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Home / Technical Articles / Testing Procedures for HV Voltage Transformers
Testing Procedures for HV Voltage Transformers
Testing Procedures for HV Voltage Transformers (photo credit:


  1. Equipment required for testing
  2. General inspection
  3. Insulation Resistance Test
  4. Polarity Test
  5. Transformer Turns ratio test

1. Equipment required

Following equipment is necessary to perform testings:

  • Polarity test kit
  • Megger 500-5000V
  • Ohmmeter
  • Multimeter
  • Autotransformers & Step-up transformers

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2. General inspection

Mechanical checks

  • General visual inspection and compliance with the drawings and manuals.
  • Check nameplate ratings and HV, LV terminal markings.
  • Check that all parts of the transformer are properly assembled and tight.
  • Check the HV connections are tight.
  • Check the cable connections on the LV side and the markings.
  • Check the oil levels and inspect for leakage. (Where applicable)

Capacitor dividers type

Check that all parts of the transformers are properly assembled.

Electromagnetic type

Check the installation of different sections.

Electrical Checks

  • Check the equipment grounding (Continuity and connection)
  • Check the fuse rating of secondary side.
  • Perform the operation described in the following

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Insulation Resistance Test

To obtain values as close as possible to the manufacturer’s specifications the insulators must be very clean. Select the megger range corresponding to the ratings of the equipment under test.

For Primary side, apply voltage depending on rating of voltage rating of VT.

  • For 6.6 kV VT (example), apply 2.5 kV and
  • For 132 kV VT (example), apply 5.0 kV.
Figure - Measurement between primary and secondary
Figure – Measurement between primary and secondary

Figure - Measurement between primary and ground
Figure – Measurement between primary and ground

Figure - Measurement between secondaries and between secondary and ground
Figure – Measurement between secondaries and between secondary and ground

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Polarity Test

The polarity is checked using the flick method (application of direct current) and check of deflection on a bi-directional milliammeter. The test is also used to check primary and secondary circuit continuity.

  • When switch k is closed, the milliammeter pointer deflects positive.
  • When the circuit is opened, the milliammeter pointer deflects in the negative direction.
Figure - VT Polarity test
Figure – VT Polarity test

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Transformer Turns ratio test

A variable AC source is applied on the primary side. The primary and secondary voltages are measured to determine the ratio V2/V1

Figure - Transformer Turns ratio test
Figure – Transformer Turns ratio test

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132kv Substation

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Reference: Contract specific procedure for testing of electrical equipment and 132kV OHL

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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.


    Nov 02, 2017


  2. Francois
    Jan 22, 2017

    This presenation is good but not all the info is 100%.
    For instance the identification of the VT Primary and secondary connections is wrong, Primary connections are labelled in Capital letters as A and N and secondary as a and n, your info might confuse first time students who wil perform these test.
    Also when you perform the insulation tests you remove the Primary N connection from earth when you test from Primary to earth and dont connnect it to the secondary as well because you want individual readings for each test performed to prove that each core is insulated on its own accordingly.
    the different secondary cores have different purposes and different VA’s , like for Metering and Protection purposes.
    When you test a CTPT unit it has a earth link in the secondary connection box that you have to remove before you start with the insulation tests.
    For the Polarity test 9 V or 12 V battery will be enough just make sure that the amp or Volt meter measures in mili amps( Volt), because due to the VT ratio you will get a miliamp or milivolt output.Use any quality galvano meter with a DC mili amp or volt selection to make your test easier.

    For the ratio test you can at least inject from 500-1000 V across the primary connections and measure output on secondary, these were tests performed on 11 kV to 132kV VTS the error was still within the spec’s allowed.But the Accuracy class was then from 0.2Cl to 0.5Cl.

    Long before the CT analyser or CPC 100 Voltano 100 Sverker 750/760 or 950 was build we used a self build injection set for mag curves that could inject max 1000 V and up to 5 amps that we used for Metering and Protection CT’s to do the mag curves and we used it for VT ratio test because it had enough VA to perform the tests.
    Edvard sorry hat I used your blog here but in the Electrical field we share our info because and help others that are new to this field ours.

  3. Vilas
    Jul 14, 2016

    Dear Sir,
    In the diag given for PT IR Measurement Sec- E Dont u think the sec S-2/S-2 Should be shorted & one end of the Pri i.e either P1 or P2 should be earthed & Megger should be connected between Shorted S-2 & earth ?

  4. mohammad hussaini
    Jan 13, 2016

    Thanks for sharing this useful article,

    but it could be better if someone could also refer to respective IEC article about details of test, for example in ratio test of PT, how much should be primary connected voltage to PT primary winding, because error percentage is different in different levels of voltage, and IEC as a comprehensive reference for sure pointed that out, if any one has such IEC respective article, it would be appriciated in advance.for sharing it

  5. Obafisoye J.O.
    Oct 21, 2015

    The information is quite good. Thanks a lot.

  6. Avnish Dubey
    Jul 18, 2015

    isn’t any concept of knee point in voltage transformer ??

  7. Capt.Waris Shaheen
    May 08, 2015

    Thanks for the helpful tips on the subject and service to the community.

    Feb 25, 2015

    So many thanks ,sir

  9. John Lovel
    May 15, 2014

    Edvard you r doin g8t job and appreciate from bottom of my heart. thanks alot

  10. John Lovel
    May 15, 2014

    Really hats up..

  11. prudhvi
    May 02, 2014

    Nice site author we are glad to u and thanx a lot

  12. Hardeep Singh Rahi
    Apr 21, 2014

    Helping Topic

  13. shahed
    Mar 26, 2014

    Dear Mr.Edvard,
    what is the votage rating of DC battery for polarity test and ratings of Auto transformer and Step down Transformer for Ratio test. Please replay as soon as.

    With Rgds

  14. Inigo Vaz
    Mar 25, 2014

    Thank you, that was interesting

  15. hoss
    Mar 22, 2014

    Thank you Mr. Edvard.

  16. kennedy mwanza
    Mar 05, 2014

    Where can i download the technical documents

    • Edvard
      Mar 05, 2014

      See the blue button ‘Get PDF’ on your left? That’s your answer! Create PDF from any technical article and save it on your disc.

  17. Alok
    Mar 04, 2014

    Topic is good but the contained matter is too short…explain with details & include balancing of transformers also…

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