During a TTR test, one actually measures the TVR. Afterward, for three-phase transformers, a correction factor that depends on the vector configuration of the windings may need to be applied.
It is impossible to measure TTR from accessible points on a transformer. An assumption is made in TTR testing that, because of no-load conditions, the voltage ratio of a transformer (TVR) is equal to the turns ratio (TTR). This, of course, ignores the reality that a true no-load condition cannot be achieved for all winding configurations. Another assumption in TTR testing is that all the flux produced by one winding links with the second winding, ignoring leakage flux. These assumptions, for some transformers, result in ‘false positives’ when looking for problems in a conventional TTR analysis.
Finally, TNR is the ratio provided on a transformer’s nameplate, or one that can be calculated from the line-to-line winding voltages provided on the nameplate. In summary, the transformer turns ratio can be expressed as:
