Mitigating Ferroresonance in Voltage Transformers in Ungrounded MV Networks

In this paper, it is shown that ferroresonant oscillations involving voltage transformers (VTs) may be initiated by switching transients in ungrounded medium-voltage networks. It was demonstrated that various modes of ferroresonant oscillations may exist for the same VT type depending on the initial conditions and on the network capacitance. As the ferroresonant oscillations pose a risk to the VTs due to large overcurrents in the primary windings, the VTs operating in ungrounded networks should be protected against this phenomenon. The analysis of the classic method of protecting the VTs by means of connecting the damping resistor to the open-delta-arranged auxiliary winding has shown that often a very low ohmic value of the resistor is required to provide reliable protection. A new method of protecting the VTs against ferroresonance with a compact active load was thus developed.