1.1 INTRODUCTION
The industrial growth of a nation requires increased consumption of energy, particularly electrical
energy. This has led to increase in the generation and transmission facilities to meet the
increasing demand. In U.S.A. till the early seventies the demand doubled every ten years. In
developing countries, like India, the demand doubles every seven years which requires
considerable investment in electric power sector.
The imperatives of supplying energy at reasonable costs coupled with the depleting reserves
of non-renewable energy sources has led to the establishment of remote generating stations–
predominantly fossil-fuel fired thermal stations at pit head. Environmental considerations also
dictate sometimes, siting of power stations at remote locations. Large hydro stations are invariably
at distances of hundreds of kilometers from load centres. The need to economize on costly
investments in generation reserves, sharing of benefits in utilizing variability in generation mixes
and load patterns have given rise to interconnection of neighbouring systems and development
of large power grids.
Remote generation and system interconnections lead to a search for efficient power
transmission at increasing power levels. The increase in voltage levels is not always feasible.
The problems of AC transmission particularly in long distance transmission, has led to the
development of DC transmission. However, as generation and utilization of power remain at
alternating current, the DC transmission requires conversion at two ends, from AC to DC at
the sending end and back to AC at the receiving end. This conversion is done at converter stations
– rectifier station at the sending end and inverter station at the receiving end. The converters
are static—using high power thyristors connected in series to give the required voltage ratings.
The physical process of conversion is such that the same station can switch from rectifier to
inverter by simple control action, thus facilitating power reversal.
The HVDC transmission made a modest beginning in 1954 when a 100 kV, 20 MW DC
link was established between Swedish mainland and the island of Gotland. Until 1970, the
converter stations utilized mercury arc valves for rectification. The successful use of thyristors
for power control in industrial drives encouraged its adoption in HVDC converters by development
of high power semiconductor devices. The device voltage rating is now in the range of 10 kV,
and current rating up to 5 kA (for 125 mm device). The highest transmission voltage reached is
± 600 kV.


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