Up Time Magazine - February/March 2009 - (Page 27) 94. °F 5 94 SP0:0°F 2. 94 92 AR18°F 0: . 94 90 88. °F 6 Figures 7a & 7b: A temperature rise of 8 F-degrees is observed on active corona on a 13kV stand-off bushing for a disconnect switch. 110. °F 2 110 110 114. °F 0 Figure 9a - Ultrasound was used to find tracking on blind side of insulation board (see arrow). AR11.° F 0: 9 14 SP0:2°F 2. 88 100 SP0:3°F 2. 95 AR12.° F 0: 4 12 105 90 100 80 76. °F 1 91. °F 1 95 Figure 8a & 8b - Delta temperatures of over 30F-degrees are seen on conductor cables that having tracking problems. Heating Patterns Associated With Corona It has been my experience that most cases of corona produce very little, if any, heat. Very faint heating patterns may result from the molecular disturbance of electrons associated with the ionization of air. Depending upon air movement and the intensity of the corona, the delta temperature may or may not be detectable with infrared technology (Figures 7a and 7b). This can be deceiving for the infrared thermographer who is unfamiliar with corona, as this serious condition may only receive a minor severity rating if evaluated solely on temperature rise criteria. In advanced stages where corona has become destructive or is advancing into tracking, I have seen very noticeable delta-temperatures, especially on conductors. Figures 8a and 8b show examples of such cases. Ultrasound equipment includes a receiver unit, headphones, and various modular listening devices that attach to the receiver for both airborne and structure-borne scanning. Airborne devices include cone-shaped collectors that capture ultrasonic wave traveling through air. Structure-borne devices include magnetic base collectors and rod attachments used to contact the surface of equipment. Through a process of ‘heterodyning” the ultrasound signal is converted by the receiver unit to a low frequency audible signal that can be heard trough the headphones. There is also a read-out display that shows the intensity of the received signal. Figure 9b: Ultrasound was used to find corona on ceramic collars surrounding 13kV bus, no visible signs of corona are present. cur and propagate in air, airborne ultrasound is the right choice to detect these problems. Ultrasonic waves are directional in their movement, which makes it relatively easy to track these problems back to their source. However, ultrasound wave will rebound off of surfaces and can be partially and completely blocked by components in the switchgear cabinet. By following the unit’s strongest received signal the operator can usually find the source of the problem. The operator can also use blocking techniques to filter out competing ultrasound noises, if necessary. Corona problems will be heard as a continuous buzzing or frying noise. The intensity of this sound will be directly related to the severity of the problem. Tracking problems will sound much like corona problems but will have pauses and possible drops and increases in intensity. Prior to opening a switchgear cabinet, it should be scanned ultrasonically using screened ventilation ports, seams around the cabinet doors, and the cabinet bolt holes once a few are removed. Typically, advanced cases of corona and tracking will be heard using this “initial” Using Ultrasound to Detect Corona and Tracking While most corona and tracking problems can be seen, I am a die-hard proponent of using ultrasound technology during a switchgear inspection. Three of the most obvious reasons are 1) ultrasound will verify the corona and tracking you see, 2) ultrasound allows you to qualify the severity of corona and tracking since they have distinct sound patterns that change with increasing stages of development, and 3) ultrasound will detect problems that you visually cannot see (Figures 9a and 9b). Because corona and tracking problems oc- Ultrasound Technology Sounds above the normal range of human hearing, 20 to 20 kilo-Hertz (kHz), is typically thought of as ultrasonic. A frequency range between 20 kHz to 40 kHz generally covers all of the ultrasonic applications used for predictive maintenance applications; leak detection, stream traps, bearings and lubrication, and electrical discharge. www.uptimemagazine.com 27 http://www.uptimemagazine.com
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