Neutral from Y Transformer

The neutral on the secondary side of the medium voltage transformer is there to stop the secondary from floating. If you only need a 3 phase (3 wire) feed then only use those. You don't have to use the neutral. I can't ever remember using the neutral on a MV system. I do use it frequently on 208/120, 480/277 and 600/347 circuits (277 and 347 lighting).

It is likely that the input transformers in your drive are floating on the secondary side which helps the gates generate a relatively smooth DC bus charge.

I am a little concerned given this question that you may be a little out of your comfort zone. Make sure you study the drawings that came with the drive and most importantly get a licensed electrical engineer that is familiar with this type of installation review and stamp the installation drawings. This is MV! A mistake can have serious consequences.

What does 'Ddyn' stand for?

The neutral may be left floating as the load is a three phase three wire load. The Ddyn is only to neutralise the 5th & 7th Harmonics at the input side of this trafo.

I'm still wondering what is a Ddyn? Is it a filter of some kind? What does Ddyn stand for?

Regards,

Frank

It's a three winding transformer, with D on the primary winding, and D + Y on the secondary winding

The correct meaning of Ddyn has been correctly pointed out by others in this discussion. The reason for -dyn is to give different phase vectors to the two windings of the secondary and hence a smoother combination to DC after rectification.