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Voltage stability requires the continuing control of the total system's supply of reactive power during emergency situation. The supply of reactive power, however, can be curtailed by the action of rotor overcurrent protection or overexcitation limiter (OEL) in reducing the rotating unit reactive power output. Practical heat run tests shows that the timing of the activation of the OEL can be very conservative, resulting in earlier than necessary operation and system voltage collapse. Significant benefit can be achieved if the timing of the OEL activation can be delayed, while ensuring sufficient margin is provided to avoid harming the rotor of the synchronous generator. This paper proposes a new thermal-based method to determine the timing of the OEL activation that is based on the thermal capacity of the rotor as the main indicator for limiting the excitation level of the synchronous generator. The proposed thermal-based method is validated using the extensive simulations of a single-machine and the Nordic power system. Simulation studies show that the system voltage collapse can be delayed significantly by delaying the OEL activation without compromising the thermal capacity of the rotor if the proposed OEL setting is used. [ABSTRACT FROM AUTHOR]