To operate two alternators in parallel, which of their quantities must be same? 

The correct answer is option 3.

Reasons for Parallel Operation of Alternators

• Several alternators can supply a bigger load than a single alternator.
• One or more alternators may shut down during the period of light loads. Thus, the remaining alternator operates at near or full load with greater efficiency.
• When one machine is taken out of service for its scheduled maintenance and inspection, the remaining machines maintain the continuity of the supply.
• If there is a breakdown of the generator, there is no interruption of the power supply.
   

Necessary Conditions for Paralleling Alternators

• The phase sequence of the busbar voltages and the incoming alternator voltage must be the same.
• The phase angle of the two systems must be equal.
• The busbar voltages and the terminal voltage of the incoming alternator must be in phase.
• The frequency of the generated voltage of the incoming machine must be equal to the frequency of the voltages of the busbar.
• The terminal voltage of the incoming alternator must be equal to the busbar voltages.