GB714237A - Improvements relating to alternating current electric generating plant - Google Patents

Abstract

714,237. Supply systems depending on the use of two or more machines. ENGLISH ELECTRIC CO., Ltd. Nov. 27, 1953 [Dec. 3, 1952], No. 30650/52. Class 38(4) An alternator particularly a standby machine, for operation on an A.C. system in parallel with other generators, is started as an induction motor supplied from the A.C. system and, as synchronous speed is approached, an exciter driven by the machine builds up and supplies D.C. excitaton to the machine so that it automatically becomes synchronized with the system, after which the prime-moverwhich had previously been turned by the machine-is automatically. rendered effective to drive the machine as a generator. In the arrangement shown, the field winding 10b of the alternator 10 is of the distributed type and is brought out to slip rings 10c so that it may be connected as a three-phase starting winding controlled by a liquid resistor 14, one phase connection including the armature 13a of a D.C. exciter mounted on the shaft of the alternator. To start. the machine, a switch 17, normally biased to the " neutral " position, is temporarily moved to the " close " position to energize a closing coil 19 for the main circuit breaker 12 which connects the stator 10a of the machine to the A.C. system 11. The energizing circuit includes contacts 14a, 15a which are closed only when the resistor 14 and exciter field rheostat 15 are in the starting positions. Operation of the circuit breaker closes auxiliary contacts 12a to energize a contactor 20 over a circuit comprising limit switch contacts 14b closed except when the resistor 14 is in the " running position and normally closed contacts 21a on a contactor 21. At contacts 20b contactor 20 energizes a reversible servomotor 22 in a direction for reducing the value of resistor 14, so that machine 10 start's up as an induction motor, driving its prime mover, which may be a Diesel engine. Exciter 13 builds up as synchronous speed is reached and machine 10 pulls into synchronism. When resistor 14 has been fully cut out limit switch 14b opens to stop motor 22 and further limit switches 14c, 14d close. 14c causing energization of a throttle controller, so that the Diesel engine becomes effective and drives machine 10 as a generator, whilst contacts 14d cause energization of a further servomotor 24 which proceeds to increase the output of the exciter by cutting out resistance 15 from its field circuit, an interlock 15b stopping the motor when the resistance has been fully cut out. An automatic regulator 16, having a control resistor in series with resistance 15. is then effective to regulate the voltage or power factor of the machine 10. A switch automatically operated by the resistance 14, on reaching the " running " position may disconnect one section of the field 10b and short-circuit it upon itself so that it acts as a damping winding. To shut down the plant, switch 17 is temporarily moved to the "trip" position to energize trip coil 12b which opens the main circuit breaker 12. Auxiliary contacts 12d, 12e cause the servomotors 22, 24 to return the resistors 14, 15 to the starting position. Contacts 25 prevent closure of contactor 12 if the system voltage is low. Specification 579,570 is referred to.