Question

A synchronous generator is running over excited with a Ef=1.40pu. This machine, with a synchronous reactance of 1.2pu, is delivering a synchronous power of 0.5pu to the bus. If the prime-mover torque is increased by 1%, by how much will the reactive power, Q change?

(a) -0.475 %

(b) 0.475%

(c) 4.75%

(d) -4.75%

I got this question at a job interview.

I'd like to ask this question from Power Angle Characteristics of Synchronous Machines in chapter Synchronous Machines of Electrical Machines

Answer 1

Right option is (a) -0.475 %

For explanation I would say: dQ/dδ = -EVsinδ/X

            dP/dδ = EVcosδ/X

            dQ/dP = -tanδ = -tan25.4 = -0.475

            dQ = -0.475%.