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Applications for

Thermal Imagers

Monitoring transformers
Application Note
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.

Power and distribution transformers change electric current from one voltage to another. They accomplish this process when electricity flowing through a coil at one voltage induces current in a second coil. The amount of change is a function of the number of windings on the coils. The following discussion focuses on monitoring externaland 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 internal problems before irreparable damage occurs. Other diagnostic technologies,including built-in temperature and pressure gauges, may be more reliable for assessing the internal conditions in dry transformers. The procedures described here should be conducted in conjunction with the recommendations of NFPA Standard 70B, Recommended Practice for Electrical Equipment Maintenance, Chapter 9: “Power and Distribution Transformers.”
At 94 °F, one of the terminals on this 1320 V to480 V main tranformer is running about 20 °F hotter than it should.

What to check?
At a minimum, use your thermal imager to look at external connections, cooling tubes and cooling fans and pumps as well as the surfaces of critical transformers.

What to look for?
In oil-filled transformers, monitor the following external components: • High- and low-voltage bushing connections. Overheatingin 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, oil flow is being restricted and the root cause of the problem needs to be determined. • Cooling fans/pumps. Inspect fansand pumps while they are running. A normally operating fan or pump will be warm. A fan or pump with failing bearings will be hot. A fan or pump that is not functioning at all will be cold. Problems with surge protection and lightning arrestors leaking to ground and current tracking over

For more information on Thermal Imagers go to www.fluke.com/thermography

insulators can also be detectedusing thermography. However, finding such problems requires the capture of subtle temperature differences often under difficultto-monitor conditions. Ultrasound or some other technology might be a more reliable monitoring technique for these problems. For thermography to be effective in pinpointing an internal transformer problem, the malfunction must generate enough heat to be detectable on theoutside. Oil-filled transformers may experience internal problems with the following: • Internal bushing connections. Note: connections will be much hotter than surface temperatures read by an imager indicate. • Tap changers. Tap changers are devices for regulating transformer output voltage to required levels. An external tap changer compartment should be no warmer than the body of the transformer.Since not all taps will be connected at the time of an inspection, IR inspection results may not be conclusive. A good approach is to create regular inspection routes that include the transformers on all essential electrical circuits. Save thermal images of each one on the computer and track temperature measurements over time, using the software that comes with the IR camera. That way, you’ll...