Correct Answer - Option 1 : 2 MJ / MVA
Two machine system:
- In a two-machine system, a synchronous generator is connected to a synchronous motor by a loss-less network.
- Initially, both the machines are in stable condition. When the load on the motor increases suddenly, the electrical input cannot change simultaneously so that the motor will get oscillations.
- When the oscillations are very high when compared to the natural frequency of oscillations the motor has become unstable.
- When the motor has become unstable, the load on the synchronous generator is zero and the generator will make oscillations.
Then the above two machine system becomes a single machine system.
If the two machines are swinging together then the equivalent inertia constant is given by
Heq = H1 + H2
As given both the machines having the same inertia constant
H1 = H2 = H
Effective inertia constant = 4 MJ/MVA
Heq = 4 MJ/MVA
Heq = H + H
Heq = 2H
4 = 2H
H = 2 MJ/MVA