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Power electronic control in electrical systems 241

supercapacitors represent a state-of-the-art technology with potential applications in
power quality. When compared with the lead±acid batteries a supercapacitor is
capable of releasing energy a lot more rapidly and can address energy storage
applications in the milliseconds to approximately 100 s. The energy storage capability
per volume unit is also higher than a conventional capacitor.
Finally, there exist systems based on hydrogen storage and technology associated
with double-layer capacitors. All these systems can offer a solution for different
energy storage needs and power requirements.

6.7 HVDC

High voltage direct current power transmission, although not part of the grid to
distribute power to customers, is a significant technology used successfully to trans-
mit power in a more economic way over long distances, to connect two asynchronous
networks and in many other cases. The idea and the relevant technology were under
development for many years and started as early as in the late 1920s. However, the
application became commercially possible in 1954 when an HVDC link was used to
connect the island of Go È tland and the mainland of Sweden. The power of that project
was 20 MW and the DC voltage was 100 kV. At the time, mercury arc valves were
used to convert the AC into DC and vice versa. The control equipment used vacuum
tubes.
Since then of course, the semiconductor field went through a revolution mainly
due to the development of the thyristor and other devices as presented in Chapter 5.
It is these continuous developments that drive the changes and the improvements in
the HVDC technology.
The thyristor or SCR was developed by General Electric and became commercially
available in the early 1960s in ratings of approximately 200 A and 1 kV. However, it
took more than a decade for the device to mature and be used successfully in
commercial high-power applications such as HVDC. The mercury arc valves then
were replaced by thyristor valves, which reduced the complexity and size of the
HVDC converter stations a great deal. The introduction of digital control and
microcomputers has also made its contribution to the further development of the
technology.
Today, further improvements can be expected and some are already in place with
the availability of the IGBT to build HVDC systems based on the VSC topologies
discussed in an earlier section of this chapter. These state-of-the-art developments
will be discussed in the following sections of this chapter.
In simple terms HVDC is the conversion of AC into DC using a phase-controlled
converter with thyristors and then transfer the power as DC into the other side which
again converts the DC into an AC with a similar converter. A simple diagram
representing an HVDC system and its major equipment is shown in Figure 6.67.
There are two AC systems interchanging their role of a sending and receiving end
power system shown as AC System 1 and AC System 2 (Figure 6.67). These systems
are connected through a transformer with the power electronics converter based on
thyristor technology. These converters (Converter 1 and Converter 2) operate as a line