A novel criterion for exactly identifying the machine leading to overvoltage of the common bus among the machines working in parallel is developed. Lose of selectivity of existing schemes in some cases is discussed. Correspondingly, an improved scheme is proposed. The change of nonload electromotive force (EMF) of generator is used to design a correction coefficient for the operation time characteristic of the overvoltage protection. The generator accompanied by the maximum positive change of nonload EMF will be judged as the one with runaway exciter. Theoretical analysis and EMTDC-based simulation tests prove that the source leading to overvoltage of the unit terminal can be correctly discriminated by means of this new criterion. A thorough comparison between the existent schemes and the proposed scheme has been made, which demonstrates that the new proposed scheme is superior to the existent ones.