Patent GB-789517-A (GB)
Title: Improvements in or relating to polyphase alternating current systems
Inventor: CHILMAN JOHN ALFRED
IPC: H02J3/46
Filed: 1954-10-12
Abstract: 789,517. Automatic control systems for alternators; synchronizing. ROTOL, Ltd. Oct. 7, 1955 [Oct. 12, 1954], No. 29400/54. Class 38 (4). Load sharing between three or more parallel connected' alternators is effected by automatically adjusting the datum setting of the governor of the prime mover driving the first alternator in order to keep the load current of that attenuator in a desired ratio to that of the second alternator, similarly adjusting the governor of the second prime mover to keep the load current of the second alternator in a desired ratio to that of the third alternator, and so forth in a closed chain back to the first alternator. The governors may also be adjusted to effect initial synchronizing, the synchronizers subsequently being switched to act either in response to a master speed governor or to effect the load sharing itself. In one embodiment, Fig. 1, air turbines T1 ... T4 drive alternators A1 ... A4 which are paralleled on to bus-bars Ba, Bb, Bc. Each turbine has a governor G1 ... G4 the datum setting of which is controlled by its two-phase servomotor M1 ... M4. Each servomotor has one phase winding connected across the output of its associated alternator, and the other phase winding in parallel with the opposed secondaries of two current transformers, e.g. CTa1, CTb1, one measuring the output current of the associated alternator, the other measuring the output current of the next alternator in the chain. Thus the governor is adjusted until the output current transformers CTa1, CTb1, &c. are equal. In a modification, Fig. 2 (not shown), each alternator is connected to the bus-bars by a circuit-breaker which carries auxiliary contacts which, when an alternator is disconnected from the bars, disconnects one phase of the governor adjusting motor, and links the preceding motor to the next current transformer so as to re-make the control chain. In another modification, Fig. 3 (not shown), the current transformers are in series opposition instead of in parallel. In a third modification, Fig. 4, each prime mover governor, e.g. G1, is controlled both by the loadsharing motor M1 and by a synchronizing machine SYN1 of the kind disclosed in Specification 516,613. For synchronizing, the two stator windings are connected to the alternator and to the bus-bars respectively to control the governor until the alternator is sufficiently near synchronism when the breaker TB is closed and contacts 22, 23 change-over to short-circuit winding 21 and connect winding 23 as a twophase winding with one phase connected across the alternator output and the other to the output of a pick-off 29 operated by a centrifugal governor 25 driven by a synchronous motor 26 from the bus-bars. Alternatively the winding may be connected to a three-phase source of constant frequency. In a fourth modification, Fig. 5 (not shown), the synchronizing machine is switched to become the load-sharing machine when the main breaker is closed. Specifications 743,669, 767,057 and 789,518 also are referred to.