MV/LV Transformer Winding Configuration

That is a particularly bad decision. High Resistance grounding offers many compelling advantages over ungrounded, so it is obvious that little to no research was executed towards this decision. If the MV system is high resistance grounded, that is unusual, because of the charging current magnitude, assuming MV is 2.3kV to 4.16kV, of higher. It would be more likely to be medium resistance grounded, 200A or higher, to provide ground fault selectivity and the low voltage system would be high resistance grounded, with all of the advantages of ungrounded operation, and none of the disadvantages.

Delta-Wye would be the obvious transformer winding, allowing Line to Neutral loads, although most facilities use additional transformers for small L-N loads attached to the main transformer. This saves carrying a full rated neutral through your distribution, cost and reliability considerations. KIS.

Yeah, it is medium resistance grounded about 55A. Actually, we don't have any line to neutral loads at low voltage (0.69kV)

You must have some new technology that allows you to sense ground faults on your MV system down to 5-6 amperes, protecting for ground faults down to the bottom 10% of your windings. Sensitivity of ground fault protective devices was the reason that no lower than 200A ground resistor was used on MV distribution systems. You need enough current so the GF sensors (zero sequence sensors, 20A at 4kV for example) actually work reliably.

The ringing and high transient voltages for certain arcing faults on your ungrounded LV system, plus the ability to trace & locate your ground fault are two of the advantages of HR grounding. Maybe your distribution system is so simple that ground faults are a rare and/or easily located event. Also KIS.

How do you plan to protect the LV phases should a ground fault occur with no earth reference on that configuration? One phase could have an earth fault and it is not until you get a second earth fault on another phase will the failures become obvious.

We have an insulation monitoring system (from bender) to monitor the ground faults.

Will the Insulation monitoring system that you propose to have, identify the faulty feeder as well or it will just say there is an earth fault in so and so phase in the system.

I too feel (like other learned contributors to the post) that High resistance grounding is the appropriate thing to adopt.