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Power electronic control in electrical systems 13

Fig. 1.11 One phase of a series capacitor. (a) mechanically controlled; and (b) electronically controlled.

HVDC Light is a very recent development in electric power transmission. It has
many technical and economical characteristics, which make it an ideal candidate for
a variety of transmission applications where conventional HVDC is unable
to compete. For instance, it can be used to supply passive loads, to provide reactive
power support and to improve the quality of supply in AC networks. The
installation contributes no short-circuit current and may be operated with no trans-
formers. It is said that it has brought down the economical power range of HVDC
transmission to only a few megawatts (Asplund et al., 1998). The HVDC Light at
Hellsjo È n is reputed to be the world's first installation and is rated at 3 MW and
10 kV DC. At present, the technology enables power ratings of up to 200 MW. In
its simplest form, it comprises two STATCOMs linked by a DC cable, as illustrated
in Figure 1.13.

1.3.3 Distribution

Distribution networks may be classified as either meshed or radial. However,
it is customary to operate meshed networks in radial fashion with the help of
mechanically operated switches (Go È nen, 1986). It is well understood that radial
networks are less reliable than interconnected networks but distribution engineers
have preferred them because they use simple, inexpensive protection schemes, e.g.
over-current protection. Distribution engineers have traditionally argued that in
meshed distribution networks operated in radial fashion, most consumers are
brought back on supply a short time after the occurrence of a fault by moving
the network's open points. Open-point movements are carried out by reswitching
operations.
Traditional construction and operation practices served the electricity distribution
industry well for nearly a century. However, the last decade has seen a marked
increase in loads that are sensitive to poor quality electricity supply. Some large
industrial users are critically dependent on uninterrupted electricity supply and suffer
large financial losses as a result of even minor lapses in the quality of electricity
supply (Hingorani, 1995).