Skip to main content

Parallel Operation of DC Generators

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.
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.
Labels:

Comments

Post a Comment

Comment Policy
We’re eager to see your comment. However, Please Keep in mind that all comments are moderated manually by our human reviewers according to our comment policy, and all the links are nofollow. Using Keywords in the name field area is forbidden. Let’s enjoy a personal and evocative conversation.

Popular posts from this blog

Practical Commutation

The e.m.f. induced in each coil of armature is alternating in nature. If load is connected, the current flowing will also be alternating. But the flow of current in a d.c. generator must be undirectional. This can be achieved by the use of commutator. When the armature conductors are under the influence of one pole they carry current in one direction whereas the current is reversed when the conductors are under the influence of other pole. This reversal of current takes place along the magnetic neutral axis. Fig. 1 Note : The reversal of current is likely to take place in short interval when a coil is short circuited by a brush so that transfer of current from one direction to other is carried out without any sparking. This process is called commutation.        Thus a process by which current in the short circuited coil is reversed while it crosses the MNA is called commutation. The time during which the coil remains short circuited is known as...
1 comment
Read more

Demagnetising and Cross Magnetizing Conductors

The conductors which are responsible for producing demagnetizing and distortion effects are shown in the Fig.1. Fig. 1        The brushes are lying along the new position of MNA which is at angle θ  from GNA. The conductors in the region AOC = BOD = 2θ  at the top and bottom of the armature are carrying current in such a direction as to send the flux in armature from right to left. Thus these conductors are in direct opposition to main field and called demagnetizing armature conductors.         The remaining armature conductors which are lying in the region AOD and BOC carry current in such a direction as to send the flux pointing vertically downwards i.e. at right angles to the main field flux. Hence these conductors are called cross magnetizing armature conductors which will cause distortion in main field flux.        These conductors are shown in the Fig. 2 Fig. 2  ...
Post a Comment
Read more

Winding Pitches

Now we will consider the definitions of different types of winding pitches, required in the design of the armature winding. 1.1 Back Pitch         The distance which is measured interns of armature conductors i.e. between top and bottom coil sides of a coil measured around the back of the armature i.e. away from the commutator is called back pitch and is denoted by . It is measured interms of coil sides. Note : Since it is difference between odd and even number, it is always odd number.        The size of the coil is decided by the back pitch. The back pitch is normally equal to coilsides per pole. It is shown for lap and wave type of winding in the Fig. 1. Fig. 1        The back pitch for the winding shown in the Fig. 1 is y b = 14 - 1 = 13 as coilsides 1and 14 are of coil number 1. 1.2 Front Pitch        It is defined as the distance measured between two c...
Post a Comment
Read more