The cheapest arrangement is a single conductor with a ground return. This, however, has various disadvantages. The ground-return current results in the corrosion of buried pipes, cable sheaths, and so on, due to electrolysis. With submarine cables the magnetic field set up may cause significant errors in ships' compass readings, especially when the cable runs north-south and unacceptable environmental impact. This system is shown in Figure 9.23(a). Two variations on two-conductor schemes are shown in Figure 9.23(b) and (c). The latter has the advantage that if the ground is used in emergencies, a double-circuit system is formed to provide some security of operation. Note that in the circuits of Figure 9.23(c), transformer connections provide twelve pulse operation, thereby reducing harmonics generated on the a.c. sides.
Figure 9.23 Possible conductor arrangements for d.c. transmissions: (a) ground return; (b) two conductors, return earthed at one end; (c) double-bridge arrangement