TTRU3 True Three-Phase Transformer Turns Ratio Tester

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.

A transformer turns ratio test checks that a transformer is converting energy in an expected way. The test is also known simply as 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-energised 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 V AC) 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: 

The TTRU3 will use ‘step up’ mode, applying voltage to the secondary/tertiary winding and inducing and measuring voltage on the primary winding. In any test mode, the instrument first performs a safety and connection test using a low voltage (less than 1 V). Auto test mode uses the results of this test to verify that testing can progress in step up mode and to determine if a 3-phase test is possible.

Maintenance should be performed only by qualified persons familiar with the hazards involved with high-voltage test equipment. Read and understand Sections 1, 2, 3, 4, and 5 of the User Guide before performing any service.The TTRU3 requires only periodic inspection. Inspect all hardware items to ensure all are in good condition.The TTRU3 may be cleaned periodically. In doing so, do not allow water to penetrate panel holes. An all-purpose, household spray cleaner can be used to clean the panel. Polish with a soft, dry cloth. Clean the cables and mating panel receptacles with isopropyl or denatured alcohol applied with a clean cloth.

You should perform a complete performance and calibration check of your TTRU3 at least once every year. This will ensure that the TTRU3 is functioning correctly over the entire measurement range. Anyone can use the optional Megger TRS1 (Transformer Ratio Standard) to calibrate a TTRU3. This high-accuracy reference ratio standard can provide ratios in both step-down test mode (for traditional ratio test instruments and the TTRU3) and step-up test mode (for the Biddle Hand Crank TTR and the TTRU3). Megger performs a TTRU3 calibration on each new or repaired unit before sending it to a customer.

Any service or repair of this equipment should be performed only by qualified persons who are aware of electrical hazards and the necessary precautions required to prevent injury.Megger offers a complete repair and calibration service and recommends that its customers take advantage of this service for routine maintenance or in the event of any equipment malfunction.In the event a service is required, contact your Megger representative for a product Return Authorisation (RA) number and shipping instructions.Ship the product prepaid and insured and marked for the attention of the Megger Repair Department. Please indicate all pertinent information, including catalogue number, serial number, and problem symptoms.