Direct Current Transmission
Power Transmission was initially carried out in the early 1880s using Direct Current (d.c.). With the
availability of transformers (for stepping up the voltage for transmission over long distances and for stepping
down the voltage for safe use), the development of robust induction motor (to serve the users of rotary power),
the availability of the superior synchronous generator, and the facilities of converting a.c. to d.c. when
required, a.c. gradually replaced d.c. However in 1928, arising out of the introduction of grid control to the
mercury vapour rectifier around 1903, electronic devices began to show real prospects for high voltage direct
current (HVDC) transmission, because of the ability of these devices for rectification and inversion. The most
significant contribution to HVDC came when the Gotland Scheme in Sweden was commissioned in 1954 to be
the World's first commercial HVDC transmission system. This was capable of transmitting 20 MW of power
at a voltage of -100 kV and consisted of a single 96 km cable with sea return.
With the fast development of converters (rectifiers and inverters) at higher voltages and larger currents, d.c.
transmission has become a major factor in the planning of the power transmission. In the beginning all HVDC
schemes used mercury arc valves, invariably single phase in construction, in contrast to the low voltage
polyphase units used for industrial application. About 1960 control electrodes were added to silicon diodes,
giving silicon-controlled-rectifiers (SCRs or Thyristors).
In 1961 the cross channel link between England and France was put into operation. The a.c. systems were
connected by two single conductor submarine cables (64km) at ± 100kV with two bridges each rated at 80
MW. The mid-point of the converters was grounded at one terminal only so as not to permit ground currents to
flow. Sea return was not used because of its effect on the navigation of ships using compasses. The link is an
asynchronous link between the two systems with the same nominal frequency (60Hz).
The Sakuma Frequency Changer which was put into operation in 1965, interconnects the 50Hz and the 60Hz
systems of Japan. It is the first d.c. link of zero length, and is confined to a single station. It is capable of
transmitting 300 MW in either direction at a voltage of 250 kV.
In 1968 the Vancouver Island scheme was operated at +250 kV to supply 300 MW and is the first d.c. link
operating in parallel with an a.c. link.
In 1970 a solid state addition (Thyristors) was made to the Gotland scheme with a rating of 30MW at 150kV.
Also in 1970 the Kingsnorth scheme in England was operated on an experimental basis. In this scheme
transmission of power by underground d.c. cable at ± 200 kV, 640 MW is used to reinforce the a.c. system
without increasing the interrupting duty of a.c. circuit breakers.
The first converter station using exclusively Thyristors was the Eel River scheme in Canada. Commissioned in
1972, it supplies 320 MW at 80 kV d.c. The link is of zero length and connects two a.c. systems of the same
nominal frequency (60Hz).