A more common dc transmission line

A more common dc transmission line has a 12-pulse converter at each terminal.
This suppresses some of the harmonics and reduces filtering requirements.
Moreover, a pair of 12-pulse converters at each terminal provides bipolar
operation. One of the lines is energized at Vdc and the other is energized
atVdc. In emergency situations, one pole of the line can operate without the
other pole, with current returning through the ground path. Figure 4-25 shows a
bipolar scheme for dc power transmission.