Open Delta Transformer

[Will96]

I have a 15KVA, 3phase auto transformer that is of the open delta configuration. How do I go about closing the delta. Would adding a third transformer to the two work and what ratings would the added transformer need?

 

[mansooratdelphi]

An open delta or 'Vee' connection utilizes only two active sets of windings. If a 3 limb core is used, the two windings are invariably placed on the outer two limbs. Alternatively, two seperate cores are used for the two windings (two identical 1-phase transformers). The rating of each is half of the total rating. If you must add another 1-phase transformer to 'close' the delta, it must be electrically identical to the existing windings (area & turns ratios), and rated for 7.5 KVA.
regards

 

[busbar]

Carefully weigh the need for using a full-delta autotransformer set. Within the existing, one coil will have to be reversed with respect to the other, and the added transformer will need an 8.66kVA rating [15/2*2/root3] to be compatible with existing pair. There will also be some high-to-low-voltage phase shift introduced that is not present in the open-delta bank; e.g., for 2:1 ratio, that will be {potentially undesirable} ~60° leading or lagging, depending upon connection.