A change in the excitation of an alternator running in parallel with another alternator:

Effect of Change in Excitation of Alternator:

• A change in the excitation of an alternator running in parallel with others affects only its KVA output it does not affect the KW output.
• A change in the excitation, thus, affects only the power factor of its output.
   

Explanation:

• Let the initial operating conditions of the two parallel alternators be identical i.e. each alternator supplies equal kW and kVAr of load, thus the operating power factors being equal to the load power factor.
• Now, let the excitation of alternator No. 1 be increased so that excitation emf of alternator 1 (E₁) becomes greater than E2.
• The difference between the two EMFs sets up a circulating current (I_C)
  • \(I_C=\frac{E_1-E_2}{2Z_S}\)

• As seen, I_C is vectorially added to the load current of alternator No. 1 and subtracted from that of No. 2.
• The two machines now deliver load currents I₁ and I₂ at respective power factors of cos φ₁ and cos φ₂.
• These changes in load currents lead to changes in power factors, such that cos φ₁ is reduced, whereas cos φ₂ is increased.
• However, the effect on the kW loading of the two alternators is negligible, but kVAR₁ supplied by alternator No. 1 is increased, whereas kVAR₂ supplied by alternator No. 2 is correspondingly decreased, as shown by the kVA triangles.