Discrimination method of power frequency ferroresonance in GIS

Ferroresonance over-voltage is one of the most important factors to impact the inner insulation of electrical appliances. The damage to appliance caused by ferroresonance over-voltage accounted for a great majority proportion in power system. A criterion in neutral-ungrounded power system was deducted in detail to estimate whether the ferroresonance will occur in power system from one manner of working transiting to another manner of working. This criterion deducted by means of analytical method expresses the relationship between circuit equivalent resistance and critical resistance, which can be used to decide the zero-sequence resistance value added to the secondary coil of potential transformer to eliminate the ferroresonance over-voltage. The availability of this criterion was verified under two working states via a practical gas insulated substation.