Ans: High voltage transmission is subdivided into HVAC and HVDC transmission systems.
(i) HVAC transmission: Advantages of HVAC transmission are as follows: As the voltage is increased, the current carried by the conductors decreases. The i2R losses correspondingly get reduced. However the cost of transmission towers, transformers, switches and circuit breakers rapidly increases with increase in voltage, in the upper ranges of a.c. transmission voltages.
(ii) HVDC transmission:
Advantages They (HVDC lines) are economical for bulk power transmission. The voltage regulation problem is much less in DC since only IR drop is involved. There is easy reversibality and controllability of power flow through a DC link. Also there is considerable insulation economy.
Limitations: The systems are costly since installation of complicated converters and DC switchgear is expensive. The converters require considerable reactive power. Lack of HVDC circuit breakers hampers network operation. Moreover there is nothing like DC transformer; voltage transformation has to be provided on the a.c. sides of the system.
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