Up Time Magazine- February/March 2008

ultrasound upload The Dynamic Duo Ultrasound & Infrared Team Up to Prevent Transformer Outages by Allan Reinstra rocessing steel requires big energy and heavy duty electrical systems. But just one electrical component failure can grind steel production to a halt, costing manufacturers huge losses in time and money. The more severe the failure, the longer it takes for systems to be restored. Maintenance crews rely on condition monitoring technologies like ultrasound inspection and infrared imaging to predict electrical component failures in all systems including substations, MCC gear, transformers, and the like. The earlier a problem is spotted, the larger the window for scheduling maintenance. Electrical repair crews prefer to make repairs on their terms, rather than on the terms of the machine because it is far less expensive and a much better option for everyone. Being alerted early that a system needs attention also means that the maintenance required is “simple maintenance” as opposed to costly overhaul, total repair, and/or replacement. By simple maintenance I mean just that – simple cleaning, dusting, and minor parts replacement using spares already stocked in stores. Most electrical faults are the result of partial discharge. Partial discharge is defined as “a localized electrical discharge in an insulation system that does not completely bridge the electrodes”1. A discharge is described as either an “arc” or a “spark” and can either be phase to phase or phase to ground. Partial discharge is destructive to the conductor or insulator, and over time, will cause the component to fail. Corrosive gases like nitrous oxide negatively impact the integrity of insulators. Partial discharge is influenced by the system voltage, the shape of the void from phase to phase, ambient temperature, the condition of the insulation material, and environmental conditions such as pollution and humidity. The combination of these factors have an impact on the time it takes for the system component to fail. Partial discharge is a common problem in high voltages, but it can also be problematic in low voltage switch gear. Since voltage is an influencing factor, the higher the voltage the more destructive partial discharge becomes. One stage of partial discharge is termed “Tracking.” Tracking is difficult to detect since it doesn’t demonstrate any heat build-up. In its early stages, it can be considered arcing. Like corona discharge and arcing, tracking exists only to seek a path to ground. Dirt, dust and moisture help tracking follow this path, which is why simple maintenance like cleaning is effective in prolonging the service life of electrical systems diagnosed with tracking. Tracking begins with a low buzzing and crackling, and builds in intensity until it reaches the point of flashover. After flashover occurs it becomes quiet again. It is this constant build up in intensity and discharge that leads to insulator breakdown and graduation to more destructive arcing. Headquartered in Brazil, Gerdau is one of the world’s leading steel producers and trades under the symbol GNA on the NYSE. Gerdau Ameristeel is the American arm of Gerdau Brazil, and there are 19 mills in North America under this umbrella. Most, if not all, of Gerdau’s mills employ various predictive technologies to ensure that electrical outages are few and far between. A good example of how combined predictive inspections prevented a transformer outage was provided for this article by Skip Young, certified infrared and ultrasound technician with Gerdau Ameristeel in Calvert City, KY. Typically, electrical faults only generate heat once they’ve reached an advanced stage. Relying solely on IR may result in a missed diagnosis, but not for Skip Young. While conducting all scheduled IR scans, Young also brings his ultrasound detector. He knows that acoustic energy is generated at any stage of discharge, and that by combining ultrasound and infrared scans, he is assured to find virtually all faults. The insulator shown in Figure 1 was damaged by tracking, and eventually, arcing. The problem was detected early with an ultrasound inspection system. One of SDT’s Certification Instructors, Debra Smith, reminds us that electricity will always follow the easiest path to ground. She teaches ultrasound inspectors how dirt, dust, and moisture enhance the opportunity for tracking to establish this path to ground. Debra also reminds us that tracking indicates the presence of an equipment fault, but when caught at an early stage, it can often be fixed with simple maintenance procedures. There are serious safety consequences to be considered by all inspectors carrying out this work. Thermal images from several 161kV to 13.8kV step down transformers were provided to us by Young. As P 74 february/march 2008

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

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.