7.1 Transformer Turns Ratio (TTR) & Acceptance
Key Takeaways
- A TTR test applies a low-voltage AC source to one winding and measures the induced voltage on the other to compute the primary-to-secondary turns ratio.
- ANSI/NETA ATS and MTS require the measured ratio to deviate by no more than 0.5% from the nameplate or calculated ratio on every tap position.
- A TTR deviation beyond 0.5% most often indicates shorted turns, a tap-changer mechanism problem, an incorrect tap setting, or winding damage.
- All tap positions must be tested on load tap changers and de-energized tap changers; a fault may only be visible at one tap.
- For a 13.8 kV:480 V transformer the nameplate ratio is 13800 / 480 = 28.75, and an acceptance reading must fall within 28.606 to 28.894.
Why the TTR Test Exists
The transformer turns ratio (TTR) test confirms that the ratio of primary turns to secondary turns matches the nameplate. Because a transformer's output voltage is set by this ratio, any deviation signals a real winding or tap-changer problem. The test is run de-energized with the transformer isolated and grounded until just before connection, then the ground is lifted on the terminals under test.
The test set applies a low-voltage AC signal (typically a few volts to a few tens of volts at line frequency) to one winding and measures the induced voltage on the other winding. The instrument calculates the ratio directly. Low voltage is used for three reasons: it keeps the test safe, it avoids saturating the core (which would distort the reading), and it lets a small, portable instrument do the work of what would otherwise require a full-voltage source.
Test Method Step by Step
- Isolate the transformer — open all terminals, disconnect neutral grounds, lift the ground only on the terminals under test.
- Select the tap — set the de-energized tap changer (DETC) or note the load tap changer (LTC) position. Test every tap.
- Connect the TTR set — H1/H2/H3 to the primary, X1/X2/X3 to the secondary on a single phase at a time. Use the test set's labeled leads; do not swap H and X leads.
- Apply test voltage — the instrument energizes one winding and reads the other.
- Record the ratio — compare to the nameplate ratio for that tap.
- Repeat for all phases and all taps.
A quality TTR instrument has an accuracy of 0.1% — well inside the 0.5% acceptance band — so a measured deviation greater than 0.5% is a real transformer problem, not test-set error.
Acceptance Criteria (NETA / IEEE)
ANSI/NETA ATS-2017 Section 7.2.2.D and the matching NETA MTS clause state that turns-ratio test results shall not deviate by more than one-half percent (0.5%) from either the adjacent coils or the calculated ratio. The same 0.5% tolerance is echoed in IEEE Std C57.12.00 (factory) and IEEE Std 62 (field testing), and in IEC 60076-1.
| Condition | NETA TTR Acceptance |
|---|---|
| Measured vs. nameplate ratio (any tap) | Within +/- 0.5% |
| Phase-to-phase comparison (3-phase) | Slight outer-phase differences normal; reject if any phase > 0.5% from nameplate |
| Tap-to-tap comparison | Each tap must be within 0.5% of that tap's calculated ratio |
| TTR instrument accuracy | 0.1% (referee grade) |
The tolerance applies per tap, per phase. A winding fault may only show on certain taps because the shorted turn lies in a section of the winding that is only in-circuit at that tap. Testing only the operating tap is a common field shortcut and a common way to miss a defect.
Three-Phase TTR and Phasing
On a three-phase transformer, test each phase pair independently (H1-H2 to X1-X2, H2-H3 to X2-X3, H3-H1 to X3-X1 on a delta, or H1-N to X1-N on a wye). It is normal for the two outer phases to read very slightly different from the center phase because of core geometry and leakage flux; rejection is warranted only when any phase exceeds the 0.5% nameplate tolerance. The test also confirms phasenameplate phasing — H1 corresponds to X1, etc. — which matters when paralleling units or connecting a differential scheme.
What a TTR Deviation Means
| Symptom | Likely Cause |
|---|---|
| One phase reads high or low by > 0.5% on all taps | Shorted turns in that phase; winding damage |
| Deviation only on certain taps | Tap changer contact problem, wrong DETC setting, or partially shorted tap section |
| All phases uniformly off by the same amount | Wrong nameplate, wrong test lead connection, or wrong voltage selection on the TTR set |
| Erratic or unstable reading | Loose tap changer mechanism, bad test lead contact, or open winding |
The TTR test does not assess insulation strength, oil quality, dielectric condition, or winding resistance — it only verifies the turns count and connection. Pair it with IR, winding resistance, and (for liquid-filled units) oil tests for a complete picture.
Worked Example: 13.8 kV to 480 V Transformer
A distribution transformer is nameplated 13.8 kV primary, 480 V secondary. The calculated ratio is:
Ratio = V_primary / V_secondary = 13800 / 480 = 28.75
The 0.5% acceptance band is:
- Lower limit: 28.75 x (1 - 0.005) = 28.75 x 0.995 = 28.606
- Upper limit: 28.75 x (1 + 0.005) = 28.75 x 1.005 = 28.894
A field reading of 28.40 on the X2 tap is out of tolerance and indicates a problem on that tap — most likely a tap-changer contact issue or a shorted turn in that section of the winding. A reading of 28.72 on every tap is in tolerance and the transformer passes the TTR portion of the acceptance suite.
NETA Level 2 Traps
- Do not test only the operating tap. A DETC with five taps must yield five ratio readings, all inside 0.5%.
- Do not confuse ratio with voltage. The nameplate ratio is a turns ratio; for a no-load ideal transformer it equals the voltage ratio, but tap voltages are listed on the nameplate and are the authoritative reference.
- Three-phase outer-phase variation is normal up to about 0.3%; only the 0.5% nameplate limit is a reject criterion.
- Lead placement matters. Swapping H and X leads on the test set gives a reciprocal ratio (e.g., 0.0348 instead of 28.75) and is a setup error, not a transformer fault.
A TTR test on a transformer primarily measures:
Per ANSI/NETA ATS and MTS, TTR results on any tap must deviate by no more than what percent from the nameplate or calculated ratio?
A 13.8 kV primary to 480 V secondary transformer has what nameplate turns ratio, and what is the acceptance band?