Load Sharing by Two Alternators in Parallel Operation

     When an incoming alternator is synchronized for parallel operation with other alternators. The alternator floats on the bus-bar i.e., it doesn't share any load or doesn't supply any load current. let us see how to make the alternator to share the load, in parallel operation depending upon its kVA rating.

     In parallel operation of dc generator when an incoming dc generator is connected to share load the machine starts floating on the bus-bar. This is because the induced emf in the incoming dc generator is equal to the bus-bar voltage. To eliminate this floating condition in dc generator the induced emf is made greater than the bus-bar voltage by varying field excitation. Thus the generator starts sharing the load with other generators.

Effect of Change of Excitation on Alternator :

     When an alternator or synchronous generator or ac generator floats on bus-bar after synchronization. By change of excitation, varying the field current changes only in the reactive power sharing but not the active power.

Load Sharing by Two Alternators in Parallel Operation

     In the above phasor diagram if VB is the bus-bar voltage and Ein is the emf induced in the alternator. Since it is a parallel operation there is a 180° ( opposite ) phase difference between them. If Ein to Ein' is the change of excitation due to floating of the machine. Then the resultant voltage and current be ER and Is supplied to the bus-bar. Here, the current Is lags the voltage ER by a phase difference of 90° because of the inductive reactance nature of the machine.

Let IL be the load current. Then the total power supplied to the load by the alternators is,
VB IL cos φL

     Since the current IL supplied by the incoming alternator is reactive. The load current IL changes to IL'. The length OC gives the active power supplied. We can see that if bus bar voltage VB is kept constant there is no change in the active power supplied by the incoming alternator by varying the field excitation.

Effect of Change of Primemover speed on Alternator :

     The active power shared by the incoming alternator can be varied by varying the prime mover speed that drives the alternator. Normally, at the generating station, the turbine mechanical output is given as input to the prime mover and the prime mover drives the alternator. Hence by varying the turbine input i.e., steam in case of thermal power and water pressure in case of hydropower the prime mover speed can be varied.

Load Sharing by Two Alternators in Parallel Operation

     The change in active power component OC' when the prime mover output is changed is shown in the above figure. Here the incoming alternator current supplied to the bus-bar has a strong in-phase component with the voltage ER.

     Therefore, when an alternator floats on the bus-bar, by varying the field excitation and prime mover output. The incoming alternator starts sharing of load with other alternators in parallel operation. The portion of the load shared by the alternator depends upon its kVA rating. The expression for load sharing between two alternators is given as, 

Load Sharing Between Two Alternators :

     Consider two alternators with identical speed-load characteristics running in parallel with a common terminal voltage of V volt and load impedance Z.
  • EA & EB = Induced EMFs of alternators A and B respectively,
  • IA & IB = Respective currents, and
  • ZA & ZB = Respective phase impedances.

Load Sharing by Two Alternators in Parallel Operation

The terminal voltage V is given by,

Load Sharing by Two Alternators in Parallel Operation

Load current I, is given by,

Load Sharing by Two Alternators in Parallel Operation

From the above diagram, the terminal V is equal to IZ i.e., V = IZ. Therefore, the emf EA induced in the alternator A is,

Load Sharing by Two Alternators in Parallel Operation

Similarly, the emf EB induced in the alternator B is,

Load Sharing by Two Alternators in Parallel Operation

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