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Home / Technical Articles / How to perform a power transformer turns ratio test

Turns ratio between the windings

Power transformer turns ratio test is an AC low voltage test which determines the ratio of the high voltage winding to all other windings at no-load. The turns ratio test is performed on all taps of every winding.

Power Transformer Turns Ratio Test
Power Transformer Turns Ratio Test

The Transformer Turns Ratio tester (TTR) is device used to measure the turns ratio between the windings (example shown below).

Voltage is applied on the H marked leads and measured of the X marked lead by the test set.

Ratio measurements are conducted on all tap positions and calculated by dividing the induced voltage reading into the applied voltage value. When ratio tests are being made on three-phase transformers, the ratio is taken on one phase at a time with a three-phase TTR until the ratio measurements of all three phases are completed.

Figure 1 - Three-phase Transformer Turns Rati (TTR) Connection diagram.
Figure 1 – Three-phase Transformer Turns Rati (TTR) Connection diagram.

Measured ratio variations should be within 0.5% of the nameplate markings.

Some TTR can perform transformer ratio measurement and also assess if on-load tap changer contacts are making satisfactorily during its transition from one tap position to the next position.

Example of single phase, hand-cranked TTR - Transformer Turns Ratio Test Set
Example of single phase, hand-cranked TTR – Transformer Turns Ratio Test Set (Measures the turns ratio and exciting current of windings in power, potential and current transformers.)

Turns Ratio Test Procedure, Step by Step

Step 1.

Isolate the equipment, apply working grounds to all incoming and outgoing cables and disconnect all incoming and outgoing cables from the transformer bushing terminals connections.

Disconnected cables should have sufficient clearance from the switchgear terminals greater that the phase spacing distance. Use nylon rope to hold cable away from incoming and outgoing terminals as required.

Step 2.

Connect the H designated three-phase test lead with the military style connector at one end to the mating connection on the test set marked with an H. Ensure that the connector’s index notch lines up properly.

Step 3.

Connect the X designated three-phase test of lead military style connector at one end to the mating connection on the test set marked with an X. Ensure that the connector’s index notch lines up properly.

Step 4.

Connect the H1, H2, H3 designated test lead to the corresponding H1, H2, H3 transformer terminal / bushing. Connect the H0 test lead if H0 terminal/bushing is present.

Refer to Figure 1.

Step 5.

Connect the X1, X2, X3 designated test leads to the corresponding X1, X2,X3 transformer terminals / bushings. Connect the X0 test lead if X0 terminal/bushing is present.

Step 6.

Perform turns ratio measurements for all tap positions.

Step 7.

Confirm that the measured ratios is within 0.5% of the calculated ratios.

Important Note:

Transformers that have wye connections but do not have the neutral of the wye brought out shall be tested for ratio with three-phase power supply.

Any inequality in the magnetizing characteristics of the three phases will then result in a shift of the neutral and thereby cause unequal phase voltages. When such inequality is found, the connection should be changed, either to a delta or to a wye connection, and the line voltages measured.

When these are found to be equal to each other and the proper values (1.732 times the phase voltages when connected in wye), the ratio is correct.

Power Transformer Testing

Automatically measuring ratio and winding resistance of all taps/phases.

Reference: Substation Commissioning Course – Raymond Lee, Technical Trainer

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More Information

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


  1. Udigwe Emmanuel chidi
    Feb 27, 2020

    Sir please,i need to learn more about ratio test and capacity testing. I get confused sometimes place hope me out. Thanks

  2. Seipribo Tom-Davies
    Dec 10, 2019

    Thanks for your brilliant explanations sir. please sir i need your help, i work with a transformer manufacturing industry here in Nigeria. i have been taxed to carryout an r&d on transformer insulation degradation using partial discharge effect on the insulation. please i need your assistance in this.

    Kind Regards

  3. AMINU A.Y
    Nov 02, 2019

    this site is really very educating keep it up

  4. Reginald 'toba ADEEYO
    Aug 13, 2019

    Thanks for educating session on this all important portal. I really fascinated by your in-dept analysis.

  5. Zia Ullah
    Oct 12, 2017

    Salaam to all..
    Can any one clear plz that if there is difference b/w the nameplate value and tested value of TTR then what fault should b in Transformer??

  6. arshad
    Aug 28, 2017

    I am electrical technician in a workshop deals with transformer repairing and overhauling. i need to learn transformer testing techniques through your discussions which are very fruitfull

  7. Samir Arikat
    Oct 26, 2016

    Very nice. Thank you

  8. shubham sachan
    May 17, 2016

    difference between line turn ratio and phase turn ratio of 3-phase transformer.

  9. mohamed mohamed
    Feb 18, 2015

    Request for how to perform timing test and contact resistance test

  10. Shaukat Ali Khan
    Sep 04, 2014

    You must be using portable transformer turns ratio (TTR) meter. I am used to applying 3-phase 415 V generator to the HV side and then measuring the induced V on the LV…. Can you explain the second method and possibly post the equipment setup diagram? I do have it but you know…Hahahaha…

    • S Arjuna Rao
      Mar 13, 2015

      Could you pls clarify the following..

      What will be the voltage ratio or turns ratio

      Yzn11, 100 kVA, 20000/400 V zigzag transformer

  11. ali hama
    Aug 09, 2014


  12. kanchi
    Apr 21, 2014

    Hai sir .
    I need to know about the load cells

  13. azm
    Jan 27, 2014

    Standard is IEC 60076

  14. Andrés Norberto Landini
    Jan 19, 2014

    Please, Could You tell me what standard describes this test?

  15. Shankar Udadange
    Jan 19, 2014


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