Pardon the pun, but why were the transformers connected WYE-WYE? Everything I have ever used was either Delta-WYE, or Delta-Delta. There are too many problems WYE-WYE which is why it is almost never used.
Sometimes, those high power gen-sets need to have a neutral grounding resistor installed to prevent that very problem. Most of my experience is with solid-state inverter units but the principals are still the same.
Harmonic currents make me think of rectifier-capacitor style power supplies - these generate a large current pulse just before the voltage crest as the filter capacitor charges. I suspect this is where the load-side 3rd harmonic current was actually coming from.
Today, we would install power factor corrected power supplies that force current to be in proportion to the voltage, and make the load (field winding power) appear almost purely resistive. A smaller field transformer would have worked quite well in that situation as there would have been little or no neutral current, even at full load.
Before PFC power supplies were common, oversized transformers or harmonic filters were just about the only way to deal with harmonic currents.
Indeed , more thorough testing by the system integrator, as you folks do, would've saved us quite a bit of midnight oil. They'dve had to notice the substantial neutral current and the transformer getting too hot.
Neutral current viewed on the oscilloscope showed no discernible 60hz content. So our design people elected to disconnect the neutral as the most expedient fix.
Thanks for your interest - i do really appreciate that ..
Focus on Fundamentals consists of 45-minute on-line classes that cover a host of technologies. You learn without leaving the comfort of your desk. All classes are taught by subject-matter experts and all are archived. So if you can't attend live, attend at your convenience.