Page 190 - Intro Predictive Maintenance

Thermography 181

tions, and rotors. Bearing problems can be found by comparing the surface tempera-
ture of like motors. Overheating conditions are documented as hot spots on the CRT
and are usually found in comparing equipment, end bell and end bell (same type bear-
ings), and stator to end bell.

Transformer—Oil-Filled
Scan transformer, transformer fins, cable connections, bushings, and tap changer. On
all transformers, the oil level should be inspected during the survey. During the
infrared survey, if a transformer appears exceptionally warm, the cooling radiators are
near ambient temperature, and the transformer is above 50 percent of full load, the oil
level is too low to circulate the oil and cooling is not taking place. Oil in the trans-
formers is cooled by convection; as the load increases, the oil expands and the level
increases until it then circulates in the cooling radiators. As a result of repeated oil
samples and oil leaks, the reduced volume of oil causes the winding to overheat, thus
reducing the life of the transformer. Plugged cooling heaters, isolated radiators, and
plugged individual cooling fins can also be detected.

Transformers—Dry-Type
Scan transfers, cable connections, bushings, and tap changer. Enclosure covers on
dry-type transformers should be removed only if there is safe clearance between the
transformer connections and the enclosure panels. Some models, especially the newer
ones, have screened openings for ventilation. Use these openings for your scanning
survey.

The iron in these transformers is hot. It will heat the bus work and cause substantial
infrared reflection. By increasing the temperature scale and adjusting the level control
on the imager, you will be able to get uniform images, which will show hot spots in
the secondary bus or the iron. A hot spot in the iron usually indicates a short. Make
certain that reflection is not a factor.

Compare all windings. If temperatures are over a winding, but there is a difference in
temperature of two windings, there may be an unbalanced load. A hot spot on a
winding may point to a shorted turn.

Transformer Bushings
As a scanner moves upward on the transformer main tank and tap changer compart-
ment, the bushings, lighting arresters, and their bus connections should be observed.
This area is also critical because the integrity of the transformer, substation, or the
complete system depends on proper installation and maintenance of each component.
A survey of the transformer bushings, comparing one to the other, will reveal any
loose connections or bushing problems. With the scanner, you can determine if the
connection is loose internally or externally.