Transformer exterior inspection and tests you MUST perform
Transformer exterior inspection and tests you MUST perform (photo credit:

Transformer exterior inspection

Periodic transformer exterior inspection (visual) reveals important condition information. For example, valves positioned incorrectly, plugged radiators, stuck temperature indicator and level gauges, and noisy oil pumps or fans. Oil leaks can often be seen which indicate a potential for oil contamination, loss of insulation, or environmental problems.

Physical inspection requires staff experienced in these techniques.

  1. To check:
    1. Oil leaks
    2. Oil pumps
    3. Fans and radiators
    4. Age
  2. To perform:
    1. Infrared temperature analysis
    2. Corona scope scan
  3. Other also important inspections

To check //

Oil leaks

Check the entire transformer for oil leaks. Leaks develop due to gaskets wearing out, ultraviolet exposure, taking a “set,” or from expansion and contraction, especially after transformers have cooled, due to thermal shrinkage of gaskets and flanges.

Many leaks can be repaired by applying an epoxy or other patch. Flange leaks may be stopped with these methods, using rubberized epoxy forced into the flange under pressure.

Very small leaks in welds and tanks may be stopped by peening with a ball-peen hammer, cleaning with the proper solvent, and applying a “patch” of the correct epoxy.

Experienced leak mitigation contractors whose work is guaranteed may also be employed.

Some leaks may have to be welded. Welding may be performed with oil in the transformer if an experienced, qualified, and knowledgeable welder is available. If welding with oil in the tank is the method chosen, oil samples must be taken for DGA, both before and after welding.

Welding may cause gases to appear in the DGA and it must be determined what gases are attributed to welding and which ones to transformer operation.

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Oil pumps

If the transformer has oil pumps, check the flow indicators and pump isolation valves to ensure that oil is circulating properly.

Pump motor(s) may also have reversed rotation, and flow indicators may still show that oil is flowing. To ensure motors are turning in the proper direction, use an ammeter to check the motor current. Compare results with the full-load-current indicated on the motor nameplate. If the motor is reversed, the current will be much less than the nameplate full-load-current.

Check oil pumps with a vibration analyzer if they develop unusual noises. Have the DGA lab check for dissolved metals in the oil and run a metal particle count for metals, if the bearings are suspect. This should be performed as soon as a bearing becomes suspect.

Ultrasonic inspection of oil pump bearings
Ultrasonic inspection of oil pump bearings

Bad oil pump bearings can put enough metal particles into the oil to threaten transformer insulation and cause flashover inside the tank, resulting in an explosive catastrophic failure of the transformer tank.

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Fans and radiators

Inspect all isolation valves at the tops and bottoms of radiators to ensure they are open. Inspect cooling fans and radiators for cleanliness and fans for proper rotation. Check for dirty or damaged fan blades or partially blocked radiators.

Fans are much more efficient if the blades are clean and rotating in cool air. Normally, fans blow cool air through the radiators and they should not be pulling air through. Check to see if fans are reversed electrically (i.e., pulling air first through the radiators and then through the fan blades).

This means that the blades are rotating in warm air after it passes through the radiator, which is much less efficient. Place a hand on the radiator opposite the fans. Air should be coming out of the radiator against your hand. Watch the blades as they rotate slowly when they are starting or stopping to determine which way they should be rotating and correct the rotation if necessary.

See IEEE 62-1995 for details. Also inspect radiators and fans with an infrared camera, see section Infrared temperature analysis.

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Transformer age is an important factor to consider when identifying candidates for replacement or rehabilitation. Age is one indicator of remaining life and upgrade potential to current state-of-the art materials. During transformer life, structural strength and insulating properties of materials used for support and electrical insulation (especially paper) deteriorate.

Although actual service life varies widely depending on the manufacturer, design, quality of assembly, materials used, maintenance, and operating conditions, the expected life of a transformer is about 40 years.

Old substation transformers
Westar Energy, Inc. 69-kV (unknown capacity MVA) single phase transformers, play an important role in this Wichita-area switchyard. The transformers are equipped with Convervator Tanks, which was a means to allow for oil expansion and drainage under high temperature conditions. These units are not forced air, nor forced oil, however do contain cooling tubes and substantial height. Their age may be around the late 1930s. They are 69-kV/40-kV phase-to-ground. They appear to have NLTC, and do not have LTC configurations. (photo credit:

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To perform //

Infrared temperature analysis

Infrared analysis should be conducted annually while equipment is energized and under full load, if possible. IR analysis should also be conducted after any maintenance or testing to see if connections that were broken were re-made properly.

Also, if IR is performed during factory heat run, the results can be used as a baseline for later comparison.

Infrared for transformer tanks

Unusually high external temperatures or unusual thermal patterns of transformer tanks indicate problems inside the transformer, such as low oil level, circulating stray currents, blocked cooling, loose shields, tap changer problems, etc. Infrared scanning and analysis is required annually for trending purposes by National Fire Protection Association 70B,

Abnormally high temperatures can damage or destroy transformer insulation and, thus, reduce life expectancy. Thermal patterns of transformer tanks and radiators should be cooler at the bottom and gradually warmer ascending to the top.

See figure 1 for a normal pattern; the red spot at the top is normal showing a “hot spot” top of B phase, about 110 degrees Fahrenheit (°F). Any departure from this pattern means a probable problem which must be investigated. An IR inspection can find over-heating conditions or incorrect thermal patterns. IR scanning and analysis requires trained staff, experienced in these techniques.

Normal transformer infrared pattern
Figure 1 – Normal transformer infrared pattern

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Infrared for surge arresters

Surge arresters should be included when infrared scanning energized transformers. Look for unusual thermal patterns on the surface of lightning arresters.

See the arrester IR image in figure 2 below. Note that the yellow in the top right of the image is a reflection not associated with the arrester. A temperature profile of the arrester is shown as black lines. Note the hot spot (yellow) about a third of the way down from the top.

This indicates that immediate de-energization and replacement must be undertaken! Catastrophic failure is imminent which can destroy nearby equipment and be hazardous to workers.

Also compare thermal patterns to sister units or earlier scans of the same arrester. Scan all high voltage connections and compare them to nearby connections for unusual temperatures.

Infrared image of defective arrester
Figure 2 – Infrared image of defective arrester

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Infrared for bushings

IR scans of bushings can show low oil levels which would call for immediate de-energization and replacement. This generally means that the seal in the bushing bottom has failed, leaking oil into the transformer. The top seal has probably also failed, allowing air and up the bushing.

Another reason a bushing can exhibit a high oil level is that the top seal is leaking – allowing water to enter. The water migrates to the bushing bottom, displacing the oil upward.

Remember, over 90% of bushing failures are attributed to water entrance through the top seal. Bushings commonly fail catastrophically, many times destroying the host transformer or breaker and nearby equipment and causing hazards to workers.

Infrared image of defective bushing
Figure 3 – Infrared image of defective bushing

Figure 4 (below) shows low-oil level in a high voltage transformer bushing. Compare previous IR scans of the same bushing with the current scan.

Doble hot-collar testing possibly may show this problem. However, Doble tests are run infrequently, and the transformer has to be out of service, under clearance, and both primary and secondary conductors removed, while an IR scan easily can be performed at any time.

Infrared for radiators and cooling systems

Examine radiators with an IR camera and compare them with each other. A cool radiator or segment indicates that a valve is closed or the radiator or segment is plugged. The IR image (figure 4) shows that the cold left radiator section is valved off or plugged.

If visual inspection shows the valves are open, the radiator or segment must be isolated, drained, removed, and the blockage cleared. Do not allow a transformer to operate with reduced cooling which drastically shortens transformer life.

Remember, an increased operating temperature of only 8 to 10 °C will reduce transformer life by one-half. IR scan all cooling systems, including heat exchangers, fans, pumps, motors, etc. Check inside control panels for overloaded wiring, loose connections, and overheated relays. Look for unusual thermal patterns and compare similar equipment.

Infrared image showing blocked radiators
Figure 4 – Infrared image showing blocked radiators

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Corona Scope Scan

With the transformer energized, scan the bushings and surge arresters and all high voltage connections for unusual corona patterns. Corona should be visible only at the top of bushings and arresters, and corona at connections should be similar to sister connections. As a bushing deteriorates due to physical defects, the corona pattern will grow progressively larger.

When the corona pattern reaches a grounded surface (i.e., the tank or structure), a flashover will occur, destroying the bushing or arrester and, perhaps, the transformer. The corona scope will reveal this problem long before a flashover.

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Other also important inspections

There are several required physical inspections that will not be covered here because they were addressed previously. Be sure to inspect the following:

  1. Winding temperature indicators
  2. Pressure relief devices
  3. Sudden pressure relay
  4. Conservator bladder
  5. Conservator breather and
  6. Bladder failure relay

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Reference // Transformers: Basics, Maintenance, and Diagnostics by U.S. Department of the Interior Bureau of Reclamation

About Author //


Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on

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