How does a transformer phase shift effect secondary fault currents 2

[rockman7892]

Does the phase shift through a transformer have an effect on the avalaibe fault currents at the secondary of the transformer.

For example for a given avalaible fault current at the primary of a transformer will a delta-wye transformer with a 30deg phase shift have a different fault current avalaible at secondary than would a wye-wye transformer with a 0deg phase sift?

For evaluation of 3-phase fault currents I do not think there would be a difference and it would only be the transformer impedance that would be the determining factor.

For unbalanced faults I'm wondering if the phase shift comes into play. I know a wye-wye will pass zero sequence currents but not sure what effect it will haave on pos seq fault currents. I suspect that the secondary fault phase angle will be different so it will combine with any other source contribution on the secondary at a different phase angle.

 

[davidbeach]

The transformer winding configuration has some impact on how faults on one side are seen on the other side, but not the phase shift as such. All wye-delta transformers will have similar affects, regardless of which of 6 different phase shifts. Like wise, all wye-wye transformers will have similar affects regardless of which of 6 different phase shifts.

 

[waross]

The available fault current is dependent on the impedance voltage which is measured by applying a voltage to the primary with the secondary short circuited. Any effect of the phase shift will be inherently included in the test results. The actual fault current will be the combination of the available fault current and the DC offset and the peak current may be much more than the available fault current.
Of more interest may be the way that a delta configuration supplies a single phase to phase fault as opposed to a wye connection.
For a wye:wye configuration the single phase fault current will be the available fault current. A delta winding may in some cases supply twice the available fault current into a single phase fault.
A phase shift implies a wye:delta or a delta:wye connection so that there may be issues.
For explanation consider a three phase transformer bank as a parallel connection of an open delta and a single transformer.
The open delta presents a virtual transformer across the open side. If you do some vector sketches of the inductive and resistive voltage drops across the open delta you will see that the resultant drops are equal to the drops of the third transformer. As a result, All three transformers contribute to a single phase load or fault but because of the phase angles of the transformers forming the open delta, the total current is twice the current expected.
When there is a phase shift there will be a delta connection and the current into a single phase fault may be greater than expected.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

As an aside to Bill, while it generally isn't done, any wye-wye or delta-delta transformer can be wired to provide vector groups 0, 2, 4, 6, 8, and 10. Vector groups 1, 3, 5, 6, 7, and 11 can be achieved with any wye-delta or delta-wye transformer if wired for that group. Thus, phase shift doesn't require a wye-delta or delta-wye transformer.

 

[rockman7892]

Thanks guys.

So it sounds like its not the phase shift itself that has the effect but its the winding configuration making up the characterisitc of the transformer that has the effect.

It sounds like the transformer impedance is determined by the transformer winding configuration with all other things considered equal. So for a given size transformer and voltages will a delta-wye transformer have a different impedance than a wye-wye unit resulting from the winding configuration? If they are then it looks like 3-phase faults will be different for each transformer type for a given size and voltage?

It also sounds like as Bill is saying that the transformer winding configuration can have an effect on unbalanced faults due to the way the transfoemr windings contribute to the fault. I'll have to spend some time digesting Bill's example.

 

[davidbeach]

Look at the transformer sequence connections and the sequence connections for the various fault types. That should help clarify what's happening for different fault types.