A conductor's DC resistance at a given temperature is influenced by its resistivity, length, and cross-sectional area. Resistivity is an inherent property of the conductor material, with annealed copper serving as the international standard for measurement. For instance, the resistivity of hard-drawn aluminum at 20 degrees Celsius is 61% of the standard conductivity of annealed copper.
Various factors impact the resistance of a conductor. Spiraling in stranded conductors increases their length and, consequently, their DC resistance by 1-2%. Additionally, resistivity varies linearly with temperature over normal operating ranges. Frequency and current magnitude also play crucial roles; the skin effect, which intensifies with frequency, increases AC resistance. In magnetic conductors, resistance is further influenced by the current magnitude.
AC, or effective resistance, is determined by the real power loss and the root mean square (rms) conductor current. The skin effect becomes significant at higher frequencies, causing current to concentrate near the conductor's surface, thereby increasing AC resistance. This effect is more pronounced in conductors with magnetic properties, where resistance varies with the current's magnitude.
Conductance, associated with real power loss between conductors or to the ground, is primarily due to insulator leakage currents and corona effects. Although these losses are relatively minor compared to conductor losses, they are significant in the overall assessment of a transmission line's efficiency. Insulator leakage current is influenced by accumulated contaminants and meteorological conditions, particularly moisture.
Corona occurs when the electric field around a conductor ionizes the surrounding air, leading to conduction. Corona loss is contingent upon weather conditions and the conductor's surface irregularities, which can exacerbate ionization effects. Understanding these factors is essential for optimizing transmission line performance and minimizing energy losses.
