Q: What is the turns ratio test, and what problems can it reveal?
A: The Turns Ratio Test (TTR) measures the ratio of the number of turns in the high-voltage winding to the low-voltage winding. For a healthy transformer, this ratio should match the nameplate voltage ratio within ±0.5%.
Why it matters:
• Detects short-circuited turns - the most common early-stage winding fault.
• Identifies incorrect tap-changer positioning or tap-changer contact problems.
• Verifies correct winding connections after re-assembly or repair.
What abnormal results indicate:
|
Condition |
Possible Cause |
|
Ratio high on one phase |
Short-circuited turns on the HV winding |
|
Ratio low on one phase |
Short-circuited turns on the LV winding |
|
All three phases off by the same amount |
Wrong tap position or nameplate error |
|
Ratio varies with tap position |
Tap-changer mechanism fault |
What is the Standard for the turn ratio test?
Per IEC 60076-1 and IEEE C57.12.90, the measured ratio deviation must not exceed 0.5% from the calculated value for power transformers.