Take a Transformer’s Temperature
Because transformers are extremely expensive and essential to the proper functioning of electrical systems in a plant, they are a good example of the effective use of thermography (Figure 5).
5. Transformer inspection. A thermogram of a 66-kV/12.4-kV transformer shows all is well during this inspection. Capture and record a baseline or footprint to compare for future inspections. You will need data for the trending. Courtesy: City Light and Power
When taking thermal images of transformers, the effect of the heat transfer in the insulating oil must be considered. Although it’s not necessarily a cause for concern on other equipment, a temperature rise of 5 degrees on an oil circuit breaker or load tap changer could indicate a serious problem, so check oil levels prior to taking thermal images of transformers.
Next, capture and record a baseline or footprint to compare for future inspections (Figure 6). You will need this data for trending. It is a good idea to take thermal images of bushings, lightning arrestors, radiators, fans, load tap changers, and ground connections. Confirm the fan settings and then operate the fans, if possible. Take thermal images of the cooling tubs and radiators to identify uneven heat distribution or blocked tubes or radiators.
6. Too hot to handle. A later inspection of the 66-kV/12.4-kV transformer cooling fans
clearly shows a failed fan motor. Courtesy: City Light and Power
In this 66-kV/12.4-kV transformer, the forced air-cooling circuit tripped out because of a bad fan motor. Some emergency switching was performed to reduce the secondary load on the transformer. Evaporative coolers were installed before the oil temperature became critical.
Write Your Reports
Once the thermal images have been taken and possible areas of concern have been identified, a report should be prepared indicating the findings and possible areas of additional investigation. This report should be retained for data comparison with subsequent thermal measurements so that changes in temperature under equivalent operating conditions can be used for preventive maintenance.
As a general rule, the document will be easier to read if similar topics are grouped together. In addition, it is useful to number each section and include a table of contents. This makes it easier to locate a specific topic. During the training process, divide the procedure into separate, distinct steps to make them easier to follow. The document should be clearly written, with the actual procedure documented separately from the ramifications of failing to test equipment. Finally, make sure the report clearly notes the differences between actual and differential temperatures.
Here are my suggested sections for a well-written thermography report:
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Location of substations, transmission distribution (overhead and underground), generation plants, circuits, or other equipment on a grid map
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Background, including previous thermography reports and results
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Purpose of the report
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Pretest procedure: test plan, equipment list, obtaining test equipment, calibration
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Test procedure: using test equipment, measurement techniques, special conditions
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Post-test procedure: reports and recommendations
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Data collection forms
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References used in the report
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Terminology
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Example calculations
More information on infrared imaging is available at www.goinfrared.com/electricalproblems. To download technical reference materials or view webcasts on IR technology and noncontact temperature measurements, visit www.goinfrared.com/IRdata.
Acknowledgements: The author wishes to thank the Infrared Training Center at FLIR Systems, City Light and Power Inc. and support staff, and Fernando Calderon, PE with AC Electric Systems for their assistance in preparing this article.
— Contributed by Gary Updegraff, a Level III thermographer for City Light and Power Inc., a second-generation lineman, and a member of the International Brotherhood of Electric Workers Local #47 for 30 years.
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