factor of 0.85 lagging was selected for analysis. Stability problems due to 

 system synchronization were not investigated since only a single generating 

 source was considered. Circuit stability was limited to the ability of the 

 system to respond from no load to full load and vice versa. To establish 

 circuit protection requirements, loads under steady-state voltage and 

 transients caused by a fault condition were evaluated. 



Early in the study it was established that overall electrical character- 

 istics could not be clearly defined that would be suitable for all lengths of 

 transmission cable under consideration. Therefore, an analysis was initiated 

 for moderate cable lengths of 600 to 30,000 feet which would be applicable 

 to surface power sources. 



Alternating Current Versus Direct Current. The design constraint 

 established for the study program required the delivery of 480-volt, 3-phase 

 60-Hertz AC power to the load module. For a DC transmission system, this 

 constraint would require additional transformation, conversion, or inversion 

 equipment at each end of the transmission cable. The primary advantage of 

 using DC is the reduction in cable size for an equivalent power level. In 

 addition, transmission cable losses are substantially reduced with DC. For 

 loads over the distance of 600 to 30,000 feet, it was determined that the 

 savings in transmission losses and cable size would not be offset by the 

 additional cost of DC terminal equipment at each end of the transmission 

 cable. 



For each voltage level, AC costs were found to be less than DC for 

 cable lengths of 600 to 30,000 feet. Therefore, an AC transmission system 

 was considered optimum for moderate cable lengths because of simplicity, 

 reliability, availability of hardware (including control devices), and low 

 acquisition cost. 



Frequency. Transmission frequencies above and below 60 Hertz were 

 evaluated in the study. As frequency increases, the impedance of the trans- 

 mission cable also increases, requiring a larger and heavier cable for a given 

 power level and cable length. In addition, frequency conversion equipment 

 would be required. 



The total impedance of the transmission cable decreases with 

 decreasing frequency down to the DC resistance at zero (DC) frequency. A 

 reduction in cable sizes is possible, but this advantage is quickly offset by 

 limited availability of low-frequency equipment for the load module. The 

 requirement for conversion equipment at the load module results in added 

 costs for a hull to house the equipment. Consequently, the 60-Hertz 

 frequency was selected as the optimum AC power transmission frequency 

 for cable lengths of 600 to 30,000 feet. 



66 



