Page 372 - Power Electronics Handbook

362 Power semiconductor circuit applications
14.9, uses a cycloconverter, operating in reverse mode to those illustrated
in Chapter 10, to provide a fixed-frequency output, which is then filtered to
give a highly stable sine wave supply. Such a system requires no
maintenance, is reliable and lightweight, and gives better performance by
use of feedback since the response time is much shorter than in mechanical
systems. Closer control of frequency is also possible, with a better match of
the amplitude and phase between different phases of a three-phase system,
even when the load is unbalanced.
x? - 3 Fixed

Variable
Variable
frequency .
from
load
alternator ~ Cycloconverter , c frequency

14.2.3 High-voltage d.c. transmission (HVDC)

A.C. power is a convenient source to generate and to convert from one
voltage level to another, for example by using transformers, and it is
therefore the supply most often used both as a power source and for
transmission. However, for very long distances, both over land and under
the sea, a.c. can result in relatively high transmission-line losses, and in
these instances d.c. is preferred. In these applications the a.c. supply is first
transformed to a high-voltage and then converted to a d.c. voltage for
transmission, the high-voltage d.c. then being converted back to a.c. at the
other end, before being transformed down to a low voltage. This is
illustrated in Figure 14.10.

D.C.

Ftgure 14.10 A high-voltage d.c. transmission system

The power source used for high-voltage d.c. transmission is usually a
thyristor converter, made from series-connected devices to obtain the
high-voltage capability. Commonly a twelve-pulse fully controlled bridge is
used, providing both the rectification and inversion operation, the ripple
being low enough to enable fairly simple filters to be used. These filters
minimise ripple voltage, which would result in additional transmission
losses.