Transformer Turns Ratio Test Sets (TTR) are portable instruments that accurately measure the Transformer Turns Ratio of power, distribution, current, and other instrument transformers. The Transformer Turns Ratio is the ratio of the number of turns in the high-voltage winding to that in the low-voltage winding in a Transformer. Transformer Turns Ratio Test Sets can also measure polarity, phase shift, and excitation current of the transformers. Correctly using this test can identify shorted or open windings, incorrect winding connections and other faults inside transformers.
The TTR Test Set operates on the principle that the voltage ratio of an unloaded transformer is almost exactly equal to the true turns ratio. The TTR Test Set applies voltage to the high voltage winding of a non-energized transformer and accurately measures the resulting voltage from the low voltage winding. High impedance input and output voltage monitoring circuits optimize the transformer turns ratio testing for the highest accuracy.
The major source of error is the primary impedance drop due to the magnetizing current in the transformer which can be kept to a minimum by energizing the transformer at a lower voltage. Although a lower excitation voltage is preferred, voltages over 100VAC can be used to test a wide variety of transformers in a number of industrial applications.
TTR Test Set Functional Test
Between yearly calibrations of the TTR Test Set, a functional test can be performed in the field to ensure that the Test Set is giving the proper results.
This simple check consists of connecting the H and X test leads from the TTR Test Set and running a typical test. The displayed turns ratio reading should be exactly 1:1 as it is only measuring the ratio of the single lead that is connected.
When performing the functional test, the H and X test connection will cause an impedance mismatch between the input and output circuits and the indicated turns ratio reading may not be exactly 1:1. A function test reading may even exceed the manufacturer’s specified accuracy for the Test Set.
The function test results should not be interpreted as a calibration check. The function test merely shows that the Test Set is functioning correctly. If the Test Set does not indicate a ratio of approximately 1:1, a calibration should be performed on the unit.
TTR Test Calibration
It is recommended that TTR Test Sets be calibrated at least once every year. The TTR Test Sets require calibration due to inaccuracies caused by electric component drift, loose connections, and mechanical wear. Regular calibration will ensure that these Test Sets are functioning properly and calibrated over the entire measurement range.
Calibration of a TTR Test Set is performed by comparison with a Turns Ratio Calibration Standard. A Turns Ratio Calibration Standard contains a set of rotary switches that offer a range of selectable turns ratios and a multiplier dial to provide higher turns ratio values.
For the highest accuracy, most Turns Ratio Calibration Standards use a toroidal transformer core to ensure a very low excitation voltage in order to achieve a high turns ratio accuracy. The accuracy of the standard will allow the verification of the TTR Test Set to a greater than ±0.1% accuracy for most ranges.