新版轮机英语unit28_paralleling_operation_of_alternators.pptVIP

Lesson 28 Parallel operation of generators (发电机的并联运行) Three phases alternators arranged for parallel operation require a considerable amount of instrumentations(仪表）. This will include ammeters, wattmeter, voltmeter, frequency meter and a synchronizing device(同步装置). Reverse power protection(逆功率保护) is provided to alternators since current protection cannot be used. Alternatively various trips(跳闸机构) may be provided in the event of prime mover failure to ensure that the alternator does not act as a motor. The operation of paralleling two alternators requires the voltages to be equal and also in phase（同相）. The alternating current output of any machine is always changing, so for two machines to operate together their voltages must be changing at the same rate or frequency, must be reaching their maximum (or any other value) together. They are then said to be ‘in phase’.（同相） Use is nowadays made of a synchroscope(同步指示器) when paralleling two a.c. machines. The synchroscope has two windings(绕组) which are connected one to each side of the paralleling switch（联络开关）. A pointer(指针) is free to rotate and is moved by the magnetic effect(磁效应） of the two windings. When the two voltage supplies are in phase the pointer is stationary（固定的，静止的） in the 12 o’clock position. If the pointer is rotating then a frequency difference exists and and the dial（表盘，刻度盘） is marked for clockwise rotation FAST and anti-clockwise rotation SLOW, the reference being to the incoming machine frequency. To parallel an incoming machine to a running machine therefore it is necessary to ensure firstly that both voltages are equal. Voltmeters are provided for this purpose. Secondly the frequencies must be brought into phase. In practice the synchroscope usually moves slowly in the FAST direction and paralleling switch is closed（合闸） as the pointer reaches the 11 o’clock position. This results in the incoming machine immediately accepting a small amount of load. A set of three lamps may also be p