in the cable sheath, and no skin effect, all of which are prevalent in an AC 

 transmission cable. Further, the DC transmission system may require only 

 one cable if the sea is used as a return circuit; however, the hazards involved 

 with so using the sea must be thoroughly investigated. Thus, the effective 

 power transmitted in a DC cable is greater than that transmitted by AC for 

 the same circuit conditions, resulting in reduced cable costs. 



A DC transmission system does have some restricting factors. There 

 is no easy way of transferring DC voltages except by AC methods at the 

 converter and inverter ends of the transmission system. Inversion equipment 

 requires a leading power factor which must be supplied by AC static or 

 synchronous capacitors. Mercury arc valves have been used as conversion 

 equipment for DC; however, this equipment requires clean room housing, 

 degassing facilities, and rebuilding approximately every 5 years. The mercury 

 arc equipment is expected to be replaced by high-voltage DC solid-state 

 conversion equipment in the near future. 



The DC transmission system has a lower cable cost than an equivalent 

 AC system but requires special transformers and converter-inverter equipment 

 at each end. A careful trade-off was therefore required before appropriate 

 transmission systems from shore-based plants could be selected for the given 

 circuit requirements. 



Figure 25 shows a preliminary system selection and indicates that 

 DC is the most cost effective system for cable lengths of 50 to 500 miles for 

 the given power levels. AC is the most cost effective for all loads up to 10 

 miles and for a 3,000-kw load at 10 and 50 miles. 



Power Level 

 (kw) 



30- 





AC 



DC 



DC 



DC 



100 — 





AC 



DC 



DC 



DC 



300 _ 





AC 



DC 



DC 



DC 



1,000 — 





AC 

 AC 



DC 



DC 



DC 



DC 



3,000 — 



AC 



- 













Figure 25. AC— DC system selection. 



74 



