In case of d.c. generating stations, there are heavy thick copper bars which acts as positive and negative terminals for the entire stations. These bars are called bus bars. While connecting the d.c. generators in parallel, the positive and negative terminals of the generators should be respectively connected to the positive and negative terminals of bus bars. If the generator is connected with reverse polarity to the bars, it results in short circuit which will cause damage to the commutators and brushes, ultimately shutting down the station. Before making the parallel operation, it should be checked for reversal polarity for the generators otherwise breakers are tripped off as a result of heavy fault current.
Consider the bus bar with positive and negative terminals as shown in the Fig. 1. The generators are connected to these bus bars for parallel operation.
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| Fig. 1 |
The generator 1 is connected to the bus bars and supplying to the load. Now generator 2 is to be connected in parallel.
The prime mover of the generator 2 is started and its speed is adjusted to the rated value. Then the switch S4 is closed. The voltmeter connected across the open switch S2 completes the circuit. Now the excitation of the generator 2 is adjusted with the help of field rheostat till the voltmeter reads zero. The zero on the voltmeter ensures that the voltage of incoming generator 2 is same as that of generator 1 or the bus bar voltage. The switch S2 is then closed which connects the incoming generator 2 in parallel with the system.
Under these conditions, the generator 2 will not take any load as its induced e.m.f. is same as bus bar voltage and there will not be any flow of current between two points at same potential. This generator is thus said to be floating on bus bars.
In order that the generator 2 should supply current, it is necessary that its induced e.m.f. should be more than the bus bar voltage. If E is induced e.m.f. and V is bus bar voltage then the current supplied I is given by,
To increase the induced e.m.f. of incoming generator 2 its field is strengthened till it takes proper share of load. At the same time the field of generator 1 is weakened to maintain the bus bar voltage constant.
In summary, following procedure should be adopted while paralleling the d.c. generators :
1. The generator which is to be connected in parallel is called incoming generator. Start the prime mover of incoming generator and adjust its speed to rated value.
2. Close the disconnected switch of the incoming generator.
3. The excitation of the incoming generator is then adjusted till it is few volts more than bus bar voltage.
4. Close the breaker of the incoming generator.
5. The shunt field rheostat of the incoming generator is adjusted to increase its voltage while the field rheostats of already connected generators are adjusted to lower their voltages till the desired load distribution which is indicated by ammeters is achieved.