Standardization of Transmission System Voltage

There is much variation in transmission voltages in different countries. A country adopts a system of voltage levels to suit its own requirements. Earlier individual attempts were made to fix voltage levels for higher power transmission but such an attempt had resulted in wastage of time and higher cost because of designs of varied nature. Thus, the transmission voltages had to be standardized. The various advantages of standardization of transmission voltage are:

1.      Standardization provides better facilities for research and development.

2.      The equipments can be manufactured with greater economy and reliability.

3.      Systems are easily interconnected.

Hence standardization enables to carry out joint efforts to tackle Extra High Voltage (EHV) or Ultra High Voltage (UHV) problems. By standardizing, the voltage level can be adopted for a reasonable period of time before next change. The choice of the highest system voltage for a country is a matter of great significance. It is not merely the economic factors that influence the next higher voltage but the site of power station, location and density of load, and the technological developments are also kept in mind. The next higher voltage level should also be selected on the basis of future load enhancements. The interval between the existing and the proposed voltage level should be judiciously spaced, as too small interval between the voltages will result in a short life of the proposed voltage level. At the same time too large interval would lead to heavy expenditure. It is therefore desirable that the next voltage selected should be at least two steps higher than the existing one.  

The various AC voltages adopted by different countries above 220 kV are 275, 345, 380, 400, 500, 735, 765, 1000, 1100, 1200 kV etc. The AC transmission voltages adopted in India are 220 kV, 400 kV and 765 kV. The next higher AC transmission voltage selected is 1200 kV.
Figure below shows a 765 kV Indian transmission line 

A number of High Voltage Direct Current (HVDC) schemes have been in operation throughout the world since 1954. Transmission voltages of HVDC line have increased from 100 kV to ±800 kV. The various voltages adopted by different countries for overhead HVDC lines are ±100 kV, ±250 kV, ±300 kV, ±400 kV, ±500 kV, ±600 kV and ±800 kV. The existing HVDC lines in India are of ±500 kV and a ±800 kV line between Biswanath and Agra has been under construction.