Parallel operation of DC generators

PARALLEL OPERATION OF DC GENERATORS

From the view of connections there are two types named series and parallel. In series connection one end is connected to supply and other end is connected to other device. In parallel connection every device is connected to both positive and negative terminals

* In series connections voltage is differ and current is same, due to this reason if one generator fails to working the other devices stop working.

* In parallel connection current is differ and voltage is same, due to this reason if one generator fails to working the other devices run without any disturbance.

Advantages of parallel connection over series connection:

1.Continuity and Reliability of service

2. Maintenance and repair

3. efficiency

4. Increase in plant capacity

The below figure shows the two generators connected in parallel:

When two generators are in running condition the current supplied by the generator is necessary that its induced EMF should be more than the bus bar voltage.

At starting position the prime mover provides the Mechanical energy and the speed is adjusted by the field rheostat. One another advantage in connecting parallel is the load sharing by the two generators.

* If any one of generators fails the other will work without any disturbance to remove the failed generator we have to open the switch S1 and S3 while the voltmeter of another generator shows zero readings or equal potential between induced EMF and bus bars.

* If the generator is connected in reverse polarity to the bars it results in short circuit which causes damages to the machines and occurs a change to shut down the station.

If E = Induced EMF

V = Bus bar voltage

Then current supplied by I is given as

Ia = E – V/Ra

Proceeding:

The below steps should adopted while operating the generators in parallel:

1. Start the prime mover of generator and adjust with rated speed.

2.Close the disconnected switch of the Incoming generator.

3. The generator excitation is adjusted to few volts more than bus bar voltage.

4. Close the generator breaker.

5. The voltage is increased by adjusting the field rheostat.

Differences between the generator operated in parallel:

	Shunt Generator	Series Generator	Compound Generator
1.	No equalizing bar is required	Equalizing bar is needed to share the load	Equalizing bar is needed to share the load (But for satisfactory operation of compound type no need of equalizers)
2.	Two parallel shunt generators having equal no load voltage share the load in such a ratio that the load current of each machine produces the same drop in each generator	If no load voltage E1 and E2 of the two generators are initially same, generators supply equal currents and have equal resistance for its field winding.	The regulation of each generator is same.
3.	The work is shared by the two generators without equalizing bar.	The work is shared with the help of equalizing bar if it is not shared the entire load will fall on single generator and there is a chance to destroy the machine	The work is shared in over compound and level compound by using equalizing bar if not the generator 1 will take load and destroys the Machine.