During the normal operation of synchronous generator, the stator magnetic pole and rotor magnetic pole can be regarded as elastic magnetic force line connection. When the load increases, the power angle will increase, that is, the magnetic line of force will be elongated; When the load decreases, the power angle will decrease, that is, the magnetic line of force will be shortened. When the load changes, due to the inertia of the rotor, the rotor power angle will not immediately stabilize at the new value, but will swing many times around the new stable value. This situation is called the oscillation of synchronous generator.

There are two types of oscillation: one is that the oscillation amplitude is getting smaller and smaller, and the oscillation of power angle is gradually attenuated. Finally, it is stable at a new power angle and still runs stably at synchronous speed, which is called synchronous oscillation; The other is that the amplitude of oscillation becomes larger and larger, and the power angle continues to increase until it gets out of the stable range, which makes the generator out of step, and the generator enters asynchronous operation, which is called asynchronous oscillation.

Phenomenon of generator oscillation or out of step

a) The indication of stator ammeter exceeds the normal value and moves violently back and forth. This is because the angle between the parallel EMFs has changed, resulting in electromotive force difference, which makes the circulating current flow between the generators. Due to the swing of rotor speed, the included angle between electromotive forces is sometimes large and sometimes small, and the torque and power are also sometimes large and sometimes small. Therefore, the circulating current is also sometimes large and sometimes small, so the pointer of stator current swings back and forth. This circulating current plus the original load current may exceed the normal value.

b) The pointer indication of stator voltmeter and other bus voltmeters is lower than the normal value and swings back and forth. This is because the angle between the potential of out of step generator and other generators is changing, causing voltage swing. Because the current is larger than normal, the voltage drop is also large, resulting in low voltage.

c) Active load and reactive load swing sharply. Because the power sent by the generator in the oscillation process without out of step is large or small, and when out of step, it sometimes sends out active power and sometimes absorbs active power.