Nov 18, 2014

Load Sharing of DC Shunt Generators



Shunt Generator are slightly dropping voltage characteristics, and so which is most fitted to stable simultaneous (parallel) operation. Their suitable operation is due to the actual fact that any propensity on the part of a generator to require additional or lesser than its correct share of load ends up in bound changes of voltage within the system that directly oppose this tendency thus restoring the first division of load. Therefore, once paralleled, they're automatically command in parallel.

In the same way, for taking a generator out of service, its field is weakened which of the opposite generator is inflated until the ammeter of the generator to be cleared reads zero. After that, its breaker and the switches are opened in consequence removing the generator out of service. This methodology of connecting in and get rid of a generator from service helps in avoiding any shock or sudden disruption to the prime-mover or to the system itself.
Shunt Generator characteristics
It is evident that the field of one generator is weakened an excessive amount of, then power are delivered to that and it'll run in its original direction as a motor, so driving its prime-mover.
Voltage shunt characteristic of two shunt generators are shown in the fig-a and fig-b above. It’s understood that for a typical terminal voltage V, the generator No. one delivers I1 amperes and generator No.2, I2 amperes. It’s also understood that generator No. 1, having additional dropping characteristic, carries less current. It’s found that 2 shunt generators can divide the load appropriately at all nodes if their characteristics are similar in kind and both has identical drop from no-load to full-load.
If it preferred that 2 generators of various power unit ratings automatically share a load in percentage to their ratings, at that point their external characteristics once planned in terms of their proportion full-load currents should be equal as shown in Fig-c. If, as an example, a 150-kW generator is functioning in parallel with a 300-kW generator to produce a complete of 360-kW, then initial generator can offer 120 KW and also the other 240 KW.
Once the singular characteristics of the generators are recognized, their mutual characteristics will be drawn by adding the detached currents at different of equal. From this combined characteristic, the voltage for any combined load will be read off and from there, current provides by every generator will be found in fig-b.
If the generators have line characteristics, then the above result will be obtained by simple calculations rather than diagrammatically.
We will see how the load sharing of two generators which have unequal no-load voltages.
Let                               E1, E2 = no load voltages of the two generators
                                    R1, R2 = Armature resistances
                                    V = Common terminal voltage
Then,
                                    I1 = (E1-V)/R1 and I2 = (E2-V) / R2
                                   I2/I1 = (E2-V)/ (E1-V) x (R1/R2)
                                    I2/I1= (K2N2Φ2 –V)/K1N1Φ1 – V) x (R1/R2)

From the above equation, it is evident that bus-bar voltage can be kept constant by increasing Φ2 or N2 or by reducing N1 and Φ1.  N2  and N1  are changed by changing the speed of driving engines and Φ1 and Φ2 are changed with the help of regulating shunt field resistances.


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