The Transformer Turns Ratio (TTR) method compares the test transformer ratio with an adjustable-ratio standard transformer by using a null balance detector to determine when the standard transformer ratio is the same as the test transformer. During test procedures transformer polarity is determined by comparison with the standard transformer.
A TTR test should be made for any new high-voltage power transformer at the time it is being installed. This test is also desirable for any transformer that has been overhauled or relocated.
Note: TTR is the preferred ratio method as its accuracy is 0.1%. All maintenance districts use the TTR data as a diagnostic base when performing transformer maintenance. The Voltmeter method is also used because it is an excellent test for verifying the proper tap changer make and break, and is easily converted to the impedance test setup that follows.
The TTR basically operates as a very accurate bridge (analogous to the Wheatstone Bridge). Measurements are dependent on the winding ratio between the excited winding and the measured winding. The test results are repeatable, a fact which makes data obtained from this test significant. This data is most useful for analyzing transformer problems where a shorted turn is suspected