Petro Wiki Wye delta

The most desirable transformer connection is the wye-delta. The transformer primary is wye-connected, and the secondary is delta-connected. Though it is done in many cases, the primary winding of the transformer, known as the Y point, should not be grounded at the wellsite because if one of the primary wires were to go to ground at some point in the primary system, the ground wire at the wellsite might be at primary voltage to ground potential, creating a danger of electrical shock for personnel. The transformer "ground" should not be connected to the grounding system at the pumping unit because the latter includes the enclosures for the electrical equipment.

The wye-delta connection has the advantage of allowing harmonic voltages in the system to have a self-canceling effect in the delta-connected secondary. It is not necessary for three single-phase units connected wye-delta in a three-phase bank to have equal impedances; however, it is important for the primary to have balanced voltage because unbalanced primary voltages can cause circulating currents in the delta secondary.

Answer:2 (Regarding YNd temporary grounding)

Floating the primary wye is a good point.  Also, when floating the wye and performing single phase switching on the transformer the last phase will experience a high over voltage. If you use lightning arresters on your transformers becareful, you should temporarily ground the wye point and then remove the ground when the transformer is completely switched in.