Voltage above 1kv but less than 66kv are denoted as medium voltage (M.V.) and 66kv and above up to 300kv are called High voltage (H.V.)Above 300 kV and up to 765 kV are designed as extra-high voltage (E.H.V.).
- Power losses in a line are inversely proportional to system voltage does for a given amount of power to be transmitted over a given distance the transmission efficiency increases the transmission voltage increase.
- The power transferring capacity of a line increases as the square of the operating voltage. The cost of the tower, insulation and terminal equipment is proportional to voltage. As a sequence, the overall capital cost of transmission line decrease as the voltage increase.
- The p.u. resistance drop and the voltage of Conductor materials decrease as the voltage of transmission increases.
- The load-carrying capability of lines are expressed in terms of “surge impedance loading” and this loading is proportional to the square of the voltage.
Thus the system voltage very much affects the capital cost of the transmission line. The weight of the conductor, the line losses and therefore the efficiency, the voltage drop in the line and the system stability all depends on the voltage of the line.
The choice of voltage, therefore, become a major factor in the transmission line design the above consideration indicates the desirability of using extra-high and ultra-high voltages if huge power is to be transmitted over long distances.
Transmission and distribution in EHV AC transmission line
EHV AC transmission lines from the mesh network system which is superior in providing the backbone for the transmission line.
Usually, it is easy and simple for load frequency control and voltage step up and step down in transmitting power.
An EHV AC transmission line is shown in the figure.
EHV AC main transmission lines : ring network is used to receive power from the main generating power station. The power is at an extra-high voltage alternative current or high voltage direct current transmission line (generally EHV-AC lines 400 kV or 765 kV ). After this power is delivered for sub-transmission or high voltage.
High voltage transmission lines : basically high voltage transmission line use to feed voltage in industrial areas, local power stations and distribution system.
Distribution transmission lines : After coming from the above transmission AC transmission lines at distribution voltage (generally 33KV, 22KV, 11KV, 6.6KV, 3.3KV and low voltage up to 1000v) is transmitted for consumer and domestic loads.