Voltage instability in stressed transmission networks is exacerbated by reactive-power deficiency, heavy loading, and contingency events that drive bus voltages below acceptable statutory limits. This study presents an Improved Strength Pareto Evolutionary Algorithm (ISPEA) framework for the optimal placement of hybrid Static Synchronous Compensator–Static Var Compensator (STATCOM–SVC) controllers for voltage stability enhancement in transmission systems. Newton–Raphson load-flow analysis establishes base-case and contingency operating conditions (90% reactive-load increase at all load buses). ISPEA then identifies the most critical load buses using a multi-objective fitness structure that simultaneously minimizes voltage deviation, maximizes loading capacity, and penalizes generator operating-limit violations. Hybrid STATCOM–SVC controllers are subsequently deployed at ISPEA-selected buses, and the post-compensation load flow is evaluated. The framework is validated on the IEEE 30-bus benchmark system and applied to the Nigerian 31-bus transmission network. For the IEEE 30-bus system, ISPEA selects Buses 5, 7, and 23, and hybrid STATCOM–SVC controllers of 5.03, 7.38, and 7.78 kVAr restore all three buses to 1.0000 p.u. while reducing total active losses from 541.66 MW (contingency) to 265.92 MW — a reduction of 50.9%. For the Nigerian 31-bus system, ISPEA selects Buses 5, 11, and 21, and controllers of 7.38, 14.34, and 14.34 kVAr restore all three buses to 1.0000 p.u. while reducing active losses from 684.52 MW to 316.10 MW — a reduction of 53.8%. The maximum loading capacities at selected buses are reduced after compensation, confirming tighter security margins. These results demonstrate that ISPEA-guided hybrid STATCOM–SVC placement provides an effective, optimization-driven approach to voltage-stability control in both benchmark and practical transmission networks.

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