Transforming Fault Levels Through A DY Tx (Fault on Y Side)

[jbond]

Is it true that a fault on the Y side of a DY Tx will translate upstream on the D side by the appropriate turns ratio for a 3Ph fault - however for a line to line or a single phase fault then the current seen upstream is as you would assume by multiplying of the transformer turns ratio and DIVIDED BY SQRT(3)?

I suspect this is because delta winding will balance out the current as seen by the upstream system - the same way that you translate between 3ph current and 1ph current?

Does this also mean that the current seen upstream will not just be the fault current divided by SQRT(3) but will also include the pre-fault load current? (ie the load on the 2 or 1 healthy phases on the star winding of the system will still be drawing their corresponding load without influence of the fault)

This may sound a bit complicated, I hope that you can understand what I'm saying!

 

[ijl]

1) Correct for the three-phase fault.

2)Almost correct for the line-to-line fault. The division by sqrt(3) is needed for the current in two windings of the D-side. No fault current in the third winding. But be careful with the currents into the D-side of the transformer. The input current in two phases of the D-side are equal to the winding currents (including the division by sqrt(3)). The input (or actually output) current in the third phase is the sum of these two.

3)Wrong for the line-to-earth fault, because there is no earth fault current in a DY-transformer.

4) Indecisive for the load current. No load current in the case of a three-phase fault. The load current is hardly equal to the prefault current in the case of a line-to-line fault, but it is transmitted normally through the transformer. (The possible saturation may also have some effect.) Normal load current in the case of the line-to-earth fault.