This model represents a simple islanded power system with primary frequency control open loop. Generator A has 30 MW nominal power, generator B has 10 MW.
The model starts at steady state, with both generators at full load supplying a 40 MW load connected at the A side of the transmission line.
At time = 2s, the load is decreased by 10%. The primary frequency controllers kick in and stabilize the frequency, with an error of 5% of 10% of 50 Hz, i.e. 0.25 Hz. Electro-mechanical oscillations are also triggered, but eventually die out thanks to the generator damping effect.