Question

a) State the necessary operations for paralleling an A.C. generator;

b) With the aid of a sketch, show how the emergency synchronizing lamps are connected;

ANS

a)Two alternators may be paralleled safely by having both their voltages the same, i.e.. Incoming voltage the same as the bus bar voltage, this is controlled by the AVR. Their frequencies must also be equal and in phase, this can be controlled by adjusting the speed control of the incoming machine. To ensure the machines are in phase, a synchroscope is used, it is selected to the incoming machine, the synchroscope should be turning in the "fast" direction ( clockwise ) slowly. When the pointer is at the five to twelve position, the breaker for the incoming machine is closed. In this way the incoming machine will immediately assume some load, this method is used to stop the machine tripping out on reverse power when the breaker is first closed.

b)Emergency synchronizing lamps are used to parallel the alternators by using a set of three lamps, when the frequencies of the alternators are different, the lamps will brighten and darken depending on the speed of the incoming machine being fast or slow. The correct moment for synchronising being when the key lamp ( top lamp ) is dark and the other two are equally bright.,