Transformer ratio measurements

Transformers are subject to mechanical impact and vibration. Problems or faults occur due to poor design, assembly, handling, poor environments, overloading or poor maintenance. Measuring the polarity and turn ratio of a transformer winding assures that the connections are correct and there are no severe mismatches or opens. Many transformers have taps built into them. These taps allow the ratio to be increased or decreased by fractions of a percent. If any of the ratio changes involve a mechanical movement of a contact from one position to another, these tap changes should also be checked during a ratio test. Regardless of the configuration, star, delta or zig-zag, the measurements are normally made phase to phase and comparisons are made to the estimated ratios defined by the transformer template (voltages and configuration).

Ratio measurements in transformers are essential to:

• Validate design specifications
• Verify quality of manufacturing process
• Establish present condition and condition trend
• Determine if damage has occurred


Take note

  • It is critical that the condition of a transformer is known.
  • The turn ratio test, if properly and accurately done, can tell a significant amount about transformer condition.
  • Off-the-shelf systems are available to measure LV and HV turn ratios.


Published as:

Transformer ratio measurements

By M Ohlen, Megger

Electricity+Control, February 2013 (pages 28- 30)

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