As large capacity units is playing an increasily important role, loss of excitation failure has become more and more dangerous to the power system stability. Especially when the power plants and grid are seperated, it is a main concern to guarantee the security operation of both generators and grid. This paper concentrates on the mal-operation or misoperation problems of loss of excitation protection, which happen frequently in system swings. Based on the different generator performances in the two scenarioes and different kinds of loss of excitation failures, three innovative criterions including voltage, reactive power and timer criterions are proposed. Further, a new loss of excitation protection scheme based on the presented criterions is designed, and the scheme parameters tuning method is also introduced. The computer simulation results show that the new scheme can accurately distinguish the phenomena of loss of excitation from power system swings, and shows the advantages of high action speed and high reliability.