Explain the Power Transmission Lines

Transmission and distribution of electric power is accomplished by using overhead lines or underground cables. Overhead lines are normally used for a.c. transmission systems with high voltage levels (e.g. 132 kV, 400 kV). In densely populated areas where overhead lines are not usually used (for safety or other reasons) underground cables are used. Underground cables are used for relatively short a.c. transmission distances or for d.c. transmission at longer length. This is due to the fact that the conductor of a cable is quite close to the earthed metallic sheath which results in very large shunt capacitance of the cable as compared to that of an overhead line. For a.c. transmission, this gives rise to capacitive current (charging current) which flows in the cable causing I² R loss and troublesome generation of lagging reactive power. For d.c. transmission, the shunt capacitance charging current ceases when the cable capacitance charges to the applied transmission voltage.