iNFRARED ELECTRICAL SURVEYS

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CURRENT AND TEMPERATURE LIMITATIONS.- The amount of heat generated in a conductor is proportional to its resistance and to the square of the current it carries, while the temperature rise depends on the rate at which the heat is dissipated through convection, radiation, and conduction.

7.1.1. Current-carrying capacity of a bus. - The current-carrying capacity of a bus is limited by the temperature rise produced by the current and other factors. Buses for generating stations and substations are generally rated on the basis of the temperature rise which can be permitted without danger of overheating equipment terminals, bus connections, and joints. ANSI C37.20C-1974 (IEEE standard 27-1974) permits a hottest spot temperature rise for plain copper buses of 30 ºC (54 ºF) above an ambient temperature of 40 ºC (104 ºF), with a hottest spot total temperature limit of 70 ºC (158 ºF).

The standard allows a hot spot temperature rise of 65 ºC (117 ºF) for metal-enclosed applications where silver contact surfaces are used on connections and a hot-spot temperature rise of 45 ºC (81 ºF) for silver-surfaces terminals of outgoing circuits. Aluminum used for bus work ordinarily has a conductivity of 63 percent, as compared to copper at 99 percent. For a given current rating and for equal limiting temperatures, the area of an aluminum bus will be about 133 percent of the area of a copper bus.

7.1.2. Allowable current density in a bus. - Allowable current density in a bus is the amount of current that the bus can carry per square inch or cross-sectional area without exceeding the permissible temperature rise. For both ac and dc buses, densities may vary from values of 9.3. x 10⁵ and 1.09 x 10⁶ A/in² (600 and 700 A/in²) in heavy current-carrying copper buses to 1.86 x 10⁶ and 2.17 x 10⁶ A/in² (1,200 and 1,400 A/in²) in light buses under favorable conditions. For aluminum, densities of 75 percent of the above values are usually permitted.

7.1.3. Current-carrying capacity of conductors. - No method has been generally accepted by the industry for the calculation of the current-carrying capacity of conductors for overhead power transmission lines. However, Reclamation designs its ACSR transmission line capacity in accordance with tables, charts, and procedures in the "Aluminum Electrical Conductor Handbook," 1971 edition or specific instructions from the conductor manufacturer.

7.2. TEMPERATURE OF CONNECTIONS.- The principal function of an electrical connection is to satisfactorily carry the electrical load over its entire service life. The electrical load can be expected to have daily fluctuations from no load to full load and frequently to very heavy overloads, thus causing wide fluctuations of operating temperature. In addition, the ambient temperature can be expected to fluctuate between daily extremes and between seasonal extremes. The effect of this heat cycling on a poorly designed or improperly installed connection is frequently progressive deterioration and ultimate failure of the connection or associated equipment. Therefore, temperature rise provides an important and quite convenient method of monitoring the condition of electrical connections.

7.4. THERMOGRAPHIC (INFRARED) SURVEYS. - Sophisticated infrared equipment in vans or helicopters is now widely used throughout the electric power industry for rapid scanning of substation and switchyard bus and equipment as well as transmission lines. Available equipment can survey an average-sized switchyard in about 30 minutes. Experience in the industry has proven the technical worth and economic justification of infrared testing as a preventive maintenance tool.

7.4.1. Thermal Imaging devices.-Thermal imaging devices are useful in permitting a rapid scan of switchyard buses and equipment terminals to detect temperature differences. Infrared signals from photovoltaic detectors are electronically amplified and transmitted to a small television viewing screen producing an image corresponding to the thermal patterns within the viewed scene. Temperature differences are shown in varying shades of gray or in different colors. An accessory instant camera may be used to provide a permanent record of temperature differences.

7.4.3. Scheduling of infrared surveys. - Scheduling of infrared surveys to detect hot spots in electrical connections and equipment is an important facet of an effective infrared testing program.  Such surveys should be conducted during periods of high system loads, which circuits in a given station are sufficiently loaded to reveal existing or potential hot spots. Subsequent surveys should be scheduled to test transfer of auxiliary buses under loaded conditions. Each hot spot detected by an infrared survey should be physically inspected and evaluated. Judging the severity of a faulty connection by temperature readings alone can be misleading. A reading of 50 ºC (90 ºF) above ambient in one case can be as serious as one 180 ºC (325 ºF) above ambient, especially if the load is low in the first case. A follow up inspection of suspect and/or repaired connections should be performed with a hand-held infrared thermometer, temperature-sensitive tape, or other suitable means.
-USBR

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