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Department of Electrical, Electronic and Systems Engineering, Universiti Kebangsaan Malaysia, Malaysia; Department of Electrical, Faculty of Engineering, Kirkuk University, Kirkuk, Iraq.
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Associate Professor, Department of Electrical, Electronic and Systems Engineering, Universiti Kebangsaan Malaysia Malaysia
Senior Lecturer, Department of Electrical, Electronic and Systems Engineering, Universiti Kebangsaan Malaysia Malaysia
Static Synchronous Compensators (STATCOMs) require precise controller tuning to maintain voltage stability in modern power grids. Fractional Order Proportional-Integral (FOPI) controllers offer enhanced flexibility over integer-order designs, yet existing tuning methods rely on weighted-sum formulations that conflate conflicting objectives through arbitrary weight selection, overlooking the inherent priority structure of power system stability requirements. This paper proposes a lexicographic bi-objective optimization framework that enforces strict priority ordering without weight selection, implemented through two formulations: a Lexicographic ITAE approach evaluated across five meta-heuristic algorithms with hard penalty constraints, and a Lexicographic PSO approach employing a dual-criteria ranking mechanism with a normalized sacrifice metric. Both were verified using an 11 kV cascaded H-bridge STATCOM in Simulink. Observations showed that there was a ''performance inversion'' effect where algorithms that provided the least cost gave oscillating or shifted voltage profiles, whereas those that were penalized resulted in improved physical performance. Obsessive settling time priority attained 0.25 s; however, it resulted in the steady-state shift by 140 V for an 82 V target voltage. Random sampling showed that only 9% of solutions gave a stable response. Between the two algorithms, Lex-ITAE with HHO performed better as it gave a smooth voltage profile close to the target 82 V despite having higher costs numerically.
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