Turns ratio between the windings
The turns ratio test is an AC low voltage test which determines the ratio of the high voltage winding to all other windings at no-load. The turns ratio test is performed on all taps of every winding.
Voltage is applied on the H marked leads and measured of the X marked lead by the test set.
Ratio measurements are conducted on all tap positions and calculated by dividing the induced voltage reading into the applied voltage value. When ratio tests are being made on three-phase transformers, the ratio is taken on one phase at a time with a three-phase TTR until the ratio measurements of all three phases are completed.
Measured ratio variations should be within 0.5% of the nameplate markings.
Some TTR can perform transformer ratio measurement and also assess if on-load tap changer contacts are making satisfactorily during its transition from one tap position to the next position.
Turns Ratio Test Procedure, Step by Step
Isolate the equipment, apply working grounds to all incoming and outgoing cables and disconnect all incoming and outgoing cables from the transformer bushing terminals connections.
Disconnected cables should have sufficient clearance from the switchgear terminals greater that the phase spacing distance. Use nylon rope to hold cable away from incoming and outgoing terminals as required.
Connect the H designated three-phase test lead with the military style connector at one end to the mating connection on the test set marked with an H. Ensure that the connector’s index notch lines up properly.
Connect the X designated three-phase test of lead military style connector at one end to the mating connection on the test set marked with an X. Ensure that the connector’s index notch lines up properly.
Connect the H1, H2, H3 designated test lead to the corresponding H1, H2, H3 transformer terminal / bushing. Connect the H0 test lead if H0 terminal/bushing is present.
Refer to Figure 1.
Connect the X1, X2, X3 designated test leads to the corresponding X1, X2,X3 transformer terminals / bushings. Connect the X0 test lead if X0 terminal/bushing is present.
Perform turns ratio measurements for all tap positions.
Confirm that the measured ratios is within 0.5% of the calculated ratios.
Transformers that have wye connections but do not have the neutral of the wye brought out shall be tested for ratio with three-phase power supply.
Any inequality in the magnetizing characteristics of the three phases will then result in a shift of the neutral and thereby cause unequal phase voltages. When such inequality is found, the connection should be changed, either to a delta or to a wye connection, and the line voltages measured.
When these are found to be equal to each other and the proper values (1.732 times the phase voltages when connected in wye), the ratio is correct.
Power Transformer Testing
Automatically measuring ratio and winding resistance of all taps/phases.
Reference: Substation Commissioning Course – Raymond Lee, Technical Trainer