Why Test Transformer Turns Ratio?
The turns ratio test (TTR test) is one of the most fundamental and revealing diagnostic measurements performed on a power transformer. By energizing one winding and precisely measuring the induced voltage in another, the test verifies the proportionality between primary and secondary windings and exposes a range of latent defects before the unit is placed in service.
1. Detects shorted turns, open circuits, and winding deformation that are otherwise invisible.
2. Confirms that the transformer matches its nameplate ratio and vector group.
3. Validates the correct operation and position of the on-load tap changer (OLTC).
4. Assesses mechanical integrity after transportation, short-circuit events, or fault currents.
5. Serves as a mandatory commissioning and routine-maintenance acceptance criterion.
Principles of Turns Ratio Measurement
The turns ratio of an ideal transformer equals the ratio of the number of turns in the primary winding (Np) to the number of turns in the secondary winding (Ns), which is also proportional to the ratio of the voltages induced across each winding:
Fundamental Relationship
Np / Ns = Vp / Vs = I(secondary) / I(primary): TTR = Vp / Vs
During the test an AC excitation voltage (typically 100 V to 500 V) is applied to one winding. The resulting induced voltage on the other winding is measured with a high-precision, high-input-impedance circuit. The instrument computes the ratio and, by energizing in both directions, determines ratio error, phase displacement, and excitation current.
Key Physical Concepts
Magnetizing (excitation) current indicates core health and detects inter-winding or turn-to-turn shorts.
Polarity and vector-group (phase displacement) confirm the correct connection of windings.
The measurement is a magnetic (no-load) test, so it is independent of load current.
Step-by-Step Test Procedure
1. Preparation and Safety
De-energize the transformer and isolate all terminals; complete lockout/tagout.
Ground and discharge all windings; verify zero voltage before handling leads.
Record ambient temperature, humidity, and the transformer nameplate data (ratings, vector group, tap table).
2. Instrument Setup
Select a certified turns-ratio (TTR) meter and power it up; run self-test and calibration check.
Enter nameplate data and the tap schedule into the instrument to enable automatic comparison.
3. Connection
Connect the excitation leads to the HV (or designated) winding and the measurement leads to the other winding.
Follow the instrument wiring diagram for the specific vector group; keep leads short and tight.
4. Measurement
Set the tap changer to the first test position; apply the excitation voltage.
Record the measured ratio, phase displacement, and excitation current for each phase.
Repeat for every tap position and for all three phases (or per the required winding combination).
5. Verification and Shutdown
Compare each reading with the nameplate ratio and the allowable tolerance.
Discharge stored energy, remove leads, and restore the transformer to its service configuration.
Document all results in the test report with pass/fail judgment.
Data Interpretation
The most widely applied acceptance criterion is the 0.5% rule: a measured turns ratio is considered acceptable when its deviation from the nameplate ratio is within 0.5%. Deviations between 0.5% and 1.0% must be investigated, while larger deviations generally indicate a genuine winding or tap-changer fault. The table below summarizes the standard interpretation.