With spreading using of switch type arc extinguishing in 10 kV distribution network

over-current occurs frequently on traditional PT（potential transformer）during arc extinguishing. Aiming to study the mechanism and suppression method of PT over-current

firstly

the mechanism of Ferroresonance and low frequency oscillation

which can be motivated by single-phase grounding and disappear during arc extinguishing

is theoretically introduced;secondly

a test platform of 10 kV neutral-ungrounded system is established

and series of single-phase grounding and disappear experiment are carried on under different capacitance current and PT;the last

comparing the traditional simulation results with experiment

a calculation method which takes characteristic of the core at deep saturation into consideration is proposed. Research results show that Ferroresonance and low frequency oscillation is likely to happen even system parameters exceeds Peterson resonance section

causing half wave inrush current and endangering PT fuse;4PT wiring and PT with good excitation characteristic can effectively suppress the resonance;the current calculated will be more accurate if characteristic of the core at deep saturation is taken into consideration.