Over Voltage Protection Pickup on an Isolated System

Dear All,

The Following scenario happened in one of the OG feeders.

Over voltage protection in line was getting picked up after opening of breaker from both side. This case happened in a GIS Substation. Initially it was doubted that the relay setting for the Transient over voltage caused due to Switching might have been on the lower side of the TOV value and hence it might have triggered the OV Pickup. But if supposing the setting may be wrong then it gives you wrong command for several seconds but in this case it gives a continuous pick up and drop command for 10 to 15 minutes Even when line was dead. The VT is located after breaker and in between there is a line isolator and hence the chances of Coupled Voltage due to CB Grading capacitance with VT has been ruled out. On further detailing I was told that it may be due to Ferroresonance between VT terminals and Grading Capacitor and it may impact the VT depending upon its duration, Frequency and voltage. How ferroresonance could exist when we have an open Isolator between CB and VT. Please Help me out