I am trying to confirm what the secondary readings would be on a 480V Delta Primary to 480V Wye connected isolation transformer when one of the primary phases is lost. Here is what I think:
Because this is an isolation transformer and the secondary wye connected L-L voltage is 480V then the L-N voltage across each winding must be 277V. So the 480V across each primary delta connected winding divided by the 277V across each secondary winding gives a winding ratio of 1.73 between the primary and secondary.
Now with a phase disconnected on the primary delta winding you will have one primary winding still reading 480V between the two remaining phases, and the other two windings would only have 240V across each of them since they are now in series between the remaining two phases. So if you take this 240V and divide it by the 1.73 winding ratio you will have aprox 138V L-N on two of the secondary phases and will have 277V L-N on the third phase on the secondary. You will also have the same L-G readings for a solidly grounded secondary. Are these expected readings correct.
Now the part that I cannot seem to see is what you would have for L-L readings on the secondary. I know you would have to subtract the vectors as you typically would but dont know what the phase angle of the 3 phases would be. Would the one full voltage 277V secondary phase be 180deg apart from the other two phases that are at 138V instead of 120deg since this is now a single phase condition?
Even if the secondary is feeding a load is it possible to read 277V L-G on the secondary through backfeeding or is this not possible since each phase still has a reduced voltage supply and in not really being backfed?
Because this is an isolation transformer and the secondary wye connected L-L voltage is 480V then the L-N voltage across each winding must be 277V. So the 480V across each primary delta connected winding divided by the 277V across each secondary winding gives a winding ratio of 1.73 between the primary and secondary.
Now with a phase disconnected on the primary delta winding you will have one primary winding still reading 480V between the two remaining phases, and the other two windings would only have 240V across each of them since they are now in series between the remaining two phases. So if you take this 240V and divide it by the 1.73 winding ratio you will have aprox 138V L-N on two of the secondary phases and will have 277V L-N on the third phase on the secondary. You will also have the same L-G readings for a solidly grounded secondary. Are these expected readings correct.
Now the part that I cannot seem to see is what you would have for L-L readings on the secondary. I know you would have to subtract the vectors as you typically would but dont know what the phase angle of the 3 phases would be. Would the one full voltage 277V secondary phase be 180deg apart from the other two phases that are at 138V instead of 120deg since this is now a single phase condition?
Even if the secondary is feeding a load is it possible to read 277V L-G on the secondary through backfeeding or is this not possible since each phase still has a reduced voltage supply and in not really being backfed?