If this was a test question, the answer would be whatever the instructor wanted it to be. Most likely no they can't be connected in parallel.
If you are in the field and faced with an overloaded Dyn1 transformer and the only transformer available to share the load was a Dyn11 transformer, then yes, it can be made to work. You cannot connect the same as you would another Dyn1 transformer but by reversing and rolling the connections you may successfully share the load.
Sort of "Can you operate an additive polarity transformer in parallel with a subtractive polarity transformer?"
Yes if you reverse the connections to one transformer.
-------------------- Ohm's law
Not just a good idea; It's the LAW!
Just like can a Dyn1 masquerade as a Dyn3 when that might be a more useful connection? Yes, it can. Be there, done that. Limit yourself to what the factory calls it, and no, they can't be paralleled. But if need be, yes, easily. Size and impedance matching are actually more important than the connection implied on the nameplate. If, that is, you're a bit nimble. But if A must always go to "1" and B must always go to "2" and C to "3" then, no, of course not. College class room vs. real life.
I’ll see your silver lining and raise you two black clouds. - Protection Operations
A true and rather embarrassing story follows. Perhaps the warning to others might be worth a bit of embarrassment.
In order to avoid low voltage delivery charges, we built a substation. We looked at the supplier's transformer nameplate, and confirmed DY11 same as the one we were installing. We extended a 115 line across the fence to our side to all new bus work. Once the substation was complete and the transformer had soaked, we went to make parallel with the low voltage delivery. The phasing sticks said don't try, with voltages consistent with a 60 degree shift. After further consultation with the supplier we found they have a strict policy of connecting ABC/abc North to South at substations. This meant hitting the transformer's H123 with CBA. To ensure that abc properly lagged ABC by 30 degrees they had reconnected the transformer internally to a DY1. Rolling the low side cables was easy enough, although time consuming. We puzzled over the high side, though. The short line came over flat with no room to roll it. The supplier's engineer pointed out where we could alter the bus so we could get back to ABC on H123. The next phasing check passed and parallel was made successfully. The theory works.
1) Answer to the first query by Zinki- No, parallel operation of two transformers with different vector groups is not possible.
2) But parallel operation of Dyn1 with Dyn11 or Ydn1 with Ydn11 (or Dyn5 with Dyn7) is possible if we interchange external connections between R & B phases on both HV and LV sides in one of the transformers. By this connection change, we are changing the vector group in one transformer from Dy1 to Dy11 or vice versa.
3) A good reference on this subject is Section 6 of IEC 60076-8 Application Guide for Transformers.
Quote- "Consequently, transformers with connection clock numbers differing by 4 or 8 can be
connected in parallel after cyclic permutation of the connections to the line on either side of the
transformer.
It is even possible to combine transformers having clock numbers 1 or 5 with transformers
having clock numbers 11 or 7 by reversing the phase sequence on both sides of either
transformer.
Parallel connections of Dyn and Yzn are not recommended because of different zero sequence impedance properties" -unquote
4)Some 30 years back, I was supplying one 630 MVA 400/21 kV generator Transformer bank (4 nos 210 MVA)to a thermal power station. Transformers were installed and processed. Isolated phase duct connections are ready. The station was getting ready to generate power. Then somebody found out that instead of Yd11 connected transformers Yd 1 is the requirement. It was not easy to transport each single phase unit (180 T transport weight) in special rail wagons to the factory 2000 km away and rectify for new requirements. Then we put an intermediate turret between LV bushing turret and the tank and the 10kA current leads to bushings were interchanged by a special criss-cross bus bar connection!
For paralleling of xfo's on secondary side, you have to match :
-Phase shifting (configuratyion type)
-impedance (or verry close, i.e. same size of xfo)
The maybe answer:
You have to modify the internal connection of the secondary side. Means: open the xfo, redo test & certification and a new nameplate by a certified manufacturer. ... It will cost $$$. And at the end you have to match the impedance...If the current impedance of both xfo are not close, you have to forget the MAYBE solution. For my point of view this not the kind of solution I reccomand.
Here we don't talk about protection and all other problems as higher short circuit level on the network from the secondary side etc.
