TRANSATLANTIC CABLE POWER SYSTEM 149 



driven ac control unit whenever the series tube plate voltage varies 

 more than 25 volts from the normal value. For example, a 2 per cent 

 change in input ac would change the rectifier output of 2,300 volts 

 (corresponding to a cable supply voltage of 2,000 volts) by 46 volts, 

 which would increase the series tube drop to 346 volts. This increase in 

 voltage would raise the signal current through the control winding of a 

 saturable reactor which forms one leg of a balanced bridge in the ac 

 motor control circuit, and thus cause the autotransformer to be driven 

 down, lowering the rectifier output until the series tube drop is restored 

 again to approximately 300 \'olts. 



Over voltage and Overcurrent Protection and Alarms 



While the power for the cable is electronically regulated, protective 

 features are provided to guard against abnormal cable current or volt- 

 age. The first order of protection is a ±2 per cent cable current alarm 

 given by a voltage relay which operates from the drop across a resistor 

 in the ground return circuit to the dc bays. A second voltage relay, set 

 for 5 per cent high cable current, also monitors the current in the ground 

 return side and in conjunction with the current-monitors in the high 

 voltage side, limits the cable current by operating the motor-driven 

 autotransformers until the current is within 5 per cent of normal. The 

 voltage of the ungrounded side, however, is much too high for direct 

 connected voltage or current relays. Therefore, magnetic amplifiers have 

 been used to obtain isolated metering of the cable current. Two of these 

 devices measure the current in the ungrounded side of the common 

 power supply lead to the cables. Of the three current-monitors available, 

 two must operate before turndown functions, to prevent false turndown 

 because of faulty metering. 



Voltage protection is provided by means of magnetic amplifiers in 

 shunt across the common power supply to the cable. Here, three monitors 

 are arranged so that any two can reduce power, by means of the turn- 

 down control, if the ^'oltage rises to the maximum allowable value for 

 which the voltage relays are adjusted. These monitors draw about 1.5 

 milliamperes each, but are connected on the supply side of the cable- 

 regulating resistance so as not to affect cable regulation. The,y guard 

 against excessive voltage resulting from an open circuit where voltages 

 around 4,000 volts could otherwise occur. They also guard against high 

 voltage caused by earth potentials or unbalance between ^'oltages at op- 

 posite ends of the cable. When set for a ceiling \-oltage of 2,600 volts, a 

 maximum of 3,000 volts occurs on open circuit on the first rise, after 



