What Does TTR Stand for in Transformer Testing?

In transformer testing, TTR stands for Transformer Turns Ratio-the ratio of turns between two windings on the same phase. Field crews and FAT engineers use TTR testing to confirm that a transformer's voltage ratio matches its nameplate and design specification.

The abbreviation also labels the instrument itself: a TTR tester (or turns ratio meter) applies a low-voltage AC signal to one winding, measures the induced voltage on another, and calculates the ratio automatically.

Definition: turns ratio vs voltage ratio

By definition:

TTR = N_primary / N_secondary

where N is the number of turns on each winding.

In practice, modern TTR instruments measure TVR (Transformer Voltage Ratio) under no-load conditions:

TVR = V_applied / V_induced

Because impedance has negligible effect at no load, TVR ≈ TTR. Industry practice validates turns ratio using instruments that measure voltage ratio-Megger, Omicron, and Huazheng TTR testers all work this way.

For three-phase transformers, apply a vector group correction factor (Dy, Yd, Yy, Dd, etc.) before comparing results to nameplate data.

What TTR testing measures

A standard TTR test reports three quantities per measurement:

Output	What it indicates
Turns/voltage ratio	Whether winding turns match design
Excitation current	Core magnetization level; spikes may signal core or winding issues
Phase deviation	Winding polarity and connection correctness

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Why TTR testing matters

TTR is one of the most common transformer diagnostic tests-performed at manufacturing, commissioning, routine maintenance, and during fault investigation.

Application	Purpose
Factory acceptance (FAT)	Verify nameplate ratio before shipment
Commissioning	Confirm correct installation and tap position
Preventive maintenance	Detect gradual winding deterioration
Troubleshooting	Identify shorted turns, open windings, tap changer faults

According to Megger and IEEE field practice, TTR results are compared against nameplate ratings throughout a transformer's life to reveal shorted turns, core heating, insulation deterioration, or improper connections.

How the test is performed

De-energize and isolate the transformer

Identify HV and LV terminals; confirm vector group and tap position

Connect the TTR tester to the windings under test (typically apply signal to HV, measure LV-or per instrument guidance)

Select tap positions - test all no-load taps; for OLTC units, test every LTC position

Record ratio, excitation current, and phase deviation for each phase

Compare to nameplate calculated ratio with applicable vector correction

Test all three phases. For auto-transformers and multi-winding units, follow the instrument manual for additional connection schemes.

Acceptance criteria

Criterion	Typical limit
Ratio deviation from nameplate	≤ ±0.5% (industry common practice)
Adjacent tap consistency	Within ±0.5% of calculated value for each position
Repeatability	Stable readings on retest after wiring verification

Results outside tolerance may indicate:

Shorted turns in one winding

Open circuit or broken conductor

Wrong tap position or OLTC malfunction

Incorrect external connections or reversed polarity

Manufacturing or transport damage to windings

If results fail, verify TTR tester connections, run instrument self-test, and retest before concluding the transformer is defective.

TTR vs related transformer tests

Test	Abbreviation	What it checks
Turns ratio test	TTR	Winding turns relationship / voltage ratio
Winding resistance test	WRT / DCR	Conductor resistance, connection integrity
Polarity test	-	Vector relationship between windings (acceptance)
No-load loss test	Po / Io	Core loss and excitation current at rated voltage
Load loss test	Pk / Uk	Copper loss and short-circuit impedance

TTR and winding resistance are complementary-TTR detects turn-count anomalies; resistance detects loose connections, broken strands, and contact resistance at bushings and tap changers.

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Standards and references

Standard	Relevance
IEEE C57.12.90	Standard test code for liquid-immersed transformers
IEC 60076-1	Routine tests including turns ratio verification
IEEE Std 62.2	Guide for diagnostic field testing on power transformers

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TTR tester selection (brief)

When choosing a TTR tester for your fleet:

Voltage range - must cover your LV side (e.g., 400 V, 690 V) and HV side through instrument design

Three-phase capability - simultaneous or sequential phase testing

Tap testing support - easy position switching for NLTC and OLTC

Accuracy - typically ±0.1% ratio accuracy on modern digital units

Portability - battery-operated units (e.g., ) for substation field work