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.

In the above phasor diagram if V_B is the bus-bar voltage and E_in is the emf induced in the alternator. Since it is a parallel operation there is a 180° (opposite) phase difference between them. If E_in to E_in' is the change of excitation due to floating of the machine. Then the resultant voltage and current be E_R and I_s supplied to the bus-bar.

Here, the current I_s lags the voltage E_R by a phase difference of 90° because of the inductive reactance nature of the machine. Let I_L be the load current. Then the total power supplied to the load by the alternators is,

V_B I_L cos Φ_L

Since the current I_L supplied by the incoming alternator is reactive. The load current I_L changes to I_L'. The length OC gives the active power supplied. We can see that if bus bar voltage V_B 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.

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 E_R.

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.

• E_A & E_B = Induced EMFs of alternators A and B respectively,
• I_A & I_B = Respective currents, and
• Z_A & Z_B = Respective phase impedances.

The terminal voltage V is given by,

Load current I, is given by,

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

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