Research on loss-of-excitation protection of large generator directly connected to UHV grid

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 E_q. 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.