The established method of transmitting large quantities of electrical energy is to use three-phase alternating-current. However, there is a limit to the distance that bulk a.c. can be transmitted unless some form of reactive compensation is employed. For long overhead lines either alternating current with reactive compensation (connected in shunt or series) or direct current may be used. If undersea crossings greater than around 50 km are required, then, because of the capacitive charging current of a.c. cables, d.c. is the only option.
Figure 9.1 shows the distances at which d.c. becomes cheaper than a.c for overhead line and submarine cable transmission. The terminal converter stations of a
d.c. scheme are more expensive than a.c. substations but the overhead line is cheaper. The choice of whether to use a.c. or d.c. is usually made on cost. With the increasing use of high-voltage, high-current semiconductor devices, converter stations and their controls are becoming cheaper and more reliable, so making d.c. more attractive at shorter distances.
The main technical reasons for high-voltage direct-current (h.v.d.c.) transmission are for the:
- 1. interconnection of two large a.c. systems without having to ensure synchronism and be concerned over stability between them (for example, the UK-France crosschannel link of 2000 MW);
- 2. interconnection between systems of different frequency (for example, the connections between north and south islands in Japan, which use 50 and 60 Hz systems);
- 3. long overland transmission of high powers where a.c. transmission towers, insulators, and conductors are more expensive than using h.v.d.c. (for example, the Nelson River scheme in Manitoba - a total of 4000 MW over more than 600 km).
Electric Power Systems, Fifth Edition. B.M. Weedy, B.J. Cory, N. Jenkins, J.B. Ekanayake and G. Strbac. © 2012 John Wiley & Sons, Ltd. Published 2012 by John Wiley & Sons, Ltd.
Figure 9.1 Costs of d.c. and a.c. transmission
The main advantages of h.v.d.c. compared with h.v.a.c. are:
- 1. two conductors, positive and negative to ground, are required instead of three, thereby reducing tower or cable costs;
- 2. the direct voltage can be designed equivalent to the peak of the alternating voltage for the same amount of insulation to ground (i.e. Vd.c. = у2Va.c);
- 3. the voltage stress at the conductor surface can be reduced with d.c., thereby reducing corona loss, audible emissions, and radio interference;
- 4. h.v.d.c. infeeds do not increase significantly the short-circuit capacity required of switchgear in the a.c. networks;
- 5. fast control of converters can be used to damp out oscillations in the a.c. system to which they are connected.
Disadvantages of h.v.d.c. are:
- 1. the higher cost of converter stations compared with an a.c. transformer substation;
- 2. the need to provide filters and associated equipment to ensure acceptable waveform and power factor on the a.c. networks;
- 3. limited ability to form multi-terminal d.c. networks because of the need for coordinated controls and the present lack of commercially available d.c. circuit breakers.