IEEE 738:2012 pdf free download – IEEE Standard for Calculating the Current-Temperature Relationship of Bare Overhead Conductors
2.Definitions
For the purposes of this document，the following terms and definitions apply. The IEEE StandardsDictionary Online should be consulted for terms not defined in this clause.1
conductor temperature: The temperature of a conductor,Tag。 is normally assumed to be isothermal (i.e.,no axial or radial temperature variation). In those cases where the current density exceeds 0.5 A/mm2(1 A/kcmil), especially for those conductors with more than two layers of aluminum strands, the differencebetween the core and surface may be significant.Also, the axial variation along the line may be important.Finally，for transient calculations where the time period of interest is less than 1 min with non-homogeneous aluminum conductor steel reinforced (ACSR) conductors, the aluminum strands may reach ahigh temperature before the relatively non-conducting steel core.
effective (radial) thermal conductivity: Effective radial thermal conductivity characterizes the barestranded conductor’s heterogeneous structure (including aluminum strands, air gaps, oxide layers) as if itwere a single, homogeneous conducting medium.The use of effective thermal conductivity in the thermalmodel simplifies the calculation process and avoids complex calculations on a microscopic level includingthe assessment of contact thermal resistances between strands，heat radiation and convection in air gapslocked between strands.
heat capacity(material): When the average temperature of a conductor material is increased by dT as aresult of adding a quantity of heat dQ, the ratio, dQ/dT, is the heat capacity of the conductor.
maximum allowable conductor temperature: The maximum conductor temperature limit that is selectedin order to minimize loss of conductor strength，and which limits sag in order to maintain adequateelectrical clearances along the lines.
Reynolds number: A dimensionless number equal to air velocity time the air density times conductordiameter divided by the kinematic viscosity of air, all expressed in consistent units. The Reynolds number,in this case, is equal to the ratio of inertia forces to the viscous force on the conductor. It is typically used todifferentiate between laminar and turbulent flow.
specific heat: The specific heat of a conductor material is its heat capacity divided by its mass.
steady-state thermal rating: That constant clectrical current which yields the maximum allowableconductor temperature for specified weather conditions and conductor characteristics under the assumptionthat the conductor is in thermal equilibrium (steady state).
thermal time constant: In response to a sudden change in current (or weather conditions), the conductortemperature will change in an approximately exponential manner, eventually reaching a new steady-statetemperature if there is no further change. The thermal time constant is the time required for the conductortemperature to accomplish 63.2% of this change. The exact change in temperature is not exponential so thethermal time constant is not used in the calculation described in this standard. It is，however,a usefulconcept in understanding line ratings.
time-varying weather and current: Neither weather conditions nor the electrical current carried by anoverhead transmission line is typically constant over time.Yet both are assumed constant in conventionalsteady-state rating calculations. Even in the transient rating calculation where the current undergoes a step-change, the weather conditions are typically assumed constant. Only real-time rating methods consider thetime-variation of line current and weather.