Transmission-line series resistance and shunt conductance cause three primary effects: attenuation, distortion, and power losses.
Attenuation
When constant series resistance and shunt conductance are present, voltage and current equations are modified. The propagation constant indicates that voltage and current waves consist of both forward and backward traveling components. These waves attenuate as they propagate, with the attenuation factor related to the resistance and conductance. In a distortionless line, where the resistance-to-inductance ratio equals the conductance-to-capacitance ratio, the attenuation factor remains constant. Waves travel without changing shape, only decreasing in magnitude.
Distortion
For sinusoidal steady-state waves, the propagation constant determines both phase velocity and attenuation. In a lossless line, the phase velocity is constant, with no attenuation. In a distortionless line, waves of all frequencies travel at a constant velocity with uniform attenuation, crucial for maintaining signal integrity over long distances. Above a certain frequency, typically 1 MHz for practical transmission lines, most lines behave as distortionless.
Power Losses
Power losses in transmission lines result from series resistance and shunt conductance. Resistance-related losses occur due to current flow through the line, while voltage across conductors causes conductance-related losses. Such losses can stem from insulator leakage, corona effects in overhead lines, and dielectric losses in cables. Analyzing transients on lossy lines with constant parameters—resistance, inductance, conductance, and capacitance—is complex, especially considering skin effects.
Overvoltages in power systems are classified as lightning surges, switching surges, and power frequency overvoltages. Lightning, a major cause, involves complex cloud interactions. Charge separation within clouds, falling raindrops carrying negative charges, and upward air drafts carrying positive charges contribute to the formation of lightning. When the voltage gradient exceeds the air's breakdown strength, a downward leader connects with an upward leader from the ground, causing a surge.
