Yes. Three-phase power transformers are often tested on a per-phase basis with a single-phase source, using relays to switch the power from one phase to another as necessary. Three-phase transformers are produced with a wide range of winding configurations and, in general, are more difficult to test accurately if the low voltage (LV) winding is delta configured. This is because TTR testing assumes that the secondary winding is an open circuit and has no load connected. With a delta-connected LV winding and measurements performed on a per-phase basis, this assumption does not hold, as the winding under test is loaded by its connection with the other two windings in the delta loop. The current circulating in the delta loop leads to internal losses affecting the accuracy of the TTR measurement.
In these cases, it is recommended either to energise the HV winding line-to-line or to use three-phase excitation. Flux distribution will be more uniform, leading to a higher coupling between windings, so the results are less sensitive to excitation voltage. Excitation losses during the test are shared by all three sources, providing higher accuracy results as compared to those obtained with single-phase excitation. Simultaneous three-phase excitation reduces testing time and improves the efficiency of resources.