High-voltage transmission systems are increasingly dominated by converter-interfaced renewable energy resources, resulting in reduced system inertia, diminished reactive support, and heightened vulnerability to voltage and frequency instability. Traditional capacitor banks offer economically effective reactive compensation, but they do not have dynamic controllability, and Battery Energy Storage Systems (BESS) can act to support both fast-response active and reactive power but are hampered by converter limits as well as range of operation. It introduces a coordinated control model of continuous BESS-based reactive and active support and discrete capacitor switching that provides grid stability. The hybrid load-flow formulation is created by complementarity functions for the purpose of explicitly describing converter limits, switching constraints, and the P–f, Q–V control interaction. The proposed approach is verified on IEEE 14- bus and IEEE 30-bus systems under different disturbance states. The findings show that coordinated BESS–capacitor control offers a substantial gain in voltage margins, maximum loadability, converter stress ratio, and frequency recovery when compared with standalone methods of compensation. The framework constitutes an effective pathway for integrating power-electronic and passive compensation resources into future high-inertia-deficient transmission grids.