Actually trying to remember decades ago, but I guess "Ddoy11" given in the specs above should be Dd0y11 as it has two secondaries, one delta in phase (0 deg) with the HV delta primary (Dd0) and the second as star rotated to 30 degrees (11 oclock) as Dy11 ?
And both are 680V (the 680/680 bit)
So maybe feeding a 6-phase rectifier?
Hopefully someone like
@Julie. can enlighten us!
You are correct, the uppercase letter (D, Y, Z, V, etc) refers to the primary winding, the lowercase letters refer to secondary winding(s)
The number(s) 0, 1, 11, 6 etc refer to the phase shift between windings (referred to the primary)
There could also be a n which indicates a star point.
So Yny0 would be a star primary winding with the star point brought out, a star secondary winding where the star point is not avaliable, and the windings are in-phase with each other.
Dd0 (the first part of the OP's tx) means Delta primary, delta secondary, both windings in-phase with each other.
Dy11 means Delta primary, star secondary (star point not available) in phase group 11 ( 30 deg - think 11 o'clock)
A multi winding such as Dd0y11 means a Delta primary, one delta secondary (in phase with the primary), and another star secondary (without star point) in phase group 11 - 30 deg at around 11 o'clock.
This is quite an unusual winding in general - I assume it is for a solid state rectifier/inverter.
Traditionally, for both Mercury arc rectifiers (and rotary converters) they would use a winding known as double star - often with a delta primary. This is because it would provide a proper star point for all 6 secondary phases, and the primary Delta would allow triplen currents to circulate (third, ninth, 15th, 21st .. harmonics ) - this forms a half wave 6 phase rectifier
Solid state rectifiers are usually arranged in a bridge, thus they don't need the transformer to have a star point - basically they are full wave over 6 phases, again triplen harmonics can circulate in the primary.
As another note, there are sometimes transformers with tertiary windings, this may also be Dd0y11, but more usually Yd1yn0
The reason for this is that if you had a Yyn - you couldn't actually get an earth fault current, or neutral current!
Appears odd? - for a current to flow in the one secondary phase (star) (say red to neutral), then current must flow in the one primary (star) phase - again red to "neutral" - but there isn't a connection!
The only path would be through the other primary windings, meaning there must be current in their secondary equivalents - so it appears open circuit.
Adding a delta winding allows this secondary earth fault to be transferred to the three primary windings equally.
In this case though the rating of the tertiary winding is zero, or small whilst the OP's case the rating of the two secondaries is identical - so it isn't a tertiary type.
If you know the exact use, and the rectifier manufacturer, they could advise if alternative windings are suitable, and/or perhaps they could provide a temporary unit.
You could solve it using two transformers
One Dd0, and one Dy11 - each would be around half size at circ 1.2MVA each these would be potentially available off the shelf.
one of the reasons we're keen to get back up & running again as quickly as possible 
