TRANSATLANTIC CABLE POWER SYSTEM 



157 



disturbances. While the system is stable in the sense that the transient 

 due to a perturbation will disappear in a finite time once the disturbance 

 has been withdrawn, the range of possible equilibrium positions (disre- 

 garding the overvoltage protective feature) is extremely wide — from 

 perhaps 1,300 to 2,500 volts at the cable terminals. The upper limit of 

 2,500 \-olts is set by the maximum output available from one power 

 plant; this also sets the lower limit of the associate power plant at the 

 far end of the cable. This situation is illustrated diagrammatically in 

 Fig. 9. 



The behaviors of the servomechanisms at the two ends of a given cable 

 are nearly enough alike that the repeated introduction of simulated earth 

 potential in the laboratory was found not to disturb substantially the 

 equilibrium point. This was true for earth potentials of either polarity 

 up to 1,000 volts and with these potentials introduced at am^ point along 

 the artificial cable. A rate of change of earth potential of 20 ^'olts per 

 second was adopted in these tests with the thought that such values 

 would be realistic. 



With regard to the long-term stability of the equilibrium condition 

 described above, it is, of course, important that the controls which es- 

 tablish the cable current at the two ends of the same cable be adjusted 

 for very nearly the same value. Unless this is done, the cable voltage at 

 (jue end will gradually increase or decrease and the voltage at the other 

 end will move equally in the opposite direction. This would eventually 



3000 



UJ 



O 

 < 



^ 2000 - 



> 



CD 

 < 



1000 



221 



1-2= _ 

 224.75 MA 



224.75 MA 



Apt B, 



->i r^— 



225.25MA 



c, 



i; 



I ^il I |Bi I 



AREA OF 

 -EQUILIBRIUM 



C, DiCpDs 



225.25 MA 



223 225 227 



CABLE CURRENT IN MILS 



229 



1-2 



Fig. 9 — Equilibrium diagram. 



