The increasing penetration of renewable energy sources into the electrical grid introduces significant challenges related to grid stability, security, and frequency regulation. These challenges primarily stem from the intermittent nature of renewable generation and the reduction in system-wide rotational inertia traditionally provided by conventional generators. This paper investigates the performance of a novel powertrain architecture that utilizes a magnetic Continuously Variable Transmission (mCVT) as a power-splitting device for a Flywheel Energy Storage System (FESS). In this configuration, the primary power exchange with the grid occurs through a grid-connected Wound Field Synchronous Machine (WFSM), thereby contributing to the grid’s physical rotational inertia. Conventional flywheels typically offer only virtual inertia, exchanging power with the grid through power-electronic converters. To manage FESS responses to grid-frequency variations, an mCVT torque control strategy is proposed. A detailed electromechanical model of the proposed FESS is developed, and simulation studies are undertaken to highlight its performance. Hardware-in-the-loop validation using a real-time target machine and a grid-connected WFSM confirms the fidelity of the proposed modelling and control framework, with close agreement observed between predicted and measured power dynamics.