Hi Juan,
Some of my comments are from the HV generator world, and there are a couple of regulars here (Bill and MikeL) who work with LV sets a lot more than I do, so where their opinion contradicts mine I would listen to them.
I would like some clarification if it is not too much trouble:
"In an ideal world they'd be Ynd transformers, solidly grounded wye on the high side"
Q: If the wye is grounded then the zigzag is useless (it can't limit earth-fault current), isn't it?
Yes, that would be the outcome.
"delta on the low/generator side"
Q: What are the advantages of using the delta on the generator side? Harmonic issues?
On an HV machine we always limit the earth fault current to a handful of amps to protect the stator core in the event of an earth fault. Fault limitation is usually done by a single phase transformer and resistor connected from the machine neutral point to the earth electrode. On LV machines it is usually a plain resistor from neutral to earth. This method of earthing the neutral would generally preclude connection of any phase-neutral loads, which isn't a problem on HV machines but should be considered on LV machines depending on what your local code allows.
"Adding to the mix, the generator neutrals all look like they are solidly grounded which would negate some of the benefits of the LV delta"
Q: Same as above
A delta winding is desirable for the generator side of the transformer because it allows the generator to operate with its own high resistance earthing scheme. The earth fault level is defined by the earthing resistance, not the number of running generators and the available system fault level.
"I have a feeling this is going to be horrible system design with Dyn* distribution transformers driven in reverse"
Q: Do you think they are trying to use distribution transformers instead of elevation transformers? (They are different, aren't they?)
Distribution transformers with Dyn windings are cheap and easily available. Generator step-up transformers with YNd windings are less common and likely more expensive / longer leadtime. If an EPC contractor can make the scheme more-or-less work with a cheap and easy solution which is technically poor then they will, if they can get away with it.
"Hopefully the LV neutral isn't grounded at both the generator and the transformer. "
Q: What are the advantages of avoiding grounding the transformer?
Don't earth a neutral in two locations, otherwise neutral current enters in the earthing system. It's very easy to do this with careless design of the system, especially if the generator can operate in both stand-alone and system-parallel modes, if the location of the neutral-earth bond is not carefully selected. Neutral current in the earthing system can cause mis-operation of the protection relays, and it is a code violation.
I would like to understand this concepts so I can try to influence the designer to change this layout
Many thanks!