In this section the author describes how the loading capacity changes for overhead lines, and what are the important points to know regarding the power transmission in overhead lines.
As you can see in the figure number 21.1,
- As the length of the overhead line increases, the loading capacity of the lines decreases.
- For a 80km line, the loading capacity is 3.0 ( pu ) while for a 960 km, the loading capacity is less than 0.5 pu
- It is non linear graph. It is not linear decrease of loading capacity vs length of line. It decreases gradually, and never goes to zero.
One thing to note is that the above line is uncompensated line. If you want to transfer power for large distances, compensation would be required. Compensation will increase the loading capacity of long distance line. But do we compensate the line? Well there are many methods available which are discussed in later sections of the chapter.
The author describes about the X/R ratio. As the resistance is very less compared to inductance of lines, the resistance can be ignored.
We all know that if you connect linear inductor and capacitor of equal value in series, we get resonance. This is linear resonance. But when we connect non-linear inductor and capacitor in series, in the even of disturbance, there is overvoltage and overcurrent in line. This effect is called Ferror resonance.
With respect to overhead lines, ferro resonance occurs when the saturation of transformer takes place. In order to prevent this, the shunt capacitors have to be disconnected quickly.