Ferroresonance: Mitgating Resistance on Open-Delta VTs

Hi to all,

All sources that I checked are expressing the necessitiy of
ferroresonance suppressing resistors connected paralelly to open-delta
secondaries of instrumentation VTs. But, I haven't find anything about
sizing calculation of this resistors, yet.

I found some application examples, let's say for a 34.5 kV system a 25
ohms, 600 Watts resistor connected parallely to an instrumentation VT.
I guess, typical sizing values of resistors are like that, but how
come, whati is the method?

Thanks in advance for all answers.

Moris

P.S. I'm not talking about high impedance grounding transformers which
are also susceptible to ferroresonance phenomenon, I'm just looking for a resistance calculation method on measurement VT applications.