Parallel Operation Of Shunt Generators

Introduction


D.C. generators are required to operate in parallel supplying a common load when the load is larger than the capacity of any one machine. In situations where the load is small but becomes high occasionally, it may be a good idea to press a second machine into operation only as the demand increases. This approach reduces the spare capacity requirement and its cost. In cases where one machine is taken out for repair or maintenance, the other machine can operate with reduced load. In all these cases two or more machines are connected to operate in parallel.

Shunt Generators 
Parallel operation of two shunt generators is similar to the operation of two storage batteries in parallel. In the case of generators we can alter the external characteristics easily while it is not possible with batteries. Before connecting the two machines the voltages of the two machines are made equal and opposing inside the loop formed by the two machines. This avoids a circulating current between the machines. The circulating current produces power loss even when the load is not connected. In the case of the loaded machine the difference in the induced emf makes the load sharing unequal. Fig. 37 shows two generators connected in parallel. The no load emfs are made equal to E1 = E2 = E on no load; the current delivered by each machine is zero. As the load is gradually applied a total load current of I ampere is drawn by the load. The load voltage under these conditions is V volt.





 



Each machine will share this total current by delivering currents of I1 and I2 ampere
such that I1 + I2 = I. Also terminal voltage of the two machines must also be V volt. This is dictated by the internal drop in each machine given by equations

        V = E1 _ I1Ra1 = E2 _ I2Ra2


where Ra1 and Ra2 are the armature circuit resistances. If load resistance RL is known these equations can be solved analytically to determine I1 and I2 and hence the manner in which to total output power is shared. If RL is not known then an iterative procedure has to be adopted. A graphical method can be used with advantage when only the total load current is known and not the value of RL or V. This is based on the fact that the two machines have a common terminal voltage when connected in parallel. In Fig. 38 the external characteristics of the two machines are first drawn as I and II. For any common voltage the intercepts OA and OB are measured and added and plotted as point at C. Here OC = OA + OB. Thus a third characteristic where terminal voltage is function of the load current is obtained. This can be called as the resultant or total external characteristics of the two machines put together. With this, it is easy to determine the current shared by each machine at any total load current I. The above procedure can be used even when the two voltages of the machines at no load are different. At no load the total current I is zero ie I1 + I2 = 0 or I1 = _I2. Machine I gives out electrical power and machine II receives the same. Looking at the voltage equations, the no load terminal equation Vo becomes

Vo = E1 _ I1Ra1 = E2 + I2Ra2

As can be seen larger the values of Ra1 and Ra2 larger is the tolerance for the error between the voltages E1 and E2. The converse is also true. When Ra1 and Ra2 are nearly zero implying an almost at external characteristic, the parallel operation is extremely difficult.

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