Load Angle, Slip and Power Swing

In sync machines, Load Angle (Torque Angle) is the angular difference between stator field and rotor field. Practically, in generators when rotor field rotates with the synchronous speed, it induces field in the stator. The induced field rotates with the same angular speed as the rotor field but the rotor fields leads the induced one by some fixed angle in no load steady state (opposite is true in case of a motor). When machines synch with a ring system, its voltage and so the rotor speed is defined by the system. It is like the machine is locked through emf field with huge system which drags it along with the system. In normal condition, steady state, the instantaneous speed of the field (dc rotor) and induced field is same. During sudden change in power system, say LOAD is increased, huge current is drawn by the system from the generator unit which results in reducing the speed of the rotor for a short time. Governor of the unit will try to increase the speed of the generator through speed droop by opening wicket gates. If the change is within the allowable limits, then speed will attain its desired (previous) value. Speed droop is the allowable speed change when load is suddenly varied or altered. Speed droop is the sensitivity of a governor i.e it allows only small (10 to 5% normally) change in speed. Sometimes, an interesting phenomenon happens called Swing. In this case when rotor speeds is reduced, governor tries open wicket gates to achieve the desired speed or frequency. When wicket gates open the speed sometimes increased too high to cross the maximum angle, so, the governor again close the gates accordingly to bring the speed within the limits or to avoid tripping, thus it reduces the speed. Speed is dropped too much to cross the lower limits and thus again an increase command is issued by the governor. This to and fro scenario is called swing. Swing will take time to reach steady state, depends upon the change in the system. If the change is so high that governor fails to achieve the desired speed, the machine will trip because interlocking between the field will be lost.