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Home / Technical Articles / 8 Recommended Periodic Inspections of a Substation Transformer

Medium power transformer routine maintenance

The following periodic tests and inspections are recommended for routine maintenance of the medium power substation transformer.

8 Recommended Periodic Inspections of a Substation Transformer
8 Recommended Periodic Inspections of a Substation Transformer (photo credit: gridsense.com)
  1. Gauge readings
  2. Cooling fans
  3. Control wiring
  4. Paint finish
  5. Fluid dielectric test
  6. Bushing and surge arrester insulators
  7. Bushing terminals
  8. Gaskets

1. Gauge readings

One month after initial energization and annually thereafter

Gauge readings, ambient temperature, and kvA load should be measured and recorded. Any abnormal reading suggests that further diagnostic testing or inspection should be done. If pressure/vacuum gauge and/or fluid level gauge readings suggest a possible tank leak, perform a pressure test according to instructions (15 Pre-energization Tests and Checklist).

Tank leaks must be repaired immediately to prevent serious damage to the transformer and danger to life.

Transformer winding and liquid temperature measuring
Transformer winding and liquid temperature measuring (photo credit: acontact.com)

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2. Cooling fans (annual)

Check the cooling fans (if any) by setting the fan “auto/manual” control switch to the “manual” position. The fans should rotate at full speed within approximately five seconds. The fans should rotate smoothly with minimal vibration.

Transformer cooling fans
Transformer cooling fans (photo credit: vostermansusa.com)

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3. Control wiring (annual)

Control wiring should be checked to insure that wire insulation is in good condition. The control cabinet and associated conduit should be inspected to ensure that weather seals are intact.

Control power supply voltage should be checked and compared to the voltage stated on the wiring diagram.

Transformer control wiring box to check
Transformer control wiring box to check (photo credit: canammachinery.com)

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4. Paint finish (annual)

Inspect the paint finish for damage or weathering that exposes the primer coat or bare metal. Repair any paint damage that might be found.

Transformer paint damage to check
Transformer paint damage to check (photo credit: railgallery.wongm.com)

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5. Fluid dielectric test (annual)

Sample the insulating fluid as described below. The dielectric strength of the insulating fluid should measure at least 26 kV.

Sampling of Insulating Fluid //

Transformers are filled with insulating fluid, which provides electrical insulation within the transformer tank and transfers heat generated in the coils to the tank wall and radiators.

The fluid is either:

  • Conventional transformer oil (mineral oil),
  • Envirotemp® FR3 fluid, or
  • Silicone fluid.
Periodically check the transformer for proper fluid level by reading the fluid level gauge. Add fluid if necessary. When adding fluid, add only the same type fluid that is in the transformer.

It is also recommended that a fluid sample be drawn annually and tested for dielectric strength. Samples should be drawn from the bottom of the tank. Use proper sampling procedures to prevent erroneous test results. Dielectric strength should measure 26 kv minimum.

Dielectric strengther / transformer oil tester
Dielectric strengther / transformer oil tester (photo credit: oil-filtration-engineering.com)

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6. Bushing and surge arrester insulators (annual)

Bushing and surge arrester insulators should be clean. If the surfaces are excessively dirty, they should be cleaned while the transformer is not energized.

Arcing horns on transformer bushings - for lightning protection
Arcing horns on transformer bushings – for lightning protection (photo credit: Wikipedia)

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7. Bushing terminals

One month after initial energization and annually thereafter

If the transformer is energized and under load, measure bushing terminal temperatures using an infrared scanner. Excessive bushing terminal temperature indicates a loose or dirty connection. If the transformer is not energized, use a torque measuring device to make sure terminal connections are tight.

Infrared (IR) scan of a transformer bushings
Infrared (IR) scan of a transformer bushings (photo credit: npm-services.com)

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8. Check the gaskets

Visually check all gaskets for cracking or other signs of deterioration. Replace as necessary.

When replacing a gasket carefully clean mating surfaces to remove any rust, dirt, transformer fluid, old gasket material, or other contamination that might prevent a good seal. Use an appropriate gasket cement when installing new gaskets.

Do not reuse old gaskets. Six months after replacing a gasket, check and retighten if necessary.

Transformer seal gaskets to check for for cracking or other signs of deterioration
Transformer seal gaskets to check for for cracking or other signs of deterioration (photo credit: powermag.com)

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Reference // Instructions Installation, Operation, and Maintenance of Medium Power Substation Transformers – Howard Industries // Substation Transformer Division

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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.

17 Comments


  1. Abdul
    Oct 16, 2021

    Please send to me softcopy


  2. khamis
    Oct 22, 2019

    i appreciate your efforts , many thanks for you.

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