What is the TTR test of transformer?
Aug 01, 2025
The TTR Test (Transformer Turns Ratio Test) is a fundamental, non-destructive electrical test performed on transformers to measure the ratio of the number of turns in the primary winding to the number of turns in the secondary winding(s). This ratio directly determines the transformer's voltage transformation capability.
Here's a detailed breakdown of the TTR test:
What it Measures:
The actual turns ratio (N<sub>p</sub> / N<sub>s</sub>) between specified windings (e.g., HV to LV).
It does this by applying a low-voltage, low-frequency AC signal (typically 8V or less, at 50/60 Hz or sometimes a special frequency like 25Hz to avoid core saturation) to one winding (usually the HV winding for safety and convenience).
The voltage induced in the other winding(s) is measured simultaneously.
The TTR is calculated as: Applied Voltage (V1) / Measured Voltage (V2). This should equal the nameplate voltage ratio and the physical turns ratio under ideal conditions.
Primary Purposes & Why it's Done:
Verify Winding Integrity: Detect shorted turns (a major fault), open circuits, high-resistance contacts, or incorrect connections within the winding.
Verify Nameplate Ratio: Confirm the transformer is built to the specified design ratio.
Check Tap Changer Performance: Test the ratio at every tap position to ensure the tap changer operates correctly and provides the expected voltage change per tap.
Determine Polarity (Additive/Subtractive): Especially important for single-phase transformers or when paralleling transformers.
Determine Phase Relation (Vector Group): Advanced TTR testers can help identify the angular displacement (e.g., Dyn11, YNd1) in three-phase transformers by testing between phases.
Quality Control: During manufacturing and after repairs.
Commissioning & Acceptance Testing: Before putting a new or repaired transformer into service.
Predictive Maintenance: Routine testing to detect developing winding problems early.
How it's Conducted:
The transformer is de-energized and isolated.
All terminals are safely grounded and discharged.
A specialized TTR Test Set is used. Modern sets are digital, automated, and display the ratio, deviation percentage, excitation current, and sometimes phase angle directly.
Test leads are connected to the designated primary (e.g., H1-H2) and secondary (e.g., X1-X2) windings.
The tester applies the low AC voltage.
The induced voltage is measured.
The tester calculates and displays the ratio (V<sub>applied</sub> / V<sub>measured</sub>).
The test is repeated for all required winding pairs (e.g., HV-LV, HV-TV if present) and at every tap position of any tap changer.
Interpretation of Results:
Exact Match: The measured ratio exactly matches the nameplate ratio (at the specific tap). Ideal, but rare.
Within Tolerance: The measured ratio deviates from the nameplate ratio by a small percentage (e.g., ±0.5% for distribution transformers, ±0.25% for large power transformers - refer to standards like IEEE C57.12.00 or ANSI C57.12.90). This is generally acceptable.
Out of Tolerance:
Lower Ratio than Expected (e.g., Measured 9.95:1 vs Nameplate 10:1): Suggests shorted turns in the primary winding or an open circuit/issue in the secondary winding. Shorted turns are the most critical concern.
Higher Ratio than Expected (e.g., Measured 10.05:1 vs Nameplate 10:1): Suggests shorted turns in the secondary winding or an open circuit/high resistance in the primary winding.
Inconsistent Results Across Phases or Taps: Points to problems with specific windings, connections, or tap changer contacts/mechanisms.
Polarity/Phase Angle: Results must match the transformer's nameplate designation (e.g., additive, subtractive, Dyn11).
Key Advantages:
Non-Destructive: Uses very low voltage.
Quick & Simple: Relatively easy to perform.
Highly Diagnostic: Excellent for detecting winding faults, especially shorted turns and tap changer issues early.
Fundamental Check: Verifies the core function of the transformer.
In essence: The TTR test is a vital diagnostic tool that ensures the transformer's windings are physically sound and configured correctly to perform its primary function of transforming voltage at the specified ratio. Deviations from the expected ratio are strong indicators of internal problems requiring further investigation.
