High voltage transmission is a combination of HVAC and HVDC transmission systems.

HVAC transmission:
In this current decreases with the increase in voltage. Hence the corresponding i2R losses reduce. The only drawback is the increased cost of transformers, transmission towers, circuit breakers & switches due to the increase in voltage in the upper ranges of a.c. transmission voltages.

HVDC transmission:
The only economical suitable lines for bulk power transmission are HVDC lines. As only IR drop is involved, the regulation of voltage is very less in DC. Power flow is controllable & easily reservable through DC link. The main drawback is the installation of DC switchgear & converters are expensive.