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Transformer Turns Ratio Test: Some Unknown Facts

Transformer Turns Ratio Test: Some Unknown Facts

NETA, Powertest Conference 2020

Megger’s article “Transformer Turn Ratio Test: Some Unknown Facts” was presented at the 2020 NETA PowerTest Conference in Chicago, Il.  This article takes an in-depth look at the transformer turns ratio test, some of the challenges associated with interpreting test results, and unknown facts related to test variables. 


Article Brief:

Transformer Turns Ratio (TTR) is one of the most common test used to assess the condition of the transformer’s windings and core.  It is performed as a part of acceptance and maintenance test procedure to determine any problems due to poor design, assembly, handling, overloading, fault conditions or poor maintenance. TTR results are compared against the nameplate ratings to determine any possible insulation deterioration, shorted turns, core heating or any other winding or core abnormalities.


TTR is a simple and easy test to perform that is often taken for granted without fully understanding the principle and basis of the test. In cases when measurements are not within expected limits, it becomes a challenging task to determine the root cause and resolve the problem. This paper will focus on some of the unknown facts associated with the TTR test. The paper covers in detail the effect of applied test voltage, comparative analysis of step up vs step down excitation, different vector configurations, differences between nameplate ratio, voltage ratio and turns ratio, sources of ratio and phase angle errors, comparison of per phase testing vs true three phase testing, extreme tap ratios being out of tolerance for On Load Tap Changers (OLTC), and TTR test correlation with other electrical tests. The paper also provides field test results and case examples to explain the above-mentioned unknown facts.


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Authors Daniel Carreno and Dinesh Chhajer
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