Wye Wye configuration and Ferro Resonance?

Does anyone have any resource info on why- Wye- Wye connected transformers are used in certain situations and how they correlate to helping or hindering Ferro Resonance? After reading what I can find, and listening to the POCO, I am confused.

Situation we have- 34.5kv Ungrounded Delta service from the POCO to customer. Customer has 4160v generators and Wye- Delta (step up) transformers to generate the 34.5kv in the event of a utility outage.
POCO states that whenever they have a fault on their line, the fault current is seeking the H0 to ground bond on customer’s transformer, instead of going back to substation… and is burning up their static lines.
They state this is a case of Ferro Resonance and customer must replace both of their Wye Delta Transformers with Wye-Wye xfmrs.
Having never heard that term before, I started doing some hw and am now more confused than I was before! Any help understanding these topics would be greatly appreciated…

Ps- Eng. firms are being brought in to analyze the situation and recommend a solution, but I would really like to understand what’s going on…

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