Up Time Magazine - February/March 2009 - (Page 24) infrared upload Infrared & Ultrasound Unite The Perfect Match for Metal-clad Switchgear Inspections by James Brady I nfrared technology has revolutionized the electrical maintenance industry by providing a means of non-contact and non-destructive testing to identify eminent failures related to compromised components and connections before they occur. But even with regularly scheduled infrared inspections in place, electrical failures of switchgear above 3500V occur; usually with little advanced warning or understanding about the cause. This leaves maintenance personnel scratching their heads and wondering if their infrared program is missing obvious problems and/or if their predictive maintenance program has deficiencies of which they are not aware. The likelihood of an infrared inspection missing obvious problems is low as long as the person(s) conducting the survey possesses the proper training and experience. However, if infrared is the only predictive maintenance technology employed to inspect high-voltage switchgear, there is a high probability that potential problems are being missed that can lead to failures. Electrical discharge in the form of corona and tracking has caused numerous shut-downs and serious damage in metalclad switchgear. Because corona and tracking conditions are voltage problems that rarely produce heat, they go undetected during a typical infrared inspection. Fortunately, the combined use of ultrasound and infrared when performing a switchgear inspection will provide early detection of both heat and non-heat producing problems. Several years ago, I was introduced to ultrasound technology at an infrared conference. After realizing the applications and benefits of this predictive maintenance tool, we were soon offering this service as a routine part of our infrared inspection business. That decision has more than paid for itself by finding critical electrical problems for our clients that would have otherwise gone undetected using infrared alone. This article will attempt to take the mystery out of the occurrence of corona and tracking in metal-clad switchgear by discussing its characteristics, what to look for during an inspection and case studies that have been documented during actual inspections. Corona Tracking Figure 1 - Corona activity advancing to the tracking stage on insulation board resting on 13kV bus. Notice the carbon deposits and light brown discoloration of the insulation board insulation board on the right. and other metals. The electromagnetic emission can be heard as interference on AM radios and the corona sound can sometimes be heard by the human ear and is detectable with ultrasonic scanning devices. Corona is a by product of voltage problems and can be present without current flow (i.e. under no load conditions). High potential in the electrical field is the major dictating factor for its presence. Corona activity is at its strongest on the positive (+) and negative (-) peaks of the 60Hz cycle. Once corona becomes active it leaves behind a conductive “tracking” path on surfaces and also creates a very conductive cloud of air around itself (Figure 1). A flash-over can occur once a “tracking” pathway is completed from phase to phase or phase to ground. It can also occur from the conductive cloud of surrounding air if it finds a path to ground. What Are Corona and Tracking? Corona refers to the faint glow surrounding an electrical conductor of 3500 volts or greater, which occurs as a result of the ionization of air as the nitrogen in the air breaks down. When corona occurs, it creates ozone (detrimental to the human lungs, eyes, etc.), ultraviolet light, nitric acid, electromagnetic emissions and sound. Ozone is a strong odorous gas that deteriorates rubberbased insulation. If moisture or high humidity conditions exist, nitric acids can also be formed that attacks copper What Causes Corona? Based upon numerous observed conditions of corona, I have concluded that there are three primary causes for its february/march 2009 24
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