The Natural Load
The characteristic impedance Z0 (equation 3.7) is known as the surge impedance if the line is considered to be lossless and all resistances are neglected. When a line is terminated in its characteristic impedance the power delivered is known as the natural load. For a loss-free line supplying its natural load the reactive power absorbed by the line is equal to the reactive power generated, that is.
At this load, V and I are in phase all along the line and optimum transmission conditions are obtained. In practice, however, the load impedances are seldom in the order of Z0. Values of Z0 for various line voltages are as follows (corresponding natural loads are shown in parentheses): 132 kV, 150 V (50 MW); 275 kV, 315 V (240 MW); 380 kV, 295 V (490 MW). The angle of the impedance varies between 0 and —15°. For underground cables Z0 is roughly one-tenth of the overhead-line value. Lines are operated above the natural loading, whereas cables operate below this loading.