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Application Notes:

Monitoring Transformers with Thermal Imaging


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Most transformers are cooled by either oil or air while operating at temperatures much higher than ambient. In fact, operating temperatures of 65 C for oil-filled units and 150 C for air-cooled transformers are common. Nevertheless, problems with transformers often manifest themselves in overheating or hot spots, making thermal imaging a good tool for finding problems.


The following discussion focuses on monitoring external and internal conditions of oil-filled transformers. Dry transformers also can exhibit both external or internal connection problems, and external connection problems can be detected as with oil-filled units. Beyond that, dry transformers have coil temperatures so much higher than ambient, it is difficult to detect.

The procedures described here should be conducted in conjunction with the recommendations.

What to check?

At a minimum, use your thermal imager to look at external connections, cooling tubes and more as seen in the application note.

What to look for?

In oil-filled transformers, monitor the following external components:

"High- and low-voltage bushing connections. Overheating in a connection indicates high resistance and that the connection is loose or dirty. Also, compare phases, looking for unbalance and overloading.

"Cooling tubes. On oil-cooled transformers, cooling tubes will normally appear warm. If one or more tubes are comparatively cool, ther may be a problem.

Problems with surge protection and lightning arrestors leaking to ground and current tracking over insulators can also be detected using thermography and thermal imagers.

For thermal imaging to be effective in pinpointing an internal transformer problem, the malfunction must generate enough heat to be detectable.

A good approach is to create regular inspection routes that include the transformers on all essential electrical circuits. Save thermal images for comparison and references of proper operating equipment.

What represents a "red alert"?

Equipment conditions that pose a safety risk should get the highest priority for repairs. However, the imminent failure of any piece of critical equipment constitutes a red alert. Key operations, maintenance and safety personnel should play roles in quantifying "warning" and "alarm" levels.

What's the potential cost of failure?

For power delivery companies, transformer failures can be very costly. A transformer failure in the summer of 2005 in Oslo, Norway resulted in a 50-minute power outage for 200,000 customers, left people trapped in subways and elevators, and cost was extreme.

Follow-up actions

Whenever you discover a problem using a thermal imager, use the associated software to document your findings in a report, including a thermal image and a digital photograph of the equipment. That's the best way to...

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