You have what is termed a single phase condition. Are you speaking of a primary phase loss or a secondary phase loss. Wye or delta?

I would like to know if it is primary phase loss..

the output will have little bit of low currents & voltages. & the input phases get imbalanced. the two phases will get more than calculated currents to give a desired output power.

If the transformer is in a balanced 3-phase system and the common point of the coils connected to the system neutral via a Petersen coil the increasing voltage drop on the Petersen will trigger the protection.

Read every word of this article and you will know everything about single phasing. You must give us a report back.

http://www.progress-energy.com/custservice/carcig/resourcectr/presentations/MotorProtection_VoltageUnbalance.pdf

Thanks Wareagle for the link. It's a very rich stuff!

If your system is delta-delta connected and you lose a leg, your available power will be 67% of normal.

Let me explain the situation with the following diagrams with a Dyn1 transformer of 11KV/380V. The three coils of the transformer are shown in Black, Red and Green color for explanation. All angles shown here are with respect to the V_BC as reference.

With Healthy condition:

HT voltages are:

V_BC = 11000 < 0

V_CA = 11000 <120

V_AB = 11000 <-120

LT Neutral to Line voltages are:

V_na = 220 < 60

V_nb = 220 < 180

V_nc = 220 <-60

LT Line to Line voltages are:

V_bc = 380 < -30

V_ca = 380 < 90

V_ab = 380 <-150

With Phase A lost condition:

Full V_BC voltage is applied in Red coil and same voltage is also applied in coils Black and Green in series. The voltage V_BC/2 is applied across each of the coils Black and Green. The voltages applied in each of the coils are:

V_BC = 11000 < 0

V_CA = 5500 <-180

V_AB = 5500 <-180

LT Neutral to Line voltages are:

V_na = 110 < 0

V_nb = 220 < -180

V_nc = 110 < 0

LT Line to Line voltages are:

V_bc = 330 < 0

V_ca = 0 < 0

V_ab = 330 <-180

Note: This analysis is done completely on my knowledge in 3 phase electrical system and I appreciate any comments on it.

-MS