The initial application of electricity started with the use of direct current. The first central electric
station was installed by Edison in New York in 1882 which operated at 110 V DC The invention of
transformer and induction motor initiated the use of AC. The polyphase induction motors which
serve the majority of industrial and residential purposes are simpler and rugged in construction and
cheaper as compared to DC motors of the same ratings. The advantages of 3-phase AC almost
eliminated the use of DC systems except for some specific applications in electrolytic processes and
adjustable speed motor drives. The main technical problems of long distance power transmission
using AC are :- voltage regulation associated with reactive power balance, steady state, transient
state and dynamic stability of the system under different load conditions and also under outage
conditions. In view of these problems with AC, the DC transmission has staged a come back in the
form of high voltage DC transmission.
The first commercially used HVDC link (20 MW, 100 KV), in the world was built in 1954 between
the mainland of Sweden and the island of Gotland. In 1961, an under water DC link was set up
between England and France. Since then the technique of power transmission by HVDC has been
continuously developed. In 1970, thyristor valves replaced the valves based on the mercury-arc
technique. To date the biggest HVDC transmission is ITAIPU in Brazil (two bipoles, 6300 MW and
300 KV). In India, the first HVDC line is Rihand-Delhi ( 500 KV, 800 MW).