Ferro-Resonance?

I work in a power station having 220KV and 500KV bus Bars.We are facing a problem in P.T of one of the two 220KV bus bars.After two three months difference appears of 20 KV in P.T out of one of the bus bars.We de energise the bus bar,ground the P.t and when re energised the difference is no longer there.Most say that this is due to switching on the system leading to ferro resonance.My point of view is that problem in burden on secondary side of the P.T.

It is worth mentioning that the difference in voltage mostly appears after rain.