The loading of overhead conductors depends upon several physical properties including the power applied to the conductor and the ambient conditions of the conductor. Electric utilities must be careful that overhead conductors not be overloaded and service interrupted.
It is a common practice for electric utilities to rate an overhead conductor by calculating the loading of the conductor as a result of certain worst case conditions. For example, the upper limits of conductor loading may be based upon a specified limiting temperature under worst case weather conditions and an expected load. When the line loading exceeds its rating, the utility is obligated to either reduce the line loading, maintain loading and reduce the service life of the line, or assume that the required ground clearances will be violated.
Oftentimes, utilities will conduct surveys and long term planning projects in an attempt to estimate expected load requirements. If, based upon the ratings of the available overhead conductors, it is determined that the rating of existing conductors will be exceeded under worst case conditions, then the utility may undertake the great expense of providing additional overhead conductors. Although this additional capital investment may often be justified in the long term, it may be possible to avoid or to postpone the expense if more accurate information were available concerning overhead conductor loading.
A factor extremely important to the loading of an overhead conductor is the temperature of the conductor. The temperature of the conductor is due in part to its internal resistance. In addition, the temperature of an overhead conductor is affected by ambient weather conditions, e.g., temperature and wind velocity. It is probable that even when the rating of an overhead conductor is exceeded, the actual temperature of a conductor will be below the harmful limit used to establish the rating. This is because it is relatively unlikely that worst case weather and line overload occur at the same time. The real line rating may in fact not have been exceeded. Thus, if the temperature of the overhead conductor could be monitored in a real time manner, there would be no need to initiate an overload alarm based on rating estimates. Instead, the actual temperature limit could be used as the operative variable.
Ideally, the utility operators would have some advance notice that the conductor temperature is approaching its limit along with some additional information such as an estimate of the time remaining before the occurrence of a critical overload. The ability to monitor the conductor temperature with reference to a temperature limit would, of course, mean that the probability of exceeding the conductor rating would be reduced.
There are numerous existing and proposed methods for measuring the temperature of an overhead conductor. One procedure is to attach a thermocouple to the overhead conductor and to supply the output of the thermocouple to a transmitter. Such a device is placed directly on the high voltage conductor and as a result is inconvenient to install and service. Moreover, the thermocouple is capable of only measuring surface temperature with the accuracy of measurement depending upon the quality of the thermal contact between the thermocouple and the surface of the conductor. Also, a single localized measurement may not be representative of the whole conductor. (Temperature variations of 10 to 15 degrees have been observed for different thermocouples on the same physical conductor span.)
An infrared pyrometer has been proposed as an additional means for monitoring the temperature of overhead conductors. Such a device measures the radiation temperature, based upon spectral peak, of the conductor, and requires a transmitter for information dissemination. There are some inherent inaccuracies in the use of infrared pyrometers because of the underlying assumptions made about the color of the conductor radiation. Also, several calibrations may be required during the life of the conductor because of changes in surface color resulting from weathering.
Another proposed method for conductor monitoring is a calculation procedure for temperature estimation. This procedure utilizes ambient weather and line loading to estimate conductor temperature. Again, this procedure merely deals with an estimate and has several drawbacks. One of these drawbacks is that weather variables must be measured locally. Typical measurements would include wind speed, wind direction, air temperature, solar radiation, and atmospheric turbulence. Such measurements are often not accurate or are incomplete. For example, typical anemometers measure only the horizontal component of wind speed even though the vertical wind speed component is significant in cooling. Moreover, the wind speed should be measured on a scale related to the diameter of the conductor which complicates measurement. Finally, temperature calculations involve the solutions of largely empirical equations based upon major assumptions and are subject to large error ranges.
Thus, it is clear that prior attempts to prevent the overheating of overhead conductors have met with only a moderate degree of success. The benefits of an accurate and inexpensive device for overhead conductor monitoring have yet to be attained.