# Open-delta connection transformers

#### nyasha

Joined Mar 23, 2009
90
So l have attached the question and my attempt to the solution. I think l answered all questions correctly....but for part c l do not know if l draw the phase angles of the winding currents and voltages with a 30 degrees phase shift between them ?

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#### t_n_k

Joined Mar 6, 2009
5,455
I would think the secondary windings are capable of carrying the full 543.5A at 460V line-line.

So the total load power would be √3*460*543.5=433kW.

Each transformer must carry half that power or 216.5kW.

The primary kVA per transformer with rated winding current & voltage is still 250kVA so each transformer is running with a power factor of 216.5kVA/250kVA=0.866

BUT ... the total 3-phase source power factor will be unity.

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#### nyasha

Joined Mar 23, 2009
90
I would think the secondary windings are capable of carrying the full 543.5A at 460V line-line.

So the total load power would be √3*460*543.5=433kW.

Each transformer must carry half that power or 216.5kW.

The primary kVA per transformer with rated winding current & voltage is still 250kVA so each transformer is running with a power factor of 216.5kVA/250kVA=0.866

BUT ... the total 3-phase source power factor will be unity.

But wouldn't that damage the windings of the transformer ? Do we want that much current flowing into the windings of the transformer ?

#### t_n_k

Joined Mar 6, 2009
5,455
But wouldn't that damage the windings of the transformer ? Do we want that much current flowing into the windings of the transformer ?
No since that value is the rated current for the secondary winding of each of the 250kVA transformers. One can draw the rated current without any damage.

#### nyasha

Joined Mar 23, 2009
90
No since that value is the rated current for the secondary winding of each of the 250kVA transformers. One can draw the rated current without any damage.
I am confused there. I thought the 250kVA was for three delta transformers connected. Which means when another phase is removed the power output decreases.

#### t_n_k

Joined Mar 6, 2009
5,455
It's spelled out in the wording of the question which you might review ...

So two single phase transformers, individually rated at 250kVA are connected in open delta.