The purpose of the study reported here was to examine generic cost differences between direct current (dc) and alternating current (ac) systems and to identify situations in which dc is clearly advantageous for long distance and bulk power transport. The study was also designed to determine the value of the dc technology when applied to transmission systems. This report presents cost comparisons between ac and dc substations and transmission lines as a function of capacity and voltage. It also presents a comparison of dc versus ac for increasing the capacity of existing corridors. Direct-current link operating stategies for enhancing the performance of the associated ac network are illustrated. Possible opportunities for simplification and cost reduction of dc converter stations are described. Both current and expected future enhancements for ac system operation are also identified to assist in making comparisons between equivalent systems. Information is presented to enable comparisons sufficiently detailed to determine ''cost break-even distance,'' which is the transmission length at which the savings in dc line costs (compared to ac) equals the additional costs of the dc converter stations (compared to ac substations). However, it is emphasized that proper use of the concept requires the inclusion of all costs of implementation of those attributes available in the two technologies which are of sufficient value to the power system to be included in determining equivalent systems. The report attempts to bring together useful cost and performance information on ac and dc power transmission for the use of electric utility system in achieving economic and reliable power system designs when considering system additions, expansions, or modifications.

The established principles of multiterminal dc transmission are applied to the planning needs of practical projects. Without further detailed review of operating techniques, possibilities and contraints that are intended to be useful in the assessment of potential projects are discussed. Reference is made to the plans for certain schemes and the status of dc circuit breakers is reviewed.

A preferred mode of operating a parallel connected HVDC transmission system would be with at least one of the major rectifier stations in voltage control. Such a station could be realized using diode rectifier units.

The feasibility of a series connection of stations in a multiterminal HVDC transmission system is demonstrated. Techniques were developed to control this system, and the operation of the system was studied on a large power system simulator. The performance of the system was shown to be similar to conventional point to point systems with most control functions being carried out by local controls. An overall system control is shown to be necessary only for optimizing load distributions. Given adequate tap changer ranges on the converter transformers the increase in valve damping and system reactive power requirements are shown to be moderate. The series connected multitermimal system is shown to have a number of advantages over the alternative parallel connected system for certain applications.

High voltage dc transmission was perhaps the earliest major application of power electronics by the electric utility industry. Modern electronics has transformed hv dc transmission technology, and different solid-state systems within converter stations now routinely handle power in the range of mW to GW.

Due to the oscillatory nature of the combination of ac system and ac harmonic filters in an HV-DC transmission scheme, transformer magnetising inrush currents have particularly important effects on the overvoltages and transient behaviour of the scheme.