[Big John]

Even completely losing a neutral wouldn't cause a flashover, because the poles are designed to withstand the full phase-to-phase system voltage continuously.

If you're getting lightning strikes and they're getting into the controls, my money says they would take out more than just the close coil, so if you're not seeing any other damage, that's not my first theory.

It could be a lot of causes: The coils themselves could be smoking because a jammed up mech, they contaminate the oil sufficiently to allow flashover, the carbon from the blowout allows phase-to-phase faults.

What protection do you have on the system? Is there anything indicating a fault? Any damage to the controllers? How old are they? How often do they operate?

 

[Big John]

Do you think ferroresonance could be playing a part as to the flashovers, where as, the "choke effect" of the ferrous conduit causing the circuit to be impedance grounded versus being solidly grounded?

No idea, I don't know enough about ferroresonance. All I can say is I wouldn't expect any failure with the neutral to impart more than line-to-line voltage, so if your equipment is designed to handle that but you're still seeing flashover failures, it's not the first thing I'd look at.

Why are your closing coils burning up? That's where I'd start.