Maintenance by Walking around

How effectively can you monitor and maintain a transformer by walking around it? How often should you walk around your transformer for inspection? What are the visual inspection items during such a ‘walk the maintenance’? What are the further actions to be taken? Read on…

Periodic inspection of power transformers and their accessories contributes to their trouble-free operation. These inspections may identify nascent problems before they become serious enough to cause equipment outages.

Sound or Transformer Noise

Station staff who regularly inspect the transformer can easily recognise any change in noise or sound from the transformer or accessories. Such change should be investigated for the root cause, and mitigation measures shall be taken if required. Transformer noise can increase under overexcitation (over-fluxing) conditions. That will happen when the rated V/ f (voltage /frequency) ratio increases when the incoming voltage is high or the grid frequency is low.

Oil Level

The table shows that oil levels in the main tank, conservator, and accessories are to be checked. When an air cell is provided, the healthiness of the air cell (from rupturing) and the system’s effectiveness are ensured by confirming that the oil sight window in the tank conservator is always full and the air cell leak detector is not showing an alarm indication. Remember that the outside of the air cell in the conservator is under vacuum and shall always be full of oil.

Oil Leak

It should be appreciated that oil leaks result not only in oil loss and tarnishing of the painting but also in water seeping inside the tank through the leaking joints during rains. The result is excess water in oil and wetting of paper insulation. Chances of insulation breakdown and excessive ageing of paper insulation are high with wet transformer insulation.


Bushings are the most vulnerable accessory in transformers, and nearly 25% of all reported transformer failures are from this component. Bushings up to 36 kV class are plain porcelain type; above that, voltage level condenser bushings (OIP-oil-impregnated paper type or RIP resin-impregnated paper type) are used. Any oil leak shall be attended to immediately, considering the low oil volume inside the bushing. If the condenser core gets exposed to air, it can lead to failure and the explosion of bushings. Check if any pull or push comes to the bushing terminal pad from external jumper connections. This pull may cause oil leaks and porcelain cracks in bushings, leading to bushing failure. Look at the terminal pad for any overheating marks (by the colour change or by infrared thermometer). Overheating can cause damage to nearby oil seals of the bushing, resulting in water entry to the bushing during rains.

Protective Accessories

Watch for any alarm flag indications in the Pressure Relief Device (PRD), air cell rupture relay, and Buchholz relay. Note the temperature indications in thermometers and maximum temperatures attained during the inspection period. Oil and winding temperatures above 80/100 deg.C shall be monitored closely. After recording, the peak indicators are to be reset.

The silica gel breathers (Main conservator, OLTC conservator, and oil-filled cable box) should be checked for the correct oil level in the seal. Ensure that breather pipes are not clogged by watching the air bubbles breathed in or out through the oil in the seal. Check the colour of the silica gel for moisture saturation. Moisture absorption capacity is drastically reduced when the gel is partially saturated. Hence, dry out the silica gel when more than 50% of the material has changed colour. Moreover, the colour change should only start from the fresh air input side (where air first enters the breather on its pathway toward the conservator). If the silica gel changes colour on the conservator side of the silica gel breather, or if the colour of the silica gel remains unchanged after several months, then this indicates an air leakage in the connection piping, allowing air to bypass the breather.

Maintaining the surrounding area of a transformer clean is very important…

Cooling Equipment

Oil Pumps should be manually energised to ensure proper operation. Any significant noises (grinding, rubbing, scraping, oil flow cavitation) should be noted, investigated, and corrected. Inspect oil flow indicators for correct flow indication. Oil flow gauges should indicate full flow without fluctuation, and fans should be manually energised to ensure proper operation. Similar to the oil pumps, significant fan motor noise or imbalance shudder should be noted, investigated, and corrected. Fan blade guards must be maintained for the safety of personnel. Obvious obstructions to the through airflow caused by debris should be noted and cleaned. Periodic infrared imaging of the
radiators should be made to ensure proper oil flow and heat transfer.

In the case of oil-to-air heat exchangers (generally used for Generator Transformers and Rectifier Transformers at metal smelters), the cooling depends on airflow through the finned tubes in the cooler. These can get clogged or fouled with debris and accumulated deposits of air-borne particles, impairing the cooling efficiency. Watch the airflow through the fins by  standing on the opposite side of the fans. Check for any oil leaks through the tube-to-tube sheet joints. For oil-to-water heat exchangers, look for oil leaks through leak detectors or in the water outlet receptacle.


Cleanliness of the surroundings of the transformer is essential. There were transformer fires from the dried leaves around the distribution transformers.


Inspect the tank grounding and neutral earthing busbars. They are still in place and have not been stolen, and the joints to the equipment and ground are intact.

Control Room

Line voltage and load current. Counter readings from load tap changer, circuit breakers, automatic reclosures, and disconnects.

Frequency of inspection

The frequency of your walking around the transformer depends on the importance of the unit, criticality to your grid, age and service record, etc. CIGRE Brochure TB 445 on Transformer Maintenance recommends the following frequency based on light, regular or intensive maintenance protocol.

Ramachandran is a Specialist in Power Transformers.

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