HVDC stands for high voltage direct
current, a well-proven technology used to transmit electricity over long
distances by overhead transmission lines or submarine cables
HVDC stands for high voltage direct current, a
well-proven technology used to transmit electricity over long distances by
overhead transmission lines or submarine cables. It is also used to
interconnect separate power systems, where traditional alternating current (AC)
connections cannot be used.
In an HVDC system, electric power is taken from one point in
a three-phase AC network, converted to DC in a converter station, transmitted
to the receiving point by an overhead line or cable and then converted back to
AC in another converter station and injected into the receiving AC network.
Typically, an HVDC transmission has a rated power of more than 100 MW and many
are in the 1,000 – 3,000 MW range.
With an HVDC system, the power flow can be controlled
rapidly and accurately in terms of both power level and direction. This
possibility is often used to improve the performance and efficiency of the
connected AC networks. There are three different categories of HVDC
transmission projects:
- Point-to-point transmission
- Back-to-back stations
- Multi-terminal systems.
The first commercial HVDC scheme, based on mercury arc
valves was commissioned in 1954. This was a link between the Swedish mainland
and the island of Gotland in the Baltic sea. The power rating was 20 MW and the
transmission voltage 100 kV There was a significant improvement in HVDC
technology in 1970 when thyristor valves were introduced in place of the
mercury arc valves. This reduced the size and complexity of HVDC converter
stations substantially. The use of microcomputer control equipment in today’s
projects has also contributed to HVDC’s current success as a powerful
alternative to AC power transmission.
WHY HVDC?
The reasons for selecting HVDC instead of AC for a specific
project are often numerous and complex. The most common arguments in its favour
are:
1. Lower investment cost
2. Long distance water crossing
3. Lower losses
4. Asynchronous interconnections
5. Controllability
6. Limited short-circuit currents
7. Environment.
In general, the different reasons for using HVDC fall into
two main groups, namely: - HVDC is necessary or desirable from the technical
point of view (that is controllability). - HVDC results in a lower total
investment (including lower losses) and/or is environmentally superior.
In many cases, projects are justified by a combination of
benefits from the two groups. Environmental aspects are also increasingly
important and HVDC has the advantage of a lower environmental impact than AC
since the transmission lines are much smaller and need less space for the same
power capacity. One of the most important differences between HVDC and AC is
the possibility to accurately control the active power transmitted on a HVDC
line. This is in contrast to AC lines, where the power flow cannot be
controlled in the same direct way. The controllability of the HVDC power is
often used to improve the operating conditions of the AC networks where the converter
stations are located.
Another important property of an
HVDC transmission is that it allows the interconnection of asynchronous
networks.
