Plant Services - March 2008 - (Page 36)

Power distribution SaFeTy spot you first detected should have gone away. It’s a myth that every hot spot is the result of a loose connection. There are many causes for hot spots. That’s why it’s wise for a qualified electrician either to perform the thermal scan, or to be present while it’s being done. Here’s what to check. Three-phase imbalance: Capture images from inside electrical panels and high-load connections at drives, disconnects and controls. If you discover higher temperatures, examine associated circuit branches and loads. Compare the three phases side by side and look for significant temperature differences. A cooler-than-normal circuit or leg might signal a failed component. More heavily loaded phases appear warmer. Hot conductors might be undersized or overloaded. However, an unbalanced load, an overload, a bad connection and harmonics can produce similar patterns, so follow up with electrical or power-quality measurements to diagnose the problem. Voltage drops across fuses and switches also can appear as imbalance at the motor and excess heat at the root trouble spot. Before you assume the cause has been found, doublecheck with both the thermal imager and multimeter or clamp meter current measurements. Connections and wiring: Look for connections that have higher temperatures than other similar connections under similar loads. That could indicate a loose, overtightened or corroded connection. Connection-related hot spots usually (but not always) appear warmest at the spot of high resistance, and they cool in relation to the distance from that spot. In some cases, a cold component is abnormal because the current is being shunted away from the high-resistance connection. You might find broken or undersized wires, or defective insulation. The InterNational Electrical Testing Association (NETA) suggests that if the temperature difference between similar components under similar loads exceeds 15°C (27°F), make repairs immediately. Fuses: If a fuse shows up hot on a thermal scan, it might be at or near its current capacity. However, not every problem shows up as hot. A blown fuse, for example, exhibits a cooler than normal temperature. Motor control centers: To evaluate a motor control center under load, open each compartment and compare the relative temperatures of key components: bus bars, controllers, starters, contactors, relays, fuses, breakers, disconnects, feeders and transformers. Use the guidelines above for inspecting connections and fuses, and identifying phase imbalance. Measure the load at the time of each scan so you can trend and evaluate your measurements against normal operating conditions. Transformers: If an imager is to detect an internal transformer problem, the malfunction must generate enough heat to be detectable from the outside. That means that a malfunctioning bushing connection, for example, will be much 36 Recommended inspection frequency High-voltage substations Transformers 440 V motor control centers (air-conditioned) 440 V motor control centers (not air-conditioned or older) Electrical distribution equipment 1 yr to 3 yrs Annually 6 mo to 12 mo 4 mo to 6 mo 4 mo to 6 mo hotter than the surface temperatures read by the imager. For oil-filled transformers, use an imager to examine high- and low-voltage external bushing connections, cooling tubes, and cooling fans and pumps, as well as the surfaces of critical transformers. This doesn’t apply to dry transformers, which have coil temperatures so much higher than ambient that it’s difficult for thermal imagery to detect problems. Use the guidelines above for connections and imbalances. The cooling tubes should appear warm. If one or more tubes appear comparatively cool, oil flow is probably restricted. Keep in mind that like an electric motor, a transformer has a specification for operating temperature that represents the maximum allowable rise in temperature above ambient (typically 40°C). A 10°C rise above the nameplate operating temperature will probably reduce the transformer’s life by half. Preventive maintenance Thermal imagers are an excellent tool for routine preventive maintenance. Technicians should measure and compare thermal signatures for equipment on the inspection route. If the temperature differs markedly from previous readings, the technician can then use other test tools (digital multimeter, power-quality analyzer, etc.) to investigate further. Michael Stuart manages thermography products for Fluke Corp. in Everett, Wash. Contact him at michael.stuart@fluke.com and (425) 446-5602. More resources at www.PlantServices.com PdM for drives – “Keeping drives healthy” Thermography – “Uncool technology” Accurate thermography – “Transportability of measurements” Thermography as a tool – “A short introduction to thermography” Cabinet coolers – “Thermoelectric enclosure cooling” Thermography program – “Thermography and its role in motor-condition monitoring” For more, search www.PlantServices.com using the keywords circuit, infrared and scan. www.PLANTSERVICES.com March 2008 http://www.PlantServices.com http://www.PlantServices.com http://www.PLANTSERVICES.com

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Plant Services - March 2008 Contents From The Editor Letters The PS Files Up and Running Crisis Corner What Works Asset Manager: Enterprise Thinking Technology Toolbox: Hydrogen In Action Cover Story: Get Real Electrical: Reliable Distribution Power Transmission: When the Belt Breaks Web Hunter: Tooting Horns In the Trenches Product Picks Classifieds Fast Facts Energy Expert: Energy versus Sustainability

Plant Services - March 2008

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