The ferro － resonance is a typical fault occurring frequently in the medium and low voltage neutral ungrounded distribution network，its triggering mechanism and inhibition method are analyzed，and a novel active resonance inhibition method based on real － time sound phase voltage magnitude tracking and open － delta resistance regulation is proposed． Furthermore， based on a full digital real － time simulation platform of power system called ADPSS/ETSDAC，according to actual parameters of a 35 kV substation，a simulation model of ferro － resonance in a distribution network is established． Feasibility and validity of the proposed resonance inhibition method are verified by the simulation results．