How to Perform a TTR Test on a Transformer?

How to Perform a TTR Test on a Transformer?

  A Transformer Turns Ratio (TTR) test verifies that the transformer's measured turns ratio agrees with its rated/nameplate ratio. It is commonly performed during transformer commissioning, routine maintenance, after repairs, and when investigating winding or tap-changer problems.

1. Isolate and Prepare the Transformer

Before connecting the TTR tester:

De-energize the transformer.

Isolate it from all power sources.

Apply the required grounding and safety procedures.

Verify that the transformer is completely de-energized.

Disconnect external connections as required by the test procedure.

Check the transformer nameplate for rated voltages, winding configuration, and tap positions.

Never connect a TTR tester to an energized transformer.

2. Select the Correct Test Connections

Connect the TTR tester to the appropriate transformer terminals according to the winding configuration.

For example, on a three-phase transformer, the tester may measure the ratio between the HV and LV windings phase by phase.

The exact connection depends on:

Transformer vector group

Wye (Y) or delta (Δ) connection

Number of phases

Available neutral terminals

Tap-changer configuration

Manufacturer's test procedure

Always follow the TTR instrument manufacturer's connection diagram and the transformer's test requirements.

3. Connect the TTR Tester

Connect the tester's excitation and measurement leads to the transformer terminals.

A typical test setup is:

TTR Tester → HV winding → Transformer magnetic core → LV winding

The tester applies a controlled AC test voltage to one winding and measures the induced voltage on the other winding.

4. Enter Transformer Nameplate Data

Modern TTR testers generally allow the operator to enter or select:

Rated HV voltage

Rated LV voltage

Transformer phase configuration

Vector group

Tap position

Expected turns ratio

The tester can then calculate the expected ratio and compare it with the measured value.

5. Perform the Ratio Measurement

Start the test from the TTR tester.

The instrument measures the voltage ratio and typically displays:

Measured turns ratio

Expected/rated ratio

Ratio deviation (%)

Phase angle

Excitation current

Polarity/vector relationship, depending on the instrument

A simplified deviation calculation is:

Ratio Deviation (%) = (Measured Ratio − Rated Ratio) / Rated Ratio × 100

The acceptable limit should be determined according to the applicable standard, transformer manufacturer's specifications, and project requirements rather than applying one universal value to every transformer.

6. Test All Phases

For a three-phase transformer, perform the test on all relevant phases.

For example:

Phase A → Phase a

Phase B → Phase b

Phase C → Phase c

The measured ratios should be consistent with the transformer's rated ratio and expected phase relationship.

Significant differences between phases can indicate a potential winding, connection, or tap-changer problem.

7. Test the Tap Positions

If the transformer has a tap changer, perform measurements at the specified tap positions.

For example:

Tap +5 → Test

Tap +2.5 → Test

Tap 0 → Test

Tap −2.5 → Test

Tap −5 → Test

The ratio should change according to the designed tap characteristics.

For an OLTC, additional tests such as dynamic resistance measurement may be required when investigating contact or switching problems.

8. Analyze the Results

• Compare the measured results with:
• Transformer nameplate data
• Factory test report
• Previous maintenance test results
• Manufacturer specifications
• Applicable standards

TTR Test vs. Winding Resistance Test

It's useful to perform both tests because they identify different problems: