When generator working normally, the connection between the stator pole and the rotor pole can be regarded as an elastic magnetic line of force. When increasing the load of generator, power angle will increase, which is equivalent to lengthening the magnetic line of force. When reducing the load of generator, power angle will reduce, which is equivalent to shortening the magnetic line of force. When the load of generator changing suddenly, the rotor power angle can not be stabilized at a new value immediately due to the inertia of the rotor. However, it goes through several oscillations around the new stable value, which is called the oscillation of synchronous generator. Oscillation is divided into two types: 1.The amplitude of oscillation becomes smaller and smaller, the swing of power angle gradually attenuates, and finally stabilizes at a new power angle, and still operates stably at synchronous speed, which is called synchronous oscillation. 2.The amplitude of oscillation be