I would scrap the entire installation and install a single, properly sized, listed ground clamp that has not been field-fabricated or field-modified like the two pictured. The top clamp has “stacked” washers/hex nuts that do not appear to be part of the original assembly, and the lower mess...
No one has cited this one yet…
From IEEE C57.13 7.7 Short-circuit capability
Voltage transformers shall be capable of withstanding for 1 s the mechanical and thermal stresses resulting from a short circuit on the secondary terminals with full voltage maintained on the primary terminals...
Do a Google search on transformer ferroresonance. At least it may be something to rule out.
IEEE std C57.105 §8 has some basic information on transformer susceptibility to ferroresonance.
You might consider something like a “Group-U” [440Δ - 220Y/127V 3ø, 50Hz] transformer listed at www.acmepowerdist.com/energy-efficient-transformers-three-phase.htm
Check out back issues of EC&M magazine. { www.ecmweb.com } There have been numerous articles on triplen and other power-system harmonics. One caveat—the online material is without charts, photos or illustrations. You have to purchase the articles to get those.
www.keybellevilles.com is one manufacturer, but beware of their healthy minimum quantities and setup fees.
www.sandc.com has, in times past, filled smaller orders for me.
A de facto document is ANSI standard IEEE C57.105-1978 IEEE Guide for Application of Transformer Connections in Three-Phase Distribution Systems §7, that be of some help.
An aside—from practical experience, voltage-mode imbalance [ANSI device 47] protection should only be applied to inhibit 3-phase motor starting—where current-mode relays [ANSI device 46] are needed to reliably interrupt motor running.