The controlled 12-pulse converter of Fig. 4-22a is the basic element for dc power
transmission. DC transmission lines are commonly used for transmission of electric
power over very long distances. Examples include the Pacific Intertie; the
Square Butte Project from Center, North Dakota, to Duluth, Minnesota; and the
Cross Channel Link under the English Channel between England and France. Modern
dc lines use SCRs in the converters, while very old converters used mercuryarc
rectifiers.
Advantages of dc power transmission include the following:
1. The inductance of the transmission line has zero impedance to dc, whereas
the inductive impedance for lines in an ac system is relatively large.
2. The capacitance that exists between conductors is an open circuit for dc. For
ac transmission lines, the capacitive reactance provides a path for current,
resulting in additional I2R losses in the line. In applications where the
conductors are close together, the capacitive reactance can be a significant
problem for ac transmission lines, whereas it has no effect on dc lines.
3. There are two conductors required for dc transmission rather than three for
conventional three-phase power transmission. (There will likely be an
additional ground conductor in both dc and ac systems.)
4. Transmission towers are smaller for dc than ac because of only two
conductors, and right-of-way requirements are less.
5. Power flow in a dc transmission line is controllable by adjustment of the delay
angles at the terminals. In an ac system, power flow over a given transmission
line is not controllable, being a function of system generation and load.