If we take a Thermal Power Plant, then steam Turbine serves the purpose of prime mover and coupled with Synchronous Generator. Now if the steam supply to the steam turbine is suddenly cut off while the Synchronous Generator is connected to Grid, what will happen?
The answer is simple. If all the Generator protections are working fine then the Generator will be isolated from the Grid by Reverse Power Protection Relay. But here we will assume that no protection is working and then in investigate the behavior of Synchronous Generator.
Here are the various possibilities for a synchronous generator connected to grid when Prime mover is suddenly decoupled. The behavior of Synchronous Generator depends on whether the field winding is excited or not.
Case1: Field Excitation is available
In case the field excitation of Generator is available when the steam supply to the turbine is cut off, the Synchronous Generator will draw power from the Grid and will behave as a Synchronous Motor rotating in the same direction.
Case2:Field Excitation is failed
In this case we will consider two types of Synchronous Generator- Salient Pole and Cylindrical Pole.
Cylindrical rotor machine: It will work as Induction Motor running in same direction with speed less than synchronous speed.
Salient pole machine: It will work as Reluctance Motor running in same direction at synchronous speed.
One more situation is possible when the Prime mover is coupled but the Excitation alone fails, What will happen then?
In this case, the Machine runs as an Induction Generator whose rotor will rotate at a speed higher than synchronous speed.
Based on construction Induction Motor and Synchronous Machine have many similarities. Both have stators wired to have rotating magnetic field. The main difference comes in rotor construction. Induction Motor requires closed path for induced rotor currents created due to flux cutting. This flux cutting arises as the machine is not rotating at synchronous speed, as Slip is not zero.
In Synchronous Machine also closed paths are available. In salient pole machine the damper bars act like squirrel cage rotor as salient poles are laminated so core does not provide proper closed paths. In case of cylindrical rotor machine there is a solid steel core which provides a closed current path. So when machine loses synchronism these will provide paths for circulating currents and create magnetic field.
Now consider a Synchronous Generator connected to Grid. When the excitation fails the power delivering capacity of machine suddenly reduces. So the mechanical power input is greater than electrical power output. This imbalance will accelerate the rotor and force it to run above synchronous speed. The circulating currents and then magnetic field are created. The machine will start to generate power by induction and hence will work as Induction Generator.
Why don’t we then operate Synchronous Generator as Induction Generator?
We cannot operate Synchronous Generator as an Induction Generator. This operation is disastrous as the circulating current will heat the core and may damage the insulations. There is also a considerable change in the reactive power flow. A Synchronous Generator which was generating lagging power is suddenly changed to a machine absorbing lagging power i.e. generating leading power. These sudden changes will certainly cause disturbances in the network.
It should be noted that failure in field does not mean rotor magnetic field is absent. The machine draws reactive energy from Grid to create magnetic field. This field is created by the circulating rotor currents.
In industries, these situations are prevented by the Generator Protection system which isolates the generator from the grid immediately after it senses the fault.