Some comments about your 11 kV system first. Consider a simple case of one line to neutral load on A phase.
The neutral will carry current and so will have an IR voltage drop. This voltage drop will move the neutral position away from the system neutral in the direction of A phase.
A phase will carry current and so will have an IR voltage drop. This voltage drop will move the A phase vector position at the load away from the system A phase in the direction of the neutral.
The result is three-fold.
1. Unbalanced phase to phase voltages.
2. Phase angle errors.
3. Displaced neutrals.
Note; The line to neutral voltages may have been corrected by voltage regulators, but these only correct line to neutral voltages and do not much affect the other issues,
That is a fact of life with unbalanced single phase loading.
The settings on your sync-check relay must be wide enough to accept the unbalances or you will have difficulty synchronizing.
The floating neutrals make your generators more able to adapt to the unbalances.
However, it is common practice and a requirement of some codes that the transformers be solidly grounded when serving line to neutral loads.
From what I understand of your situation I would solidly ground all neutrals.
I may be wrong. I have learned to be wary of unknown systems. There may be factors of which I am unaware.
In regards to multiple NERs:
For example if your NERs are 5 Amps, With one transformer online a ground fault will be limited to 5 Amps. With 4 transformers online the ground fault current will be limited to 20 Amps.
In the event of an arcing fault 20 Amps will do more damage than 5 Amps. In some systems the extra current may interfere with ground fault coordination somewhere else on the system.
I must stress again that I am unaware of the details of your system and I am unaware of local practices.
I have tried to explain some of the basic effects of an unbalanced system.
By the way, A delta-wye transformer works well, but a wye-delta transformer is a bad idea.
Your generators are feeding the system with a delta-wye. No problem.
However if the system back feeds through the transformers they become wye-delta transformers.
Under some conditions, depending on site specifics such as the capacity of your plant compared to the overall system capacity and the PU Impedance of your transformers, I may consider high amperage NERs.
In the event of a line to neutral fault somewhere on the 11 kV system, The your transformers will act as wye-delta transformers and back feed into the fault.
The current will be limited by three times the transformer impedance.
If you have protection in place to cope with this safely, no problem.
If your protection will not provide adequate protection you may consider limiting the neutral current.
The size of the NERs in this case may be based on 3 times the transformer impedance plus enough reactance to limit the current to the transformer's maximum allowable current. In this case the allowable current may be based on 150% or more of rated full load current.
As to the transformer impedance and an NER value.
The transformer impedance may have six time as much inductive reactance as resistance (X:R ratio). If you add resistance you have to add it to the small value of transformer resistance with a small contribution from the inductive reactance.
If you add reactance you will not need as high an ohmic value.
Again, subject to site conditions.
I would investigate wireless reactors. This is an old old trick for increasing the impedance of a transformer for parallel operation with a transformer of higher PU impedance.
These are no longer commercially available, BUT.
You can add reactance by passing the grounding conductor through an old window type CT core. Passing the conductor through more than once multiplies the effect.
Again subject to site conditions. It depends on how much impedance you need.
I hesitate to suggest major changes to a working system.
However, you may consider changing the settings on your sync-check relay. That may solve your most pressing issues.
Bill
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"Why not the best?"
Jimmy Carter