Paralelle Operation of Alternators:
Synchronizing:The operation of connecting two alternators in parallel is known as synchronizing.Certain conditions must be fulfilled before this can be effected. The incoming machine must have its voltage and frequency equal to that of the bus bars and, should be in same phase with bus bar voltage. The instruments or apparatus for determining when these conditions are fulfilled are called synchroscopes.
Synchronizing can be done with the help of
(i) dark lamp method or (ii) by using synchroscope.
Reasons for operating in parallel:
a) Handling larger loads.
b) Maintenance can be done without power disruption.
c) Increasing system reliability.
d) Increased efficiency.
Conditions required for Paralleling Operation of Alternators:
The figure below shows a synchronous generator G1 supplying power to a load, with another generator G2 about to be paralleled with G1 by closing switch S1.What conditions must be met before the switch can be closed and the 2 generators connected in parallel?
Paralleling 2 or more generators must be done carefully as to avoid generator or other system component damage. Conditions to be satisfied are as follows:
a) RMS line voltages must be equal.
b) The generators to be paralleled must have the same phase sequence.
c) The oncoming generator (the new generator) must have the same operating frequency as compared to the system frequency.
General Procedure for Paralleling Generators:
Consider the figure shown below. Suppose that generator G2 is to be connected to the running system as shown below:
1. Using Voltmeters, the field current of the oncoming generator should be adjusted until its terminal voltage is equal to the line voltage of the running system.
2. Check and verify phase sequence to be identical to the system phase sequence. There are 2 methods to do this:
i. One way is using the 3 lamp method, where the lamps are stretched across the open terminals of the switch connecting the generator to the system (as shown in the figure below). As the phase changes between the 2 systems, the lamps first get bright (large phase difference) and then get dim (small phase difference). If all 3 lamps get bright and dark together, then the systems have the same phase sequence. If the lamps brighten in succession, then the systems have the opposite phase sequence, and one of the sequences must be reversed.
3. Check and verify generator frequency is same as that of the system frequency. This is done by watching a frequency of brightening and dimming of the lamps until the frequencies are close by making them to change very slowly.
4. Once the frequencies are nearly equal, the voltages in the 2 systems will change phase with respect to each other very slowly. The phase changes are observed, and when the phase angles are equal, the switch connecting the 2 systems is closed.
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