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 Standards Dictionary Online should be consulted for ternu not deli ned in this clause.
conductor trmprrature: The temperature of a conductor, is normally assumed to be isothermal (i.e.. no axial or radial temperature variation) In those cases where the current dcnsity exceeds tt.5 k”nim’ (I A”kcmil). especially for those conductors with more than two layers of aluminum strands, the difference betwccn the core and surface may be significant. Also, the axial variation along the line may be ‘nsortini. Finally, for transient calculations where the time period of interest is less than I mm with nonItomogencous aluminum conductor steel reinforced (A(’SRt cotsducti’r,s, the aluminum strands may reach a high temperature befose the relatively non-conducting steel core.
rffretisr (radlall thrrnsal cunduetislty: Effective radial thermal conductivity charactenres the bare stranded conductor’s heterogeneous structure t including aluminum strands. air gaps. oxide layers) as if it were a single. hitmogenootas conducting medium. The use of effective thermal conductivity in Lhe thermal model simplifies the calculation process and avoids complex calculations on a microscopic level including the assessment of contact thermal resistances between strands, heat r.adiatism and convection in air gaps locked between strands.
heal capacity (nsaterial): When the average tenperaturc na conductor material is increased by dT as a result of adding a quantity of heat dQ. the ratio, dQdT, is the heat capacity of the conductor.
nia’,imant allowable conductor temperature: the maximum conductor temperature limit that is selected in order to minimize loss of conductor strength, and which limits sag in order to maintain adequate electrical clearances along the lines.
Reynolds numbrr: A dimensionless number equal to air velocity time the air density times conductor diameter divided by the kinematic viscosity of air, all expressed in consistent units. The Reynolds number. in thia case, is equal to the ratio of inertia forces to tile viscous force on the conductor. It is typically used to dilIerentiatc between laminar and turbulent how.
specitle hrat: The specific heat of a conductor niaterial is its heat capacity divided by its mass.
steady-slate tlsernsal rating: That constant electrical current wInch yields the maximum allowable conductor tempctature for specified weather conditions and conductor characteristics under the assumption that the conductor is in thermal equilibrium (steady state).
thernaal timr constant: In response to a sudden change in current (or weather conditions), the conductor temperature will change in an approsimately exponential manner, eventually reaching a new steady.stare temperature if there is no further change. The thermal time constant is the time required for the conductor temperature to accomplish 63.2% of this change. The exact change in temperature is not exponential so the thermal time constant is not used in the calculation described tn this standard. It is. however, a useful concept in understanding line ratings
time_srarving weather aad current: Neither weather conditions nor the electrical current earned by an overhead transmission line is typically constant over time, Yet both are assumed constant in conventional steady-state rating calculatiomuc Even its the transient sating calculation where the current undergoes a step- change, the weather conditions are typically assumed constant. Only real-time rating methods consider the time-variation of lute current and weather.
transirnt thermal rating: The line current can change suddenly. The conductor temperature cannot, For short-time emergency line currents, the delay in heating the conductor ny allow relatively high currents to be applied (or s[s’rt tunes leg.. less thai1 thenmial time constant’.)IEEE 738 pdf download.