TTRU3

To upgrade from 2.2.4 to 3.x.x, the TTRU3 must be returned to a factory or ASC.

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, it is more difficult to test accurately if the 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 energize 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.

A Transformer Turns Ratio (TTR) test checks that a transformer is converting energy in an expected way. The test is also known as simply a turns ratio test. A TTR test is performed by a ratio meter (ratio tester). This test validates the transformer design, the transformer nameplate, and the transformer’s fixed transforming abilities throughout its service life. A TTR test should be performed to confirm that the transformer’s de-energized tap changer is properly positioned and that shorted windings turns do not exist. The ratio meter provides convenient and accurate readings of power transformer ratios and polarities.

A Transformer Turns Ratio test works in accordance with the same fundamental electromagnetic phenomena that the transformer operates. The difference is that the TTR test typically uses a low voltage (LV) AC excitation signal (< 250 VAC) on a per-phase base or as a three-phase simultaneous excitation.

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: