The monitoring principle of field windings inter-turn short circuits based on the unbalanced current root mean square（RMS）value in the main protection overcomes defects of the fault monitoring using the single harmonic detections，and can maximize the fault characteristics information extracted.The sensitivities of the Three Gorges and Pengshui generators are analyzed through simulations.Factors affecting their sensitivities are analyzed with respect to measured data.Based on the analysis，a monitoring scheme considering influences of natural unbalanced currents is proposed.Through phase shift and superposition of the unbalanced current，non-fault characteristic components in the steady-state unbalanced current can be eliminated to further improve the monitoring sensitivities.Anti-malfunctioning measures，a two-segment value setting principle and floating threshold operation characteristics are discussed.An on-line monitoring device manufactured based on the scheme has already been applied in Zhejiang Xinanjiang Hydropower Station.