27.2
Resistance and Conductance
A conductor's DC resistance at a given temperature is influenced by its resistivity, length, and cross-sectional area. Resistivity is an inherent prop…
A conductor's DC resistance at a specific temperature depends on its resistivity, length, and cross-sectional area.
Resistivity varies by conductor metal, with annealed copper as the standard for measurement.
Hard-drawn aluminum's resistivity at 20ºC is 61% of copper's conductivity.
Factors like spiraling, temperature, frequency, and current magnitude impact resistance. Spiraling in stranded conductors increases their length and DC resistance by 1-2%.
Resistivity changes linearly with temperatures.
AC resistance is determined by real power loss and rms current.
The Skin effect increases with frequency, causing a rise in AC resistance, while for magnetic conductors, resistance depends on the current magnitude.
Conductance, accounting for real power loss between conductors or to the ground, is primarily due to insulator leakage currents and corona effects. Though these are minor compared to conductor loss.
Insulator leakage current is affected by contaminants and moisture.
Corona, caused by high electric field strength ionizing air, depends on weather conditions and conductor surface irregularities.
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Q1: What factors determine a conductor's DC resistance?
A conductor's DC resistance at a specific temperature depends on three primary factors: resistivity, length, and cross-sectional area. Resistivity is an inherent material property, with annealed copper serving as the international standard for measurement. Hard-drawn aluminum's resistivity at 20°C is 61% of copper's conductivity. These relationships form the foundation for calculating resistance in transmission line design considerations.
Q2: How does spiraling in stranded conductors affect resistance?
Spiraling in stranded conductors increases their physical length and consequently raises their DC resistance by 1-2%. This geometric effect occurs because the helical arrangement of strands creates a longer current path compared to a straight conductor of equivalent diameter. This factor is important when calculating actual resistance values for practical transmission line applications.
Q3: What is the skin effect and how does it influence AC resistance?
The skin effect causes current to concentrate near a conductor's surface at higher frequencies, increasing AC resistance. AC resistance is determined by real power loss and root mean square (rms) current. The effect becomes more pronounced in conductors with magnetic properties, where resistance also varies with current magnitude. This frequency-dependent behavior is critical for understanding transmission line performance.
Q4: How does temperature affect a conductor's resistivity?
Resistivity changes linearly with temperature over normal operating ranges. As temperature increases, a conductor's resistivity increases proportionally, which directly increases its resistance. This temperature-dependent relationship is essential for predicting conductor behavior under varying thermal conditions in transmission systems and ensuring reliable operation.
Q5: What causes conductance losses in transmission lines?
Conductance, accounting for real power loss between conductors or to ground, is primarily due to insulator leakage currents and corona effects. Insulator leakage current is influenced by accumulated contaminants and moisture. Corona occurs when high electric field strength ionizes surrounding air and depends on weather conditions and conductor surface irregularities. Though minor compared to conductor losses, these effects impact overall transmission line efficiency.
Q6: Why is corona loss dependent on weather conditions and conductor surface?
Corona loss occurs when the electric field around a conductor ionizes surrounding air, leading to conduction. This ionization is contingent upon weather conditions, which affect air density and moisture, and conductor surface irregularities that can exacerbate ionization effects. Understanding these dependencies is essential for optimizing transmission line performance and minimizing energy losses.
Q7: How do resistance and conductance relate to series impedances in transmission lines?
Resistance is a key component of series impedances in transmission systems, working alongside other parameters to determine total line impedance. Conductance represents shunt losses to ground or between conductors. Together, resistance and conductance characterize series impedances three phase line behavior, affecting voltage drop, power loss, and overall system efficiency.