index (WVSI), and its settings are determined using a hybrid particle swarm optimization-simulated annealing (HPSO-SA) algorithm. The proposed methodology aims to maximize the voltage stability in the power system by minimizing a multi-objective function (MOF). The IEEE 57 bus test network is used for evaluation under various operating scenarios with an N-1 contingency. Three UPFCs were optimized separately and in combination. Separate optimization achieves a remarkable improvement in the voltage stability of the test system by reducing power losses, voltage deviation, and the voltage stability index, and it also shifts the operating state from a critical stable to a marginal stable condition for most component outages. Under combined optimization, better MOF results are obtained under various operational scenarios. Comparative analysis shows that HPSO-SA outperforms the standalone PSO and SA techniques. The proposed hybrid technique demonstrated superior performance in terms of convergence and statistical robustness, as evidenced by the mean and standard deviation results, in addressing multi-UPFC optimization in large-scale, contingency-prone power systems. • A robust methodology is developed for optimal UPFC location and sizing under renewable energy variability, load variations, and N − 1 contingencies. • A weighted voltage stability index is proposed by integrating Lmn, MLVSI, and VDI to accurately identify weak buses and critical lines for the UPFC location. • A hybrid PSO-SA optimization approach is applied to enhance the convergence speed and balance the global–local search performance in UPFC sizing. • The optimized UPFC configuration effectively reduces power losses, minimizes voltage deviation, and enhances overall voltage stability using a multi-objective function.