Monograph, the 1st edition, John Wiley & sons, Somerset (UK), 1991, pg. 741The primary function of overhead transmission lines and substation busbars is to transfer electric power. Because the voltage is usually fixed, the power that can be transferred depends on the current and its phase. Each conductor or busbar has resistance to the flow of electric current; hence it will heat up to a temperature which is determined by its physical characteristics; the magnitude of the current and the time for which it flows; and the degree of cooling (and heating) by the atmospheric conditions.
In the case of a long transmission line, the maximum current that can be carried may be determined by considerations of system stability, voltage regulation or economic power loss. On the other hand, the current-carrying capacity of shorter lines may be determined by the maximum permissible temperature of the conductor, which determines the maximum sag, and the time distribution of the conductor temperature, which determines the creep, the rate of annealing and the total loss of tensile strength of the conductor.
In calculating steady-current ratings, it is necessary to consider heat gains and losses in detail and also those factors which determine the temperature distribution within the conductor. Until recently, steady-current ratings were calculated using deterministic methods, i.e. fixed values were used for the maximum conductor temperature and the more important atmospheric variables. This approach is conservative, because the conductor operates at well below the design temperature for a good deal of the time. In the present economic climate, there is pressure to utilise transmission-line and sub-station conductors to their fullest extent; hence there is a trend towards the adoption of dynamic and probabilistic methods for rating conductors.CONTENTS:
Introduction
The steady state - Internal heat conduction
The heat gains
The heat loss by natural convection
The heat loss by forced convention
The heat loss by mixed convention
The heat loss due to vibration
The heat loss by radiation
The heat loss by mass transfer
Sensitivity analysis of the steady state
The unsteady state with heat loss (dynamic rating)
Probabilistic current rating
The unsteady state with no heat loss (fault rating)
Worked examples
Appendixes
Glossary
Nomenclature
Index
