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Home / Technical Articles / Do Not Energize Oil-Filled Transformer Without Performing These 15 Tests and Checks!

Obligatory Checklist

The following tests and checks should be performed at a minimum to ensure that the transformer is ready to be energized. Do not energize the transformer without performing these tests and checks.

Do Not Energize Oil-Filled Transformer Without Performing These 15 Tests and Checks!
Do Not Energize Oil-Filled Transformer Without Performing These 15 Tests and Checks! (on photo 3-core type oil-immersed rectifier transformer 380kV; via

If any of below explained tests fail and transformer is energized anyway, it may lead to the serious hazard, and that’s not good ;) That’s not what we want.

Let’s see these 15 tests…

1. Insulating Fluid Test

Draw a fluid sample and test its dielectric strength. Dielectric strength of new fluid should be 26 kv or greater.

How well insulating oil can withstand an electrical load
The breakdown voltage indicates how well insulating oil can withstand an electrical load and is therefore decisive for the operational efficiency of a transformer. The breakdown voltage is measured according to VDE 0370 part 5 (IEC 60156); photo credit: SIEMENS

2. Pressure Test

Check the transformer tank for leaks by pressurizing the tank with dry air or dry nitrogen through the pressure test fitting to a pressure of 3 to 4 PSIG. let the tank stand under pressure for one to two hours, then examine the tank and fittings for leaks. leaks above the fluid level can be detected by applying soap solution to all welds, joints, pipe fittings, and cable connections.

Upon completion of this test, reduce the internal pressure to 1 or 2 PSIG.

Repairing transformer oil-tank for leakage
Repairing transformer oil-tank for leakage (photo credit:

3. Insulation Megger Test

Perform a 1000-volt Megger test and a power factor test to ensure that none of the windings is grounded. See the complete procedure here.

Regular insulation testing is one of the most cost effective methods of identifying aging of transformer
Regular insulation testing is one of the most cost effective methods of identifying aging of transformer

4. Ratio Test

Perform a ratio test at each tap position to ensure that transformer coil ratios and tap changer connections are correct. Read the complete procedure of transformer turns ratio test step by step.

5. Continuity Test and Resistance Test

Perform a continuity check on each winding. Measure the winding resistance of each winding and compare results to factory test values.

An increase of more than 10% could indicate a loose internal connection.

6. Line Connections

In preparation for making line connections, check to make sure that all mating connector surfaces are clean and smooth. Connections must be tightened appropriately to prevent overheating and possible subsequent failure of the connection.

Connections to should be made with care to avoid placing undue stress on the bushings.

Oil-filled transformer bushings
Oil-filled transformer bushings (photo credit:

7. Tap Changer Setting

Check the tap changer setting to ensure it is set to the proper position for the required voltage. See an example of transformer tap-changer correct adjustment.

On-Line Oil Filtration for Load Tap Changer Compartment
On-Line Oil Filtration for Load Tap Changer Compartment (photo credit:

8. Delta/Wye and Series/Multiple Switch Settings

Check delta/wye and series/multiple switch settings to make sure they are set correctly.

If these connections are made using an internal terminal board, check to ensure that these connections are made properly according to the chart on the transformer nameplate.

GE distribution transformer nameplate
GE distribution transformer nameplate (photo credit:

9. Grounding

Check to ensure that the transformer tank is permanently and effectively grounded. The transformer tank ground pad is located near the bottom of the tank.

10. Wiring

Check wiring of control and alarm circuits (if provided) to make sure there are no loose connections and no damage to insulation.

Oil-filled transformer terminal box
Oil-filled transformer terminal box

11. Fluid level

Liquid Level Gauge
Figure 1 – Liquid Level Gauge

Check to make sure the fluid level as indicated by the fluid level gauge is as follows:

The fluid level gauge indicates the fluid level inside the tank. The transformer tank is filled with fluid at the factory to the normal level at a reference temperature of 25°C.

12. Tank Finish

Check all painted surfaces to make sure that there is no damage or corrosion.

13. Bolted Connections

Check all bolted connections for tightness.

Oil transformer bolted connections
Oil transformer bolted connections (photo credit:

14. Tools

Check to make sure that all tools and equipment are accounted for and have been removed from the transformer.

15. Fluid Temperature

Read the fluid temperature gauge and make sure the temperature is no lower than minus 20°C before the unit is energized.

Liquid Temperature gauge
Figure 2 – Liquid Temperature gauge

Reference: Installation, Operation, and Maintenance of Medium Power Substation Transformers – Howard Industries

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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. Tom Tumun
    Oct 24, 2019

    How do I pressurizing the tank with dry air or dry nitrogen through the pressure test fitting?
    any procedures?

  2. A L Yusuf
    Oct 17, 2019

    Thanks a great deal for this.

  3. Fairuz
    Sep 11, 2018

    Was the CT ratio coil being connected properly to the meter gauge. Chk the passing through cable to the CT coil

  4. Yahya Mohamed
    Jan 06, 2018

    Was 2012 I worked with al GAhaz trading company one year WITH SWEET progact THIS IS MY STORY

  5. Janga Prasad Reddy
    Jan 06, 2018

    Just I want to know about ELR. We have connected ELR With Incoming panel(800A). That ELR is getting Trip within Seconds. Equipments are not getting any Earth Leakages. No shock effect from equipments… I am not able to Understand what is exact problem..Pls Let me know Reason.

    Prasad Reddy

  6. Ahmed Sobih
    Aug 31, 2017

    Very thanks for this article

  7. Raphael kongolo
    Aug 02, 2016

    I’m very glad about this, but if i want know more, how to discussaed with you as electrical engineer?

  8. Manish Kohli
    Aug 30, 2015

    I am a protection engineer and I carry out all such tests . I just want to know that why magnetic balance test of 160MVA tfr is diferrent from < = 100 MVA tfr.

  9. Amitava Nag
    Aug 30, 2015

    Good information for an executive.

  10. yogaamurthy
    Aug 30, 2015

    Very very important. Articles for sub station maintenance. Great thank u

  11. Pasha
    Jun 02, 2015

    Thanks for such a informative article

  12. kulsekar
    May 31, 2015

    nice information

  13. Fadi
    May 11, 2015

    Thanks a lot for such a useful article. I have a question related to the resistance test where i have to read through the Megger. Is it right to compare with short circuit test and open circuit test which will provide me the magnetization Resistance and core loss resistance??

    I will aprreciate answering my question

    • Moeez
      Jun 02, 2015

      For Comparison, this can be used and usually is

  14. Nikolay
    Apr 25, 2015

    Also need to check if:
    – Radiator valves are open
    – Cooling fan motors rotate in the correct direction
    – There are no visible oil leaks
    – Off-load tap changer handle has been locked

  15. Deep
    Apr 14, 2015

    I believe that there are other important test like core balance, short circuit etc

    • Moeez
      Jun 02, 2015

      Exactly. But for Beginners this is quite helpful for getting the idea of testing involved in the process.

  16. Mhmoud refaat
    Apr 14, 2015

    Many thanks

  17. ASIM
    Apr 13, 2015

    thank you Mr.Edvard ,, very nice one but i think there is other tests to be added here like the polarization index and tan delta test.

  18. Abdullah Gwani
    Mar 25, 2015

    Thank you for your articles are so rich.

  19. muthamil Selvan
    Mar 23, 2015

    Its useful data.Thanks

  20. Francisco Alvarez
    Mar 14, 2015

    Amazing post, brother. Thanks.


  21. Balamurugan
    Mar 13, 2015

    Good app it is..I am Electrical Engineer who wants to improve the knowledge day by day..Thanks for this great app…Mr.Edvard..

    Mar 01, 2015

    many thanks sir

  23. jesus perez
    Feb 27, 2015

    Excellent article. You must add something about funcional test. For example test of ventilation system. It’s my suggestion.

  24. Mel Wright
    Feb 26, 2015

    Please remove my photo, the first photo (DGA Syringe), from your publication. It is copyrighted and used without my permission.
    Thank you.

    • Edvard
      Feb 26, 2015

      My apologize Mel. Your photo has been removed.

  25. Naresh Kinger
    Feb 25, 2015

    Very good article. Air cell in conservative to be included in final checking list.

    Feb 25, 2015

    Its helpful

    Feb 24, 2015

    Your article is very nice and useful..
    Thanks for every….

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