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Ultra high voltage grid has a large time constant. Its surge current attenuates slowly. These could lead to tripping delay of under-voltage relay and impedance relay with offset mho characteristic comprised in loss-of-excitation relay, and affect high-speed loss-of-excitation detection. In this paper a novel algorithm and scheme of loss-of-excitation protection is developed using excitation voltage and generator synchronous electromotive force Eq. With this scheme, the influence of large time constant of ultra high voltage line and variation of power system operating conditions to loss-of-excitation protection could be reduced, the operation speed is improved. The simulation results demonstrated the effectiveness of the proposed algorithm.