ANSI standards for power equipment, and a vast store of technical literature, describe various methods by which thermal ratings may be adjusted if actual weather conditions are known or if the "overload" is to be applied for a limited period of time. These methods have been given various names including dynamic thermal rating, on-line rating, and dynamic ratings to describe the process of adjusting thermal ratings of power equipment for actual weather conditions and actual electrical load patterns. This paper discusses in detail a project undertaken by the Electric Power Research Institute (RP 3022-7) as part of its research on Flexible AC Transmission. This project avoids dependence on temperature measurement, instead, calculating critical equipment component temperatures based solely on real-time weather and electrical current. Inexpensive, commercially available weather stations, digital data loggers, and IBM-compatible PC computers are combined with sophisticated thermal algorithms to yield a portable, flexible, instrumentation method which can rate several transmission circuits simultaneously, including underground cable, overhead lines, power transformers, current transformers, switches, bus, line traps, and circuit breakers. Useable increases of 5% to 15% in the thermal capacity of transmission equipment circuits result.
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