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Home / Technical Articles / 10 general guidelines for installing power transformers

Operation of power transformers

When your transformer arrives on site, various procedures should be carried out to assure successful operation (installation, testing and various checkings). The successful operation of a transformer is dependent on proper installation as well as on good design and manufacture.

Guidelines For Installing Power Transformers
Guidelines For Installing Power Transformers (photo credit:

The instructions mentioned in the manufacturer manual or in Standards MUST be followed to ensure adequate safety to personnel and equipment.

This technical article will provide 10 general guidelines for installing and testing both dry-type and liquid-filled power transformers for placement into service:

  1. Standard and special transformer tests
  2. Site considerations
  3. Preliminary inspection upon receipt of transformer
  4. Plan for the prevention of contaminants
  5. Making connections that work
  6. Controlling sound level
  7. Make sure the transformer is grounded
  8. Final inspection and testing
  9. Applying the load
  10. Adjustment for correct tap setting

1. Standard and special transformer tests

Standard transformer tests performed for each unit include the following:

  • Ratio, for voltage relationship;
  • Polarity for single- and 3-phase units (because single-phase power transformers are sometimes connected in parallel and sometimes in a 3-phase bank);
  • Phase relationship for 3-phase units (important when two or more power transformers are operated in parallel);
  • Excitation current, which relates to efficiency and verifies that core design is correct;
  • No-load core loss, which also relates to efficiency and correct core design;
  • Resistance, for calculating winding temperature
  • Impedance (via short circuit testing), which provides information needed for breaker and/or fuse sizing and interrupting rating and for coordinating relaying schemes;
  • Load loss, which again directly relates to the transformer’s efficiency;
  • Regulation, which determines voltage drop when load is applied; and
  • Applied and induced potentials, which verify dielectric strength.

There are additional tests that may be applicable, depending upon how and where the transformer will be used. The additional tests that can be conducted include the following:

  • Impulse (where lightning and switching surges are prevalent);
  • Sound (important for applications in residential and office areas and that can be used as comparison with future sound tests to reveal any core problems);
  • Temperature rise of the coils, which helps ensure that design limits will not be exceeded;
  • Corona for medium voltage (MV) and high-voltage (HV) units, which helps determine if the insulation system is functioning properly;
  • Insulation resistance (meg-ohmmeter testing), which determines dryness of insulation and is often done after delivery to serve as a benchmark for comparison against future readings; and
  • Insulation power factor, which is done at initial installation and every few years thereafter to help determine the aging process of the insulation.
Download Guidelines to Power Transformer Type, Routine and Special Tests //

Download Guidelines

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2. Site considerations

When planning the installation, the location is selected, that complies with all safety codes yet does not interfere with the normal movement of personnel, equipment, and material. The location should not expose the transformer to possible damage from cranes, trucks, or moving equipment.

A powerful crane and worker dexterity
A powerful crane and worker dexterity – that’s what is needed to precisely position the 87-ton power transformer onto the supports that span the gravel-covered pit (credit:

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3. Preliminary inspection upon receipt of transformer

When received, a transformer should be inspected for damage during shipment. Examination should be made before removing it from the railroad car or truck, and, if any damage is evident or any indication of rough handling is visible, a claim should be filed with the carrier at once and the manufacturer notified.

Subsequently, covers or panels should be removed and an internal inspection should be made for damage or displacement of parts, loose or broken connections, dirt or foreign material, and for the presence of water or moisture.

Important note // If the transformer is moved or if it is stored before installation, this inspection should be repeated before placing the transformer in service!

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4. Plan for the prevention of contaminants

Develop a procedure for inventory of all tools, hardware, and any other objects used in the inspection, assembly, and testing of the transformer. A check sheet should be used to record all items, and verification should be made that these items have been properly accounted for upon completion of work.

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5. Making connections that work

The connections shall be made, between the transformer’s terminals and the incoming and outgoing conductors, carefully following the instructions given on the nameplate or on the connection diagram. Check all of the tap jumpers for proper location and for tightness.

Re-tighten all cable retaining bolts after the first 30 days of service.

Before working on the connections make sure all safety precautions have been taken. Arrangements shall be made to adequately support the incoming/outgoing connecting cables, so that there is no mechanical stress imposed on transformer bushings and connections. Such stress could cause a bushing to crack or a connection to fail.

Solar power transformer and cable connections
Solar power transformer and cable connections (credit:

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6. Controlling sound level

All power transformers, when energized, produce an audible noise. Although there are no moving parts in a transformer, the core does generate sound. In the presence of a magnetic field, the core laminations elongate and contract. These periodic mechanical movements create sound vibrations with a fundamental frequency of 120 Hz and harmonics derivatives of this fundamental.

The location of a transformer relates directly to how noticeable its sound level appears.

For example, if the transformer is installed in a quiet hallway, a definite hum will be noticed. If the unit is installed in a location it shares with other equipment such as motors, pumps, or compressors, the transformer hum will go unnoticed. Some applications require a reduced sound level, such as a large unit in a commercial building with people working close to it.

Occasionally, the installation of some method of sound abatement will be called for.

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7. Make sure the transformer is grounded

Grounding is necessary to remove static charges that may accumulate and also is needed as a protection should the transformer windings accidentally come in contact with the core or enclosure (or tank for wet types). Note that for MV power transformers, the secondary neutral is sometimes grounded through an impedance.

Ensure that all grounding or bonding systems meet NEC and local codes.

Grounding of a 45KVA step-down transformer enclosure
Grounding of a 45KVA step-down transformer enclosure to remove static charges (credit:

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8. Final inspection and testing

Once the transformer has been located on its permanent site, a thorough final inspection should be made before any assembly is accomplished and the unit is energized. Before energizing the unit, it’s very important that all personnel installing the transformer are alerted, that lethal voltages will be present inside the transformer enclosure as well as at all connection points.

The installation of conductors should be performed only by personnel qualified and experienced in high-voltage equipment. Personnel should be instructed that should any service work be required to the unit, the lines that power the transformer must be opened and appropriate safety locks and tags applied.

A careful examination should be made to ensure that all electrical connections have been properly carried out and that the correct ratio exists between the low and high-voltage windings. For this test, apply a low-voltage (240V or 480V) to the high-voltage winding and measure the output at the low-voltage winding.

However, for low-voltage (600V and below) transformers, this is not practical! Here, a transformer turns ratio indicator should be used to measure the ratio. Any control circuits, if any, should be checked to make sure they function correctly. These include the operation of fans, motors, thermal relays, and other auxiliary devices.

Correct fan rotation should be visually verified as well as by checking indicator lights if they are installed. Also, a one-minute, 1200V insulation resistance test of the control circuits shall be done. (If the power transformer has CT circuits, they should be closed.)

All windings should be checked for continuity. An insulation resistance test shall be carried out to make certain that no windings are grounded.

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9. Applying the load

Before energizing a 3-phase transformer, arrangement for monitoring the voltages and currents on the low-voltage side shall be done. Then, without connecting the load, energize the transformer. The magnitude of the voltages shown (line-to-ground and line-to-line) should be very similar. If this is not the case, de-energize the transformer and contact the manufacturer before proceeding further.

Next, connect the load and energize the transformer. While monitoring the voltages and currents, gradually increase the load in a stepped or gradual application until full load is reached. Both the voltages and currents should change in a similar fashion. If this does not happen, de-energize the transformer and contact the manufacturer.

The maximum continuous load a transformer can handle is indicated on its nameplate.

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10. Adjustment for correct tap setting

After installation, check the output voltage of the transformer. This should be done at some safe access point near or at the load.

WARNING! Never attempt to check the output voltage at the transformer. Dangerous high voltage will be present within the transformer enclosure.

When changing taps, the same changes must be made for all phases. Consult the transformer diagrammatic nameplate for information on what tap must be used to correct for extra high or extra low incoming line voltage. The same adjustment should be made to compensate for voltage drop in the output due to long cable runs.

When the load-side voltage is low, tap connections below 100% of line voltage must be used to raise the load voltage.

If the load-side voltage is high, tap connections above 100% of line voltage must be used to lower the load voltage.

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Reference // Best practice manual for power transformers

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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.


    Aug 01, 2020

    good ARTICLE

  2. li kewen
    Feb 29, 2020

    hello,could i have the elecrical installation stardard for the 220kV suabstation,thank you!

  3. David
    Feb 25, 2020

    Good article

  4. aravind
    Nov 30, 2019

    If power transformer installed on the uneven surface in the foundation. what will happen?

  5. Albert.Hure
    Oct 16, 2019


    Could you kindly sent me a PDF file on transformers and electrical isolation.(SOP)

    Kind Regards,

  6. Abumbe Francis Abuga
    Aug 28, 2019

    Please can I have a step by step PDF file of how to install an Electrical Distribution power transformer sent to my mail.

  7. Miguel Ángel
    Aug 01, 2019

    Can you help me? I have a doubt, we have a new 165MW Power Transformer (132kV/30kV) in a photovoltaic plant, because of delays of construction of the Substation that will feed the transformer, they want to make the first energization of the Power Transformer from the secundary, taking the voltaje from the photovoltaic plant, is this posible? (energize the transformer without voltaje on the line, feeding from the secundary), and if they decide this (I think is an error do it) the transformer will suffer, can be damage?

  8. kapil
    Jun 11, 2019

    For installing Power transformer, what is standard dimensions of Oil soak pit

  9. Anil Paul Jacob
    Jun 08, 2019

    Dear ED., Very informatic presentation. Very good for the designing staffs as well as for the project teams

  10. Edis
    Dec 26, 2018

    Thanks for sharing practical knowledge…
    I have questions for the author…

    1-Then, without connecting the load, energize the transformer. The magnitude of the voltages shown (line-to-ground and line-to-line) should be very similar.

    -I am wondering how can values of line-line and line-ground voltage be the same (where is x1,73 difference)?

    -second questions: how can the load be increasing in steps from the low value to nominal power?

  11. PowerLite Electricals
    Dec 12, 2018

    Thanks for sharing, keep it up the good work.

  12. Malia Davis
    Aug 22, 2018

    I never knew it was important to ground the transformers to remove static charges. I’m so glad I found this article so that I know before looking any further into power transformers. I’ve been trying to do my research, but sometimes all of this is hard to understand for someone like me with little experience in electrical engineering. It might be good to just look at hiring a professional to handle everything for me so I don’t forget important details like making sure the transformer is grounded.

  13. Arif
    Apr 13, 2017

    Keep it up. Good work.Thanks for the article.

  14. Jin Sim
    May 18, 2016

    Edvard, please don’t be discouraged. You do a great service to the industry by providing variety of technical information people find very interesting. What readers do with these information is up to them. Keep up the good work.

    • Edvard
      May 18, 2016

      Thank you Sim! I just don’t like this kind of advertising…

  15. Gordon N
    Feb 11, 2016

    Remind me never to have the author of this article install a transformer for me….

    FAT: These tests are determined by the standard that the transformer is being manufactured to. For type tests, all transformers will have heat run rests, impedance tests, usually Partial Discharge tests, Applied Voltage, resistances during field collapse (this is not for WTI calibration!!! FFS) plus many more such as Sweep Frequency Response, C1 & C2 (Tan Delta) testing of bushings

    SAT: At a minimum for a new power TX – SFRA, Excitation, Power Factor, C1, C2, leakage Reactance, Ratio, resistances and Insulation resistance.

    I am left scratching my head when garbage such as this is published and people take it as a procedure for future works

    • Edvard
      Feb 11, 2016

      Yes, Gordon you can divide these tests into FAT and SAT, that’s true, but why to emphasize this? This is just the list of guidelines in short.

      Are you advertising your company? And ‘garbage’? Are you trying to do some conversation or you’re just spitting on article and author?

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