IEEE Std 738:2012 pdf free download

IEEE Std 738:2012 pdf free download.IEEE Standard for Calculating the Current-Temperature Relationship of Bare Overhead Conductors.
4.4.6.3 Magnetic core effect on AC resistance
Within steel-core conductors such as AC’SR and AC’SS. the flow of AC’ current is primarily through the aluminum strands Since the helically-wound aluminum strands surround the steel core in an alternating left-hand and right-hand by direction, magnetic flux is generated in the steel core, much like a solenoid. increasing with the magnitude of current. The core’s impact on conductor resistance depends on the construction of the bare stranded conductor
For ACSRIACSS conductors (Drake. Ibis. Bluebird) the magnetic core flux produced by the current in each layer essentially cancels, and level of magnetic flux in the core is quite low.
For single-layer ACSRIACSS conductors, the magnetic flux in the steel core is quite high and the resulting magnetic hysteresis and eddy current losses in the core can increase the effective resistance by as much as 20Y. at high current levels.
For three-layer ACSR ASS. there is partial magnetic field cancelation in the steel core so that the losses in the core are much smaller, but the solenoid effect couples the layer currents, making the current densities unequal and increasing the overall AC resistance of the conductor by as much as 5% at high current levels.
The resistance values for single-layer AC’SR and ACSS in the referenced handhooks include the effects of magnetic core losses at each temperature but only for certain assumed weather conditions. The resistance values for three-layer ACSRJACSS conductors must be supplemented by “correction curves” to obtain a resistance multiplier as a function of current density. Of the two recotnniended correction schemes, 18291 is the easiest to model and is slightly more conservative, Engineering judgment is required in thermal calculations involving these steel-core conductors.
4.4.6.4 Radial thermal gradient component of ACIDC resistance
Since this standard was first published in 19X6. the maximum conductor temperatures used to determine line ratings with conventional bare stranded conductors has gradually increased from the range of 50 °C to 75 °C. to the range of 95 °C to ISO °C. In addition, high-temperature, low-sag conductors rated at from 150 °C to 254°C have come into widespread use. As a result, the internal temperature difference between core and surface of conductors can no longer be neglected in all eases. This phenomenon is discussed in more detail in 4.4.7.
In terms of conductor resistance, if the core of the stranded conductor is more than a few degrees hotter than the surface, the resistance of the conductor should he calculated for the average conducting wire.