Up Time Magazine- February/March 2008

Figure 1 - The insulator shown here was damaged by tracking and eventually arcing. shown in Figure 2, there were no apparent hot spots visible on the A, B, and C phases but a vigilant ultrasound scan produced a sound file2 with obvious indication of early tracking. Figure 3 – The demodulated sound ﬁle can be viewed in both the time (top image) and spectrum (bottom image) domain. The top shows tracking picked up in the ultrasound range from the A-Phase bushing of the transformer. We also listened to sound files taken from the B and C Phase bushings. Neither showed any signs of heat when scanned with Young’s infrared camera. However, both projected signs of tracking when scanned with the ultrasound collector. Once a diagnosis was made on the suspect transformers, the decision to perform simple maintenance during the next planned outage was made. Infrared scans showed no signs of heat so we felt confident that the discharges discovered during ultrasound scanning was, indeed, tracking in its early stages. The problem was discovered at an early enough stage that simple maintenance could be done on human terms, not terms dictated by a failure. According to Young, their simple maintenance merely included cleaning and tightening all connections on A, B, and C phase. Looking at the time signal in Figure 5, we Figure 2 - Thermal images of A-phase bushing and incoming connection on this 161Kv to 13.8Kv step down transformer showed no apparent hot spots. We can have a good look at Young’s ultrasonic data using SDT’s Ultranalysis software. This program looks at captured ultrasound files in the time and spectrum domain as shown in Figure 3. The top image illustrates the time domain showing the build up and release of the ionization discharge as it finds a path to ground. Ultrasonically we hear the build up, and then an interruption or neutralization of the air surrounding the problem. Heat does not build up here until the situation progresses and there is sufficient flow or current to produce heat along the discharge path. The bottom image illustrates the spectrum domain from Young’s ultrasonic data. There are two things to note here. First, the obvious repetition of 60 Hz events clearly tells us that, in addition to tracking, nuisance corona is present. Secondly, the noise level between the 60 Hz peaks confirms there is tracking activity. Figure 4 – B-Phase Bushing prior to cleaning. Note in the time signal the short but violent bursts at the 22 second mark and again at the 28 second mark. These are clear signs of tracking without heat build up. In the spectrum domain there are clear 60hz peaks once again but nowhere near as strong as in image 3. www.uptimemagazine.com 75 http://www.uptimemagazine.com

Table of Contents Feed for the Digital Edition of Up Time Magazine- February/March 2008

Up Time Magazine- February/March 2008 Contents Upfront Upclose: Lessons Learned from the Walter Reed Story Information Technology: Parenting and Asset Performance Infrared: It's Alive! Lubrication: A Sense of Change Maintenance Management: PdM, Standardized Motor Testing: Being Well Connected Precision Maintenance: How to be Marketable Reliability: The Voyage of Wells Dairy Ultrasound: Two is Better Than One Vibration: Diagnosing a Cracked Shaft Upgrade: A Different Kind of Journey

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