Q9ANS

 

Answer

 

They cannot be operated in parallel.

Vector group can be modified by changing internal connections for delta of primary winding.

It is not possible at site. It can be done at works only.

 

In Dy01, secondary phase voltages are lagging by 30 degree to respective primary phase voltages.

In Dy11, secondary phase voltages are leading by 30 degree to respective primary phase voltages.

 

However there is simple practical solution.

 

It is seen that vector diagrams DY01 and DY11 are mirror image of each other. If vector diagram DY01 is flipped horizontally i.e. left–right, it will be similar to DY11. Of course phase sequence is reversed.

 

Consider two moving persons A and B at some distance. B is at right of A as under.

Consider them moving from left towards right.

In this case A appears behind (lagging) B.

But with persons in the same position, consider them moving from right towards left.

In this case A appears ahead (leading) of B.

 

Change in vector group with change in phase sequence.

Dy01 act as DY11 when phase sequence is reversed.

Similarly Dy11 act as DY01 when phase sequence is reversed

 

Therefore phase sequence of one of the transformer is to be reversed in primary as well secondary sides. Thereafter both the transformers have same vector group and can be connected for parallel operation.

 

This is not simple as appears. Corresponding changes for instrumentations required for the transformer with changed sequence.